TECHNICAL REFERENCE FOR HAYES (TM) MODEM USERS

    15 Sept 1992
    Version P2.0

    Hayes Microcomputer Products, Inc.
    P.O. Box 105203
    Atlanta, Georgia 30348 U.S.A.

    (c) 1990, 1992 Hayes Microcomputer Products, Inc.
    All rights reserved.

    44-00012 AA H32 (BBS Version)

Note: This version of the "Technical Reference for Hayes Modem Users" is a special version edited for bulletin-board downloading. As a plain ASCII character file, it therefore cannot contain any of the illustrations and graphic elements provided in the printed version.

## I - Internal Memory Tests

The various forms of the I command instruct the modem to query its memory for information about itself. The results of these tests are frequently used by programmers to determine compatibility with software. Because these commands request information about the modem's firmware, they are not run when a connection has been established with a remote modem.

## I0 - Display Product Code

This option reports the product code of the modem to the DTE. The modem produces information text dependent upon its highest DCE line speed. The responses below are examples:

    Result Codes     Description
    ------------------------------------------------------------------------
    300              Smartmodem 300.
    120              Smartmodem 1200, Smartmodem 1200B, Smartmodem 1200C,
                     Smartmodem 1200A.
    240              Smartmodem 2400, Smartmodem 2400B, Smartmodem 2400P,
                     Smartmodem 2400Q, Smartmodem 2400M, V-series Smartmodem
                     2400, V-series Smartmodem 2400B, V-series Smartmodem
                     2400 Quad, V-series Smartmodem 2400M, Smartmodem OPTIMA
                     (TM) 24, Smartmodem OPTIMA 24 + FAX96,
                     ACCURA (TM) 2400 EC/FAX96.
    960              Smartmodem 9600, V-series Smartmodem 9600, V-series
                     Smartmodem 9600B, V-series ULTRA (TM) Smartmodem 2400,
                     V-series ULTRA Smartmodem 9600, V-series ULTRA 24 with
                     Express 96, Smartmodem OPTIMA 9600, Smartmodem
                     OPTIMA 96 + FAX96, ACCURA 9600 EC/FAX96.
    14400            V-series ULTRA Smartmodem 14400, Smartmodem OPTIMA 144,
                     Smartmodem OPTIMA 144 + FAX144, ACCURA 14400 EC/FAX144.
    ------------------------------------------------------------------------

## I1- Display ROM Checksum

The I1 command instructs the modem to calculate the value of the ROM checksum. The response is a number, the sum of all of the bytes in ROM.

## I2 - Perform ROM Checksum

This command instructs the modem to verify the ROM checksum. Depending on whether the ROM checksum has been found to be correct, the modem produces text that resembles a result code. The modem memory test compares the ROM checksum and tests it against the correct sum, also stored in ROM. Rather than returning a numeric value as in I1, the I2 command generates a result code. When the checksum is valid, the response is: OK. When the ROM checksum fails, the modem responds with ERROR.

## I4 - Identify Product Features

The capabilities and features of the modem are encoded into a string of text that consists of several strings that are ASCII character representations of hex numerals which are bit-mapped. The first character of each string identifies which bit maps are in that string. For example, the "a-string" starts with a lower case "a" and identifies most of the basic modem capabilities such as modulation standards supported and support for AutoSync.

Since the following tables identify features for Hayes modem products, the values included here and the number of strings are subject to change and expansion. The maximum length per string is 40 characters.

The I4 text is displayed in the following form:

         a097800C204C264
         bF60410000
         r1031111111010000
         r3000111010000000

surrounded by additional <CR> and <LF> characters as are required by the V command option in effect. According to convention, all <CR> and <LF> characters are defined by S3 and S4, respectively. The meanings of the a, b, r1, and r3 strings currently defined are described below.

### I4 "a" String

The first string, the a-string, is composed of the ASCII character "a" followed by a series of hexadecimal characters (D1-D16 in this example). The bit map for each byte is defined below.

    a D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15 D16 D17 D18....

    ------------------------------------------------------------------------
    D1, D2     Reserved
    D3         Bit 3: Indicates modem based on SM1200FE commands
               Bit 2: Indicates modem based on SM2400 commands
               Bit 1: Indicates that modem supports &J commands
               Bit 0: Indicates that modem supports &L commands
    D4         Bit 3: Indicates that modem supports AutoSync (&Q4)
    D5         Bit 3: Plug-in board modem product
               Bit 2: Standalone modem product
               Bit 1: Supports &H0 through &H4
               Bit 0: Supports &I0 through &I4
    D6         Bit 3: Reserved
               Bit 2: Supports M4 command
               Bit 1: Supports 32-bit FCS (frame check sequence
               for AutoSync)
               Bit 0: Supports AutoSync II
    D7         Bit 3: Supports V.22 at 1200 bps (B0)
               Bit 2: Supports 212A (B1)
               Bit 1: Supports ASB in V.23 75 bps xmt/1200 bps rcv (B2)
               Bit 0: Supports ASB in V.23 1200 bps xmt/75rcv (B3)
    D8         Bit 3: Supports V.23 1200 bps xmt/75 bps rcv (B4)
               Bit 2: Supports V.23 1200 half duplex (B5)
               Bit 1: Supports V.23 75 bps xmt/1200 bps rcv (B10)
               Bit 0: Supports V.23 75 bps xmt/600 bps rcv  (B11)
    D9         Bit 3: Supports V.21 100/300 bps (B15)
               Bit 2: Supports ASB in V.23 75 bps xmt/600 bps rcv (B20)
               Bit 1: Supports ASB in V.23 600 bps xmt/75 bps rcv (B21)
               Bit 0: Supports V.23 600 bps xmt/75 bps rcv  (B22)
    D10        Bit 3: Supports V.23 600 bps half-duplex (B23)
               Bit 2: Supports V.22bis at 2400 bps (B30)
               Bit 1: Supports V.27ter at 2400 bps (B31)
               Bit 0: Supports V.27ter at 4800 bps  (B40)
    D11        Bit 3: Supports V.32 full duplex at 4800 bps (B41)
               Bit 2: Supports Express 96 at 4800 bps (B42)
               Bit 1: Reserved
               Bit 0: Supports V.29 half duplex at 4800 bps (B44)
    D12        Bit 3: Supports V.29 half duplex at 7200 bps (B50)
               Bit 2: Supports V.32 full duplex at 9600 bps (B60)
               Bit 1: Supports Express 96 at 9600 bps (B61)
               Bit 0: Reserved
    D13        Bit 3: Supports V.29 half duplex at 9600 bps (B63)
               Bit 2: Supports 103 110/300 bps (B16)
               Bit 1: Supports V.32bis (7200/12000/14400 bps)
               Bit 0: Reserved
    ------------------------------------------------------------------------

    ------------------------------------------------------------------------
    D1, D2      Reserved
    D14         0: Reserved
                1: Personal Modem 1200
                2: Personal Modem 2400
                3: Personal Modem 1200 Plus
                4: Personal Modem 2400 Plus
                5: Pocket Edition
    D15         Bit 3: Reserved
                Bit 2: Supports $ dial modifier and NO DIALTONE result code.
                Bit 1: Supports S95 Bit 5 for COMPRESSION: result code
                Bit 0: Supports S95 Bit 4 for AUTOSTREAM: result code
    D16         Bit 3: Supports S95 Bit 3 PROTOCOL: result code
                Bit 2: Supports S95 Bit 2 CARRIER result code
                Bit 1: Supports S95 Bit 1 CONNECT/ARQ result code
                Bit 0: Supports S95 Bit 0 CONNECT XXXX (for DCE rate)
    ------------------------------------------------------------------------

### I4 "b" String

The second string, the b-string, is composed of the ASCII character "b" followed by a series of hexadecimal characters (D1-D9). The bit map for each character is defined as follows:

    b D1 D2 D3 D4 D5 D6 D7 D8 D9....

    ------------------------------------------------------------------------
    D1          Bit 3: V.42 Alternate Protocol Supported
                Bit 2: V.42 LAPM Protocol Supported
                Bit 1: X.25 Protocol Supported
                Bit 0: LAPB (Point-to-point error control) protocol supported
    D2          Bit 3: Reserved (should be set to zero)
                Bit 2 MNP (TM) Class 5 supported
                Bit 1: V.42bis supported
                Bit 0: Compression through the X.25 network supported
    D3/D4       These combine to indicate the number of AutoStream
                Type A channels which are supported. The formula
                (D3*16 + D4) is used. Zero means AutoStream is not supported
    D5          Bit 3: Reserved
                Bit 2: Reserved
                Bit 1: Adjustable startup (ASU) is supported
                Bit 0: Negotiates adjustable startup
    D6          Bit 3: &K5 not supported
                Bit 2: &K5 supported
                Bit 1: Unidirectional flow control
                Bit 0: S105 N104 parameter supported
    ------------------------------------------------------------------------

### I4 "r1, r2" and "r3" strings

These ID strings allow software to determine the available speeds that may be used to send AT commands. The r1-string contains a bit map that indicates at which DTE rates the autobaud process is supported.

The presence of the r2-string in the I4 result infers that S87 is supported and is required for autobauding at the higher speeds. To autobaud at one of the speeds indicated in the r2-string requires that S87 be set to match that speed.

The r3-string is issued if synchronous DTE speeds are supported. The map indicates which DTE rates are supported in synchronous modes. Each r- string begins with the lower case letter "r" and may be followed by as many as 39 additional characters, not counting <CR><LF>'s that will be used to separate them from other strings. Those 39 additional characters are limited to the ASCII-HEX alphabet 0-9 and A-F.

After the two lead-in characters (i.e., r<n>), all subsequent characters contain the DTE rate maps. All maps have the same mapping for convenience to software. (Refer to the following chart.) Not all bit assignments are possible: for example, the split speeds have no meaning in the r3 synchronous map and are always filled with zeros.

In r1, if the bit is filled with a 1, the corresponding DTE rate is supported for sending AT commands to the device. A zero indicates that DTE rate is not supported for AT commands.

In the r3 map, the bits simply indicate which DTE rates are supported for synchronous operation on-line. This does not indicate, however, the supported rates for synchronous V.25bis commands.

Split speeds if available are marked in the r1 string only when the appropriate B command option has been selected. Future expansion of these strings may include new speeds that are not in strict ascending order.

### DTE Rate Bit Map for r1, r2 and r3 Strings

    Character    Bit #       DTE Rate
    ------------------------------------------------------------------------
     3:  1       bit 0       45.45 bps
         2       bit 1       50
         4       bit 2       75
         8       bit 3       75/600 (xmt is 75, rcv is 600)
     4:  1       bit 4       75/1200
         2       bit 5       110
         4       bit 6       134.5
         8       bit 7       50
     5:  1       bit 8       300
         2       bit 9       450
         4       bit 10      600
         8       bit 11      600/75
     6:  1       bit 12      1200
         2       bit 13      1200/75
         4       bit 14      1800
         8       bit 15      2000
     7:  1       bit 16      2400
         2       bit 17      3000
         4       bit 18      3600
         8       bit 19      4200
     8:  1       bit 20      4800
         2       bit 21      5400
         4       bit 22      6000
         8       bit 23      6600
     9:  1       bit 24      7200
         2       bit 25      7800
         4       bit 26      8400
         8       bit 27      9000
    10:  1       bit 28      9600
         2       bit 29      12000
         4       bit 30      14400
         8       bit 31      16800
    11:  1       bit 32      19200
         2       bit 33      21600
         4       bit 34      24000
         8       bit 35      26400
    12:  1       bit 36      28800
         2       bit 37      31200
         4       bit 38      33600
         8       bit 39      36000
    13:  1       bit 40      38400
         2       bit 41      43200
         4       bit 42      48000
         8       bit 43      52800
    14:  1       bit 44      56000
         2       bit 45      57600
         4       bit 46      62400
         8       bit 47      64000
    15:  1       bit 48      67200
         2       bit 49      72000
         4       bit 50      76800
         8       bit 51      81600
    16:  1       bit 52      86400
         2       bit 53      91200
         4       bit 54      96000
         8       bit 55      100800
    17:  1       bit 56      105600
         2       bit 57      110400
         4       bit 58      115200
         8       bit 59      reserved ("0")
    ------------------------------------------------------------------------

## W - Negotiation Progress Message Selection

The W command works in conjunction with S95 (where supported) to determine which result codes will be used to describe the type of connection and protocol, etc., that resulted from handshaking and negotiation.

The W command supports extended result codes in addition to the CONNECT result code. When the modem is operated in error-control mode (&Q5 is in effect), the W command and S95 together allow the user to select these additional result codes:

    CARRIER
    PROTOCOL
    AUTOSTREAM
    COMPRESSION
    CONNECT

Any result codes enabled by the W command and S95 will be generated in the order indicated above. If AutoStream is not being used, no AUTOSTREAM result code is returned. Result codes not enabled when the W command option is in effect may be turned on by setting certain bits in S95. The W command options below are available when S95 is configured for any setting other than the factory-setting of 0.

    ------------------------------------------------------------------------
    W0     CONNECT result code reports DTE speed, and if S95=0, then disable
           all extended result codes.
    W1     CONNECT result code reports DTE speed, and if S95=0, then enable
           the CARRIER and PROTOCOL extended result codes.
    W2     CONNECT result code reports DCE speed, and if S95=0, then disable
           all extended result codes.
    ------------------------------------------------------------------------

Refer to the S95 bit map description in the next section. Note that selecting W0 and setting S95=12 is the same as selecting W1; and that selecting W0 and setting S95=1 is the same as selecting W2. S95 cannot be configured to force W2 to report DTE speed in the CONNECT result code; there is no setting of S95 that will force W1 not to produce the CARRIER and PROTOCOL result codes. S95 extends the functionality of the W command. The W command with S95=0 (factory setting) maintains backwards compatibility with previous Hayes Products. However, selecting W0 and setting S95 as required allows you to tailor result code characteristics to your own requirements.

## Z - Soft Reset Command

The modem can be reset by issuing the Z command. The command tells the modem to go on hook and restore the selected stored profile. Any non- storable parameters previously set by commands are returned to their factory settings. The modem aborts execution of all commands following the Z command on the same command line. Subsequent commands on the same line are ignored. Refer to the &W command for description of which modem parameters are included in a stored profile. Z0 recalls stored user profile 0, stored with &W0; Z1 recalls stored user profile 1, stored with &W1.

    ------------------------------------------------------------------------
    Z0     Recall stored profile 0
    Z1     Recall stored profile 1
    ------------------------------------------------------------------------

## &T19 - Perform RTS/CTS Cable Test

Software can use &T19 to help determine whether the DTE-to-Modem cable supports the RTS and CTS signals necessary for hardware flow control, which is selected by the &K3 command.

&T19 in the modem works interactively with the DTE to confirm the RTS and CTS signal. Only the DTE can detect CTS, and only the modem can detect RTS. The modem cannot verify both signals without the active involvement of the DTE software.

The modem performs the following algorithm in response to the AT&T19<CR> command:

1. Turns OFF CTS (normally ON).
2. Starts a 500 millisecond timer.
3. Monitors RTS for ON and OFF states.
4. If both states of RTS are detected before the 500ms timer expires then:
     a. Restore CTS to the ON state.
     b. End the test (stop the timer).
     c. Return the OK result code (meaning the modem has verified RTS).
5. If the timer expires before both states of RTS are detected then:
     a. Restore CTS to the ON state.
     b. Return the ERROR result code (meaning the modem has not verified RTS).

Note: By itself, the modem cannot verify either signal. The DTE must toggle RTS during the test in order for the modem to be able to detect a change. Also, the DTE should look for CTS to drop during the test to verify it is connected.

The following algorithm is used by Hayes Smartcom software in its use of the &T19 test:

1. Issue AT&T19 <CR>

   Note: For more predictable timing, do not combine other commands on the same command line with &T19. Also, higher DTE port speeds improve the timing resolution.

2. Wait 500ms for CTS to drop. If CTS does not drop in that time, then the test fails.
3. If CTS drop is detected, then:
     a. Drop RTS, wait 50ms, raise RTS, wait 50ms, repeat up to 6 times, or until CTS is detected high.
     b. If CTS is detected returning high before 6 iterations, then the test is passing (and an OK result code may be expected soon), or if all 6 iterations did not result in CTS being raised, then the RTS part of the test is failing (and an ERROR result code may be expected after the 500ms timer expires).
4. Process the result code from the modem. Success during Step 3 above and an OK result code implies that RTS and CTS are both wired correctly. Failure during Step 3 or an ERROR result code implies that either RTS or CTS is not wired in the cable.

Note: Keep in mind that the result code may arrive while you are still in Steps 2 and 3 above.

A modification to the software algorithm could be made to permit software to detect which signal is missing from the cable. After issuing the AT&T19 <CR> command, give the modem about 100ms to drop CTS. Then, even if the CTS drop is not detected, proceed to toggle RTS anyway, keeping in mind that the CTS part of the test has failed.

RTS/CTS flow control is preferred because it is "out of band", that is, it does not consume any of the 256 serial codes from the user data. XON/XOFF flow control uses DC1 (ASCII 17) and DC3 (ASCII 19). If RTS/CTS is not supported, then Transparent Flow Control (&K5) is the next best option (IF the software supports it) because it accomplishes Xon/Xoff flow control without interfering with the user data (e.g. during binary file transfers).

## 1.2 Result Code Listing

This section defines the result codes returned by Hayes modems in response to commands.

The table below shows the various formats in which modem responses can be presented. Note that the "text" of the info-text may consist of multiple lines of text. The formats depicted here only refer to the <CR><LF> characters between info-texts and not within them.

    ------------------------------------------------------------------------
                      V0            V1
    Information Text  text          <CR><LF>
                      <CR><LF>      text
                                    <CR><LF>
    Result Codes      numeric code  <CR><LF>
                      <CR>          verbose code
                                    <CR><LF>
    ------------------------------------------------------------------------

### 1.2.1 Command Response and Call Progress Monitoring

This set of result codes includes responses to commands and call progress monitoring responses. They are available to all modems within the capabilities of the modem. For example, the result code CONNECT 9600 is not available to Smartmodem 2400. The factory setting for all high- speed modems enables the extended set of call progress monitoring (X4). When set up in this way, the modem performs and reports full call progress monitoring (RING, NO CARRIER, NO DIALTONE, and BUSY). It also indicates the speed of the connection (CONNECT 1200 as opposed to simply CONNECT). The factory setting for Smartmodem 300, Smartmodem 1200, and all others whose highest speed is 1200 bps is basic call progress monitoring (X0).

The command response and call progress monitoring result codes are defined below:

0 - OK
This result code indicates that a command or command string was executed. Note that if more than one command were included on a line and an ERROR result code received, this means that one or more of the commands was not processed. If one or more were executed properly, but even one was invalid, no OK will be issued, only the ERROR.

1 - CONNECT
This result code indicates a connection was made between the DTE and the modem. If X4 (extended set of call progress monitoring) were selected, the code indicates that a connection from at 0 to 300 bps was made. However, if X0 (basic set of call progress monitoring) were selected, the connection could be 0-300, 1200, 1200/75, 75/1200, 2400, 4800, 7200, 9600, 14400, 19200, or 38400 bps. If the modem is not operating in error-control or ASB mode, this is the same as the line speed. See other CONNECT messages and CARRIER messages.

2 - RING
This result code indicates the modem has detected a ring signal. No distinction can be made as to whether this is a voice call, a modem call, a fax call, or other type.

3 - NO CARRIER
This result code indicates that no carrier signal was detected, or that the signal was lost. This is the response the modem will give when no connection is made; see CONNECT result code. The modem will also return this message when the connection is broken, either intentionally as when the hangup process completes, or if line difficulties break the connection.

4 - ERROR
This result code indicates that an invalid command was issued, or that there was an error in the command line. For example, if the command line exceeds the character limit for your modem, this result code will be returned. See your user documentation to determine the character limit for your modem. This result code is also returned in response to the I1 command requesting a ROM checksum, if the modem detects an error in the computation.

5 - CONNECT 1200
This result code indicates a connection has been established at 1200, 1200/75 or 75/1200 bps between the modem and the DTE. If the modem is not operating in error-control mode, this is the same as the line speed. This result code is disabled by X0. Only CONNECT is reported.

6 - NO DIALTONE
This result code indicates that no dial tone was detected when the modem went off hook. Dial tone detection and this result code are enabled by X2 or X4, or the W dial modifier.

7 - BUSY
This result code indicates that the modem detected a busy signal when it attempted to connect with the modem at the number dialed. Busy signal detection and this result code are enabled by X3 or X4.

8 - NO ANSWER
This result code indicates no silence was detected when dialing a system not providing a dial tone. Enabled by the @ dial modifier.

10 - CONNECT 2400
This result code indicates a connection has been established at 2400 bps between the modem and the DTE. If the modem is not operating in error- control mode, this is the same as the line speed. This result code is disabled by X0. Only CONNECT is reported.

11 - CONNECT 4800
This result code indicates a connection has been established at 4800 bps between the modem and the DTE. This result code is disabled by X0.

12 - CONNECT 9600
This result code indicates a connection has been established at 9600 bps between the modem and the DTE. This result code is disabled by X0.

13 - CONNECT 14400
This result code indicates a connection has been established at 14400 bps between the modem and the DTE. This result code is disabled by X0.

14 - CONNECT 19200
This result code indicates a connection has been established at 19200 bps between the modem and the DTE. This result code is disabled by X0.

22 - CONNECT 1200/75
This result code indicates a connection has been established at 1200 bps when transmitting data and 75 bps when receiving data between the modem and the DTE.

23 - CONNECT 75/1200
This result code indicates a connection has been established at 75 bps when transmitting data and 1200 bps when receiving data between the modem and the DTE.

24 - CONNECT 7200
This result code indicates a connection has been established at 7200 bps between the modem and the DTE. This result code is disabled by X0.

25 - CONNECT 12000
This result code indicates a connection has been established at 12000 bps between the modem and the DTE. This result code is disabled by X0.

28 - CONNECT 38400
This result code indicates a connection has been established at 38400 bps between the modem and the DTE. This result code is disabled by X0.

### 1.2.2 Negotiation Progress Messages

Hayes products report special result codes during error-control negotiation. Whether or not these messages are displayed is selected with the W command (not to be confused with the W dial modifier). The factory setting is messages disabled (W0) to avoid conflict with software programs that do not support this additional level of call progress monitoring.

Negotiation progress messages are reported in the following order:

    CARRIER
    PROTOCOL
    AUTOSTREAM
    COMPRESSION
    CONNECT

If AutoStream is not used, no message is reported.

40 - CARRIER 300
This message indicates that a carrier signal has been detected at 300 bps (modem-to-modem line speed).

44 - CARRIER 1200/75
This message indicates that a carrier signal has been detected at 1200 bps when transmitting and at 75 when receiving (modem-to-modem line speed).

45 - CARRIER 75/1200
This message indicates that a carrier signal has been detected at 75 bps when transmitting and at 1200 bps when receiving (modem-to-modem line speed).

46 - CARRIER 1200
This message indicates that a carrier signal has been detected at 1200 bps (modem-to-modem line speed).

47 - CARRIER 2400
This message indicates that a carrier signal has been detected at 2400 bps (modem-to-modem line speed).

48 - CARRIER 4800
This message indicates that a carrier signal has been detected at 4800 bps (modem-to-modem line speed).

49 - CARRIER 7200
This message indicates that a carrier signal has been detected at 7200 bps (modem-to-modem line speed).

50 - CARRIER 9600
This message indicates that a carrier signal has been detected at 9600 bps (modem-to-modem line speed).

51 - CARRIER 12000
This message indicates that a carrier signal has been detected at 12000 bps (modem-to-modem line speed).

52 - CARRIER 14400
This message indicates that a carrier signal has been detected at 14400 bps (modem-to-modem line speed).

66 - COMPRESSION: CLASS 5
This message indicates that data compression using MNP Class 5 has been negotiated for the connection.

67 - COMPRESSION: V.42BIS
This message indicates that data compression using CCITT V.42bis has been negotiated for the connection.

68 - COMPRESSION: ADC
This message indicates that data compression using Hayes Adaptive Data Compression has been negotiated for the connection.

69 - COMPRESSION: NONE
This message indicates that data compression was not negotiated for the connection.

70 - PROTOCOL: NONE
This message indicates that no protocol was negotiated for the connection. A standard asynchronous connection was made.

71 - PROTOCOL: ERROR-CONTROL/ LAPB
This message indicates that an error-control connection was negotiated with LAPB protocol.

72 - PROTOCOL: ERROR-CONTROL/ LAPB/HDX
This message indicates that a half-duplex error-control connection was negotiated with LAPB protocol.

73 - PROTOCOL: ERROR-CONTROL/LAPB/AFT
This message indicates that an error-control connection was negotiated using the Hayes Asynchronous Framing Technique. This protocol is used for connections between modems such as Smartmodem 1200 that do not communicate synchronously across the telephone line. AFT enables an error-control protocol to be used.

74 - PROTOCOL: X.25/LAPB
This message indicates that an error-control connection using the X.25 protocol was established with a carrier speed of 1200, 2400, 4800, or 9600 bps.

75 - PROTOCOL: X.25/LAPB/HDX
This message indicates that a half-duplex error-control connection using the X.25 protocol was established with a carrier speed of 4800 or 9600 bps.

76 - PROTOCOL: X.25/LAPB/AFT
This message indicates that an asynchronous error-control connection using the X.25 protocol was established with a carrier speed of 1200 bps. The Hayes Asynchronous Framing Technique was used.

77 - PROTOCOL: LAP-M
This message indicates that an error-control connection using the V.42 LAPM protocol was established.

78 - PROTOCOL: LAP-M/HDX
This message indicates that a half-duplex error-control connection using the V.42 LAPM protocol was established.

79 - PROTOCOL: LAP-M/AFT
This message indicates that an asynchronous error-control connection using the V.42 LAPM protocol was established with a carrier speed of 1200 bps. The Hayes Asynchronous Framing Technique was used.

80 - PROTOCOL: ALT
This message indicates that an errorge indicates that an error-control connection using the V.42 LAPM alternative protocol was established. This protocol is MNP Classes 2, 3, and 4 compatible.

91 - AUTOSTREAM: LEVEL 1
This message indicates that Hayes AutoStream Level 1 has been negotiated for the connection. This technique provides for multiplexing of multiple virtual channels.

92 - AUTOSTREAM: LEVEL 2
This message indicates that Hayes AutoStream Level 2 has been negotiated for the connectionsage indicates that Hayes AutoStream Level 2 has been negotiated for the connection. This technique provides for multiplexing of multiple virtual channels, with transparent control of one PAD (non- simultaneous). Level 3 has been negotiated for the connection. This technique provides for multiplexing of multiple virtual channels, with transparent control of all PADs (simultaneous).

93 - AUTOSREAM LEVEL 3
This message indicates that Hayes Autostream Level 3 has been negoitiated for the connection. This technique provides for multiplexing of multiple virtual channels, with transparent control of all PADs (simultaneous).

## S95 Negotiation Message Options

S95 enables various result codes that indicate the sequence of events in the establishment of an error-control connection. This register does not affect the way in which the modem negotiates the connection; it merely enables message options. The factory setting for this register is value 0, no bits selected. To enable any combination of the bits, add the value(s) to the right of the bit number and set the register to this sum.

Note: The bit values of S95 may be set to override some of the characteristics of the Wn command. Setting any of the S95 bits to "1" enables the corresponding result codes regardless of the Wn command in effect. Changing the Wn command setting does not affect the value set for this register.

    Bit   Value    Explanation
    ------------------------------------------------------------------------
    0     1        Verbose CONNECT result code indicates the DCE speed
                   (rather than DTE speed). Numeric result codes are also
                   different when CONNECT reports DCE speed.
    1     2        Append "/ARQ" to CONNECT result code when an
                   error-control connection is made.
    2     4        Add CARRIER messages
    3     8        Add PROTOCOL messages
    4     16       Add AUTOSTREAM messages
    5     32       Add COMPRESSION messages
    ------------------------------------------------------------------------

For example, if you want to add the compression result code (with W1 selected), you would select bit 5 (value of 32). The command line ATS95=32J<CR> will then enable the COMPRESSION negotiation messages.

Refer to the Wn and Xn commands for additional and related information.

## APPENDIX D: APPLICATION SUGGESTIONS

This appendix offers suggestions for developing applications software using the Hayes Standard AT Command Set. The techniques described apply to Hayes modems in general except where specifically indicated. Although provided here, this information is intended for experienced programmers who want assistance in modem application development.

### D.1 Modem Identification

The initial concern for most communications software is modem identification. Before the software determines the type of modem (e.g., is it a Hayes modem, a high-speed modem, what features does it support - error-control or compression?). By limiting the AT command controller portion of the software to work with a known set of modems, you can limit the complexity of your software.

Because the type of modem that will be present, certain assumptions can be made regarding modem characteristics, such as maximum transmission rate, support of AT commands or specific commands such as L or X. If a more general application is being designed for an environment about which assumptions cannot be made regarding type or brand of modem that might be used, the software's first task should be to identify the modem.

The I0 and I4 command options make this process simple. In the initial versions of Smartmodem 1200, I0 returned the three digit response: 120. Since then, responses have been extended for several groups identifying modem supporting 2400 bps, 9600 bps, and other products. The I0 response simply indicates the speed category of the modem.

The result of the I0 command is a number which identifies the category of modem product. Some unique I0 values can be used to identify a unique product which has specific behaviors. 960, for example, identifies a Hayes product capable of 9600 bps, which has additional commands and behaviors.

The I4 command provides a reliable means of communicating specific features and modulation protocols to software. The responses to the I4 command are strings delimited by <CR> and beginning with a lowercase letter and typically followed by a hex-character bit-map. The I0 and I4 responses currently defined are detailed in the description of the I4 command in Chapter 1. The tables show the decoding of the hex- map returned in the "a", and "b" bit-mapped strings. If I4 is used to identify features of the modem, consider that new result strings are periodically defined that may be returned in addition to those expected. Fields once designated as "reserved" that held a zero may now have values assigned. The strings themselves may also be of different lengths than previously implemented.

In spite of the modifications to this command necessary to be current with new modems, the I4 command is the best way for software to determine the modem type and capability, if the guidelines below are considered:

- I0 or I4 commands should be issued at 1200 bps. All Hayes products (including the Smartmodem 300) respond to AT commands at 1200 bps. Most other brands also respond at 1200 bps. You can switch to a higher transmission rate once the modem has been identified.
- Result codes should be parsed as strings surrounded by <CR><LF>. The string will begin with a lower-case letter followed by up to 39 additional characters.
- After all result strings have been sent, an OK result is returned that obeys the V and Q command settings.
- ERROR, OK, or a numeric result in response to the I4 command should be expected. These results may be returned by products shipped before the I4 command was introduced, or by non-Hayes products.
- The length of the strings may be different than anticipated. If shorter than expected, empty positions should be presumed zeros. If longer than expected, extra characters should be ignored.
- Some non-Hayes brand modems return unpredictable results in response to I0 or I4 commands. One brand of modem actually responds with its configuration when the I4 command is sent.

  An example I4 command and response is shown below:

                           AT E0 V1 Q0 S0=0 I4 <CR>

  response:

           <CR><LF>a087840C004424<CR><LF>
           <CR><LF>bF60410000<CR><LF>
           <CR><LF>cUS<CR><LF> J
           <CR><LF>m0000000001001FFFF<CR><LF>
           <CR><LF>OK<CR><LF>

  Note: Each I4 result is surrounded by <CR><LF>; not all responses are hex-strings; and some responses may not be expected at all.

### D.2 Result Code Recognition

Modems that are equivalent to Hayes modems support verbose and numeric forms of result codes. Unless echo may be a problem and you will be installing the controller in a limited-growth environment, verbose results rather than numeric results are preferred. Numeric result codes were originally intended to make it easier for software to control the modem, but there are two primary reasons they should not be used:

Software can be confused by a command echo. For example, if the following command were sent with echo on (E1) and numeric results (V0) on:

                           AT ... S9=20 <CR>

The resulting data, echoed by the modem, would be followed by the numeric result code zero, meaning OK:

                          AT ... S9=20<CR>0 <CR>

Software may become confused by seeing a 0<CR> result which is actually part of the command echo, then another 0<CR> which is the numeric result. A program can become unsynchronized with the command processor in the modem.

Turning off echo mode (E0) in the initial setup string would solve this problem; however, do not end that command with any digits (simply E).

Another shortcoming of numeric results is that the software must anticipate all possible responses. This requires updating controller software whenever new result codes are added. For example, suppose a CONNECT 115200 result were added with a numeric value of 31. If verbose results were used instead, and the controller directed to interpret the number after the CONNECT result as simply the connection speed in bits per second, no changes to the driver are necessitated by the new result code. If, however, numeric result codes were used, the result code 31 must be added to the table, and the controller modified to interpret it appropriately.

As characters are received, they should be processed through a state machine providing the functionality of the following one. This state machine recognizes strings surrounded by <CR><LF> characters and stores the string in a character array. <CR><LF> are defined by S3 and S4.

    Sample State Machine
    Initialize with: state = 1 ;

    ch = <next character from the input>
    switch( state )
    {
         case 1:     /*-- Scanning for leading CR --*/
                   if( ch == CR ) state = 2 ;
                   i = 0 ;
                   break ;
         case 2:     /*-- Scanning for leading LF --*/
                   if( ch == LF ) state = 3 ;
                   else if( ch == CR ) state = 2 ;
                   else state = 1 ;
                   break ;
         case 3:     /*-- Buffer result, watch for trailing CR --*/
                   if( ch == CR ) state = 4 ;
                   else buf[ i++ ] = ch ;
                   if( i > LIMIT ) state = 1 ;
                   break ;
         case 4:     /*-- Scanning for trailing LF --*/
                   if( ch == LF ) state = 5 ;
                   else if( ch == CR ) state = 2 ;
                   else state = 1 ;
                   break ;
    }
    if( state == 5 )
    {
         buf[ i ] = 0 ; /* Null terminate buffer */
         <process result in 'buf'>
         state = 1 ;
    }

This state machine can be imbedded within a loop that reads all received data one character at a time, checks for a timeout, and also checks for user abort. Once a result is recognized, that loop can be exited or continued if additional results are expected.

Once a result code string is returned, it can be compared against the known result code strings. Some strings may incorporate wild-card suffixes. For example CONNECT followed by any numeric value indicates a successful connection at the indicated transmission rate. Even if a result such as CONNECT 38400 is not anticipated, if the controller has been coded for wild-card recognition, the controller will be capable of interpreting such responses correctly. This practice also facilitates interpretation of connection failed messages that are preceded by NO followed by any other character string such as DIALTONE, CARRIER, or ANSWER.

### D.3 Modem Preparation

Once the modem has been identified, the controller can continue to program any registers or user-defined values into the modem necessary prior to initiating the connection process. Typically, the setup operation is separated from the connection processing because it is performed independently of whether the call establishment will be in the originating or answering mode.

Setup commands can be issued at the highest transmission rate the modem supports as determined from the identification process or it may be fixed at a certain value if the modem is not identified.

### D.3.1 Reset

Before issuing any other commands to the modem, it is advisable to issue a Z or &F command to the modem before the identification or setup process. No specific response should be anticipated. The modem may be set up to return numeric, or no result codes. If a reset will be used, the following points should be considered:

- Even if a recognizable result within 2.6 seconds, the program should continue. (Some modems do a lengthy reset process before responding with a result; others may be in Q1 or V0 mode.)
- Following an OK result, an additional 600ms delay should be imposed. Some modems will respond with an OK then do lengthy reset processing, in which case they are unable to accept additional commands.

After the modem is reset, the first setup string (e.g., verbose rather than numeric result codes) should be issued, then the identification command.

### D.3.2 Setup

Software should generally provide some modem setup. However, the software can be written to rely on modem configuration via a stored profile recalled on reset, or by DIP switches set depending on the product. In this case, any unique settings must have been set up prior to running the software, and all the program does is send the Z command to recall the desired profile. Even more basically, software can assume the modem is in the power-up state. However, unless the software will be used within a very predictable environment, these assumptions may result in failures with the controller software.

Some commands will always be overridden by the controller in order to ensure its proper functioning. Other command options should either default to the factory setting, or simply act as the "transfer agent" for the commands specified by the user. Menus and dialogs can be provided to prompt the user for specific activities; the program can then interpret these requests and configure the modem accordingly, as Hayes Smartcom products do, or provide the user opportunity to enter AT command strings.

Commands frequently set by a modem controller:

    E0         Turn off echo mode to avoid having command echoes pass
               through the result code scanner.

    &F         Recall the factory profile.

    Q1         Enable result codes to ensure that commands are being
               processed, and to synchronize with the modem command
               processor (except for synchronous communications where result
               codes may cause the DTE confusion).

    V0 or V1   Use either numeric result codes or verbose (recommended)
               result codes.

    S0=0       Disable auto-answer during the setup process to avoid
               inadvertent disruption by an incoming call.

    H0         Ensure modem is on hook before continuing to the answer or
               originate process.

    S12=10     Set the escape sequence guard time to 200ms to hasten the
               escape for hang-up process. Also reduces the probability of
               inadvertent user escapes.

    S2=*       Change the escape sequence character for two reasons: To
               avoid inadvertent user escapes, and to provide different
               escape sequence characters for answer and originate ends.
               This prevents inadvertent escaping when data is echoed.

    S4=*       Modify the linefeed character to make the <CR><LF>NO
               CARRIER<CR><LF> result code more unique if you scan for it
               to detect carrier loss.

    S95=60     Enables the result codes which will provide the maximum
               amount of information about the connection when it is
               established.

Two typical setup sequences using these recommendations are shown below:

Recommendation for software design based on using pre-existing user settings:

                  AT E Q V S0=0 H S12=10 S2=28 S4=31 S95=60 <CR>

Recommendation for software design based on starting from a known factory setting:

                  AT &F S2=28 S4=31 S12=10 S95=60

Note: Where the zero suffix is used, it may be omitted from a command. Spaces are shown above for readability, but the use of spaces between commands is not recommended.

Once this setup command has been sent, and the OK response returned, the controller can continue to the originate or answer processing.

If user-programmed settings are included in additional setup strings, or the user is permitted to enter AT setup strings, the software should anticipate ERROR result codes. If an ERROR is returned in response to such a command, the result does not have to be reported to the user, but the controller should not be prevented from continuing in either case. Many times a connection can be made even though some setting is in error or is inappropriate for the class of modem being addressed.

### D.3.3 Establishing the Desired Connection and Fallback Strategy (S36 Developers' Tips)

S36 determines which fallback action will occur if the protocols and procedures set by S46 and S48 do not produce a LAP-based (LAPB) error- controlled connection.

The optimum fallback control strategy depends on which Hayes error- control modem you have. The fallback behavior is determined by S36, S46, S48, and &Q5.

There have been two improvements in the V-series, OPTIMA and ACCURA EC modems that affect S36: The first added &Q6 Asynchronous Speed Buffering (ASB); ASB provides the same fixed speed DTE interface and local flow control as in the error control mode but without the error control or remote flow control. The second improvement added V.42 with its Alternate Protocol (MNP).

Previously, S36 only consisted of one bit, Bit 0, which permitted the user to decide whether the modem would hang up or not if it did not succeed in negotiating an error-controlled connection. The factory default for S36 was 1, which meant fall back to direct async rather than hang up if an error control protocol is not established in &Q5 mode.

When &Q6 (ASB) was added, a fall back option for &Q5 mode, which can be controlled with S36, was also added. If S36 Bit 1 is set, and bit 0 is also set, the modem will fall back to ASB instead of direct async if no error control protocol is established in &Q5 mode. The factory default for S36 remained 1.

When V.42 was added, it added another fall back option, MNP, to S36. If S36 Bit 2 is set, then MNP will be attempted if the primary protocol selected in S48 is not established. Bit 2 is evaluated before Bits 0, then Bit 1. If MNP is established as the protocol, then Bits 0 and 1 are ignored. The factory default became 5 and later was changed to 7 to take more advantage of ASB.

Now that we know that there are three versions of S36, we may need a way to tell which version our software is controlling. First, verify that it is a Hayes error-control modem. This is done by issuing an I4 command and looking for the existence of a "b-string". A b-string begins with a lower case letter "b" and is followed by several upper case letters or numbers forming a hex value.

The easiest way for software to identify whether a Hayes modem supports Asynchronous Speed Buffering (also known as ASB, Buffered Async, or Normal) is to issue the following AT command, AT &Q6 &Q5 <CR>. If the result code is OK then &Q6 is supported, and ASB is also supported as a fall back. If the result code is ERROR, then &Q6 is not supported. If this is the case, then software should be prepared to fall back to a direct async connection.

The easiest way for software to identify whether a Hayes modem supports V.42 (and MNP) is to decode the first character after the leading "b" in the b-string of the I4 ID command response. The characters following the "b" in that line are "ASCII-hex" (0-9, A-F), which decode into 4 bits of ID code (3-0). If Bit 3 is set, then MNP is supported; Bit 2 is V.42.

Armed with the knowledge of which S36 bits are supported, the software may now safely configure the modem and properly anticipate the fall-back action.

It should be safe to set S36 bit 2 to enable V.42 (and MNP) even if those protocols are not supported in the modem.

If the modem does not support ASB, then software should be prepared to follow the CONNECT XXXXX speed result code and change the DTE port speed to match the indicated line speed of the direct connection when no error controlled connection was negotiated.

If software will not change port speeds in response to the CONNECT message, then when software has identified a 'pre-ASB' modem, it should set S36 to 0 or 4 so that if no protocol is negotiated, the modem will hang up.

The following table shows the order in which the bits of S36 are evaluated: (Remember, these steps only occur after the S46/S48 selections have failed to make a LAPBased error controlled connection in &Q5 mode.)

    ------------------------------------------------------------------------
    S36      Bit 7-3=0       reserved
    First    Bit 2 (4)       If set, means try MNP protocol; reset means
                             don't use MNP.
    Third    Bit 1 (2)       If set, means fall back to ASB; reset means
                             direct async.
    Second   Bit 0 (1)       If set, means fall back based on Bit 1;
                             reset means hang up.
    ------------------------------------------------------------------------

The following table shows the meaning of each setting:

    ------------------------------------------------------------------------
    S36=7   Try MNP, then fall back to ASB.
    S36=6   Try MNP, then hang up.
    S36=5   Try MNP, then fall back to direct async.
    S36=4   Try MNP, then hang up.
    S36=3   Don't try MNP; fall back to ASB.
    S36=2   Don't try MNP; hang up.
    S36=1   Don't try MNP; fall back to direct async.
    S36=0   Don't try MNP; hang up.
    ------------------------------------------------------------------------

There are three types of fallback stratagies which Hayes products use.

Type 1: Hayes Modem Operation in &Q5 Communications Mode Applies to V-series Smartmodem 2400 up to and including Version 1.3 and V-series Smartmodem 9600 up to and including Version 1.4.

Type 2: Hayes Modem Operation in &Q5 Communications Mode Applies to V-series Smartmodem 2400 Version 1.4 and V-series Smartmodem 9600 Versions 1.5 and 1.6.

Type 3: Hayes Modem Operation in &Q5 Communications Mode Applies to all V-series products newer than Types 1 and 2 and all OPTIMA and ACCURA EC products.

    ------------------------------------------------------------------------
                   ATI3 Response contains...
                   V-SERIES SMARTMODEM 2400    V-SERIES SMARTMODEM 9600
    Type 1         04-00005-10                 04-00015-10
                   04-00005-11                 04-00015-11
                   04-00005-12                 04-00015-12
                   04-00005-13                 04-00015-13
                                               04-00015-14
    Type 2         04-00005-14                 04-00015-15
                                               04-00015-16
    Type 3         Does not contain any        Does not contain any
                   of the above numbers        any of the above numbers
    ------------------------------------------------------------------------

## D.4 Connect Processing

Once the setup operation has been completed, the commands to establish the connection can be issued. The instruction can be either to originate (using the D command), or to answer (using the A or S0 commands).

### D.4.1 Originating a Call

If the D command is issued with the desired phone number, several possible result codes can be returned. The list below outlines some results to expect:

    Result Code        Meaning
    ------------------------------------------------------------------------
    NO CARRIER         Connection failed
    NO ANSWER          No response to '@' dial modifier
    NO DIALTONE        No dial tone in X4 mode
    NO ____            Connection failed for some other reason
    BUSY               Busy signal detected
    CONNECT ____       Connection successful; you may need to change DTE
                       speed to the indicated baud rate. (See Wn and S95)
    CARRIER ____       * DCE carrier speed. (See Wn and S95)
    PROTOCOL: ____     * Error-correction protocol being used by the modems.
                       (See Wn and S95)
    COMPRESSION:____   Compression technique in use by modems. (See Wn
                       and S95)
    AUTOSTREAM         Autostream technique in use by modems if selected by
                       user. (See Wn and S95)
    ------------------------------------------------------------------------

Ignore other responses, but continue to wait for CONNECT ___, BUSY, or NO ___ responses. The CARRIER and PROTOCOL results are intermediate results and precede either a CONNECT ____ or NO ____ result. These results are only returned by Hayes modems when configured to use an error-correcting protocol.

If you recognize any numeric value for the baud rate after the CONNECT result, you will have a much more robust controller able to handle many situations. You can use the PROTOCOL result to determine if the flow control requested by the &K command is in effect.

Note: If you want to manually dial a call and then have the modem connect, enter the command string ATX1D (X1 disables dial tone detection) when you hear the dial tone.

### D.4.2 Answering a Call

The simplest technique for answering an incoming call is to set S0 and wait for a CONNECT ___ result. You may get several RING results, and possibly a NO CARRIER result if the caller hangs up before connecting. These results should not cause your controller to abort. Continue to wait for a CONNECT result code.

If you set S0, you may want to set it back to zero after your controller finishes the call to prevent inadvertent answering when your software is not running. By setting S0 to the number of rings you desire before the modem answers, you utilize the ring detection technology already built into the modem.

You should not use the A command to answer after counting RING results because the command may collide with another RING result from the modem and be missed. The RING results may be generated in pairs depending on the ringing cadence of the phone system.

### D.4.3 Using the CD Line

Monitoring the Carrier Detect (CD) line of the RS-232 interface is another technique for carrier detection in answer or originate mode. This assumes that &C1 or the corresponding DIP-switch has been set and the cable is wired properly. Both are risky assumptions. You will have a more robust controller if you use result code scanning rather than the RS-232 lines.

If you use CD, you do not know when the modem has given up waiting for the carrier, or why. If the line is busy, you may want to re-try the dial operation. If there is no dial tone, the user needs to know this.

### D.4.4 Aborting a Connect Request

Once the D or S0 command has been issued, the modem goes off hook (or may be off hook for S0) and it must be put back on hook (hangPup) before the abort is completed. To abort an in-progress connect command, send any character to the modem. This will typically result in a NO CARRIER response. The result code scanner should be called after the abort character is sent to prevent additional commands from being sent before the controller and the modem are again in sync.

Smartcom products send AT<CR> to abort an in-progress connect command. This elicits a result code regardless of whether the modem were off-hook or not. If the modem was off-hook attempting to connect, this will abort the connect operation and return NO CARRIER. If the modem was on-hook in command mode, this simply returns <CR><LF>OK<CR><LF>.

## D.5 Carrier Loss Detection

You want your application to be able to detect when the carrier has been lost so you can determine when the connection is complete. You might be unable to put this part of the code in your controller software, since the controller is typically running only during the connect or hang-up process. Once the application has detected the carrier loss event, it can call the modem controller and restore the modem settings.

### D.5.1 Using the CD line

If you are confident of the communications environment and cabling requirements, and have access to the RS-232 signal status, then monitoring the CD line is the easiest carrier loss detection method to implement. This requires &C1 to be programmed at setup time, or be stored in the modem as the value recalled on reset or power-up.

However, this is the most restrictive and risky choice. It requires a properly wired cable and support of &C1 by the modem's command set or proper DIP-switch settings.

### D.5.2 Scanning the Incoming Data Stream

In cases where you cannot depend on 100% Hayes compatibility or want to be independent from the cable wiring, then scanning for the NO CARRIER result code is more reliable. It is also more complex to implement.

Typically, at the low-level of the program all received data is retrieved through one subroutine. This subroutine can be augmented or layered to provide the service needed. As data passes through, the last fourteen characters are buffered, typically in a circular buffer. If more data passes in each call, only the last fourteen need to be copied. At a time when the processor is free, such as after 100 ms of idle time or the receive routine has returned no data for 30 to 100 calls, then the buffer is compared against the <CR><LF>NO CARRIER<CR><LF> result code. If a match is found, the carrier lost event is triggered.

By only checking when there is idle time, or after no data has been received for a while, you reduce the CPU overhead and ensure that the modem is not falsely triggered when the string is imbedded in an actual data stream.

You can also modify the linefeed character by using S4 to a different value such as S4=31 to make the result code sequence more unique. This action, however, affects other result codes generated by the modem.

## D.6 Escape and Hang Up

When your controller has been instructed to terminate the connection, you must put the modem back in command state and issue the hang-up (H) command. In addition to hanging-up, you will also want to restore settings you changed to their factory-set values, or issue an ATZ<CR> to undo the effects of your changes. In any case, restoration of the modem settings is necessary even if the connection was terminated due to loss of carrier.

### D.6.1 Escaping the Modem to Command State

To escape the modem, the controller must first delay the escape sequence guard time (specified by S12), then issue the escape sequence character three times (specified by S2); then wait for an OK result. Waiting for the result also enforces the required guard time after the escape sequence. Once the OK result is received, the modem has entered command state. The controller can then hang up and restore the modem.

The controller software must be sure to wait the required guard time before sending the escape sequence characters. Your controller may have been called just after data was transmitted and, without the delay, your characters will just be sent without triggering the escape sequence recognition process. It is important that the serial transmitter be permitted to be idle for the escape sequence guard time, plus a few extra milliseconds to allow for error, before sending characters.

For example, if S12=10, a delay of at least 200 milliseconds is required before sending the escape sequence characters. After sending the escape sequence characters, the OK result will be received after another 200 millisecond wait. This completes the escape sequence process in slightly over 400 milliseconds. If S12=50 (factory setting) is used, one full second must pass before the characters can be sent, then another second delay must transpire prior to the OK result. This completes the process in slightly over two seconds. For this reason, it is recommended that S12=10 be issued to speed up this process.

A delay slightly longer than that stored in S12 should be used to allow for errors in the system clock as well as in the modem clock. 100ms is an adequate safety margin.

### D.6.2 Using DTR to Escape or Hang Up

The DTR RS-232 signal can be used to escape the modem to command state, or to reset the modem depending on the &D command set or DIP-switch settings. This also requires the cable to be properly wired. Unless the software will operate in a highly-controlled environment, this technique is discouraged over using the escape sequence process because of the requirements to make it function properly. Leaving a call connected simply because the cable was not properly wired can be potentially expensive. The escape sequence is reliable in all environments if it is properly utilized.

## D.7 Modem Re-configuration

When a call has been completed, a "clean-up" command should be issued to return the modem a more known configuration. For example, if verbose result codes were selected when the modem was reset, and the controller selected numeric result codes, on completing the session, the controller should reset the modem to re-select verbose result codes. In the same way, if the linefeed character were changed to suit the software or environment, the character should be set to its former value. Any other command options that were modified, should be restored to their factory- set values.

The minimum the controller should do when through with the modem is issue a Z command option to ensure the modem is restored to its power-up state.

## D.8 Timing Considerations

A modem controller inherently has a sense of time. Usually all that is needed to utilize the timing part of the controller is access to a time reference. For example, the number of milliseconds since power-up or program launch, or a "system tick" value can be used.

Under DOS, the INT 1C timer tick produces an interrupt every 55 milliseconds. An ISR can be installed on this interrupt to add 55 to a long integer every time it is called. This will provide a millisecond counter.

On the Macintosh, the "Tick Count" function will return the number of vertical-retrace ticks since computer power-up. Each tick represents 1/60 of a second.

### D.8.1 Programming for Time

The time value is used to determine relative time. For example, if a loop should be executed for only 2 seconds it could be coded as:

    timeout = Tick Count( ) + 120 ; /* 60 tics per second = 2 seconds */
    do
    {
         got_one = Check_Result( ) ;
    }
    while( ( ! got_one ) && ( SystemTick( ) < timeout ) ) ;

This code fragment continues to call the Check_Result function until it returns a true value, or until two seconds have elapsed.

This technique is independent of processor speed. A faster processor may make thousands of trips through the loop, where a slower one would only make a few hundred. Any anticipated result code would arrive within that two-second real-time window.

Care should be given to considering when to start the timing loop. If an AT command string is sent, then a loop executed, the time interval may also include the time required to send the AT command (if data is buffered and sent by an interrupt service routine).

At 300 bps, where each character takes 33ms just to transmit (10/300), a 40 character AT command would take over a second to transmit. This means a two second loop spends more than half of its time waiting for the AT command process to complete, leaving only a fraction of a second for the modem to respond with the result (again at 33ms per character).

One way to avoid this is to wait until all data has been transmitted by an ISR before entering the result code scan loop. Alternatively, more time can be provided for loops to process results. Another option is to measure idle time rather then elapsed time.

### D.8.2 When to Consider Time

The use of timing varies from command to command and operation to operation. Some commands take longer to execute. The guidelines below can be used to determine the best amount of time to wait.

- For the Z command, wait two seconds for a response, then wait an additional 600 ms, whether a response were received or not.
- For general setup commands, wait two seconds for the response.
- For the hang-up command (H), wait up to 20 seconds for a response. Some modems may take longer to hang-up if data buffered within the modem is still waiting to be transmitted and acknowledged. This time is controlled by S38.
- For dial commands (D or O) wait at least one minute or more. Values set for carrier detect time, tone versus pulse dialing, commas in the phone number, all can take additional time.

If the software times out, the modem may, in fact, not be connected to the computer, disconnected, or turned off. If this is the case, enforce a reasonable timeout to the first setup or identification command. That will determine whether a modem is attached and functioning.

A timeout may also occur when the software receives a result code it does not recognize. The software may continue to wait until it receives a code it does recognize. If this is the case, the controller should proceed as if an ERROR response were received. The only instance in which it is not prudent to continue is when a connect (D, A, or S0) command was issued.

Before implementing a timeout, the advantages, if any, to this level of program interruption should be considered. For example, if the program times out from a dial command in one minute when it may take two minutes to complete the call, the timeout defeats the purpose of the command. The modem always responds with a result code, whether BUSY, NO CARRIER, or CONNECT, after some length of time.

Idle time is the time since data was received. Elapsed time is the time since the software started looking for the result. Idle time can be measured by resetting the timeout clock each time the software receives a character. Rather than exiting the loop after two seconds of elapsed time, the logic changes to exit after no further data has been received for two seconds.

### D.8.3 Recovering when "Out Of Sync"

Another disadvantage of timing out is that an early timeout can put the software out of sync with the modem command processor. The controller may be interpreting results sent in response to previous commands as the response to later commands. To avoid this condition, any pending receive data should be flushed before the next command is issued.

## D.9 Transparent XON/XOFF Local Flow Control

The use of flow control during error-control modem connections is essential to avoid loss of data during error-control protocol retransmissions. Use of flow control allows the DTE-DCE interface to be run faster than the line speed, permitting the additional benefit of increased throughput due to stripping of start and stop bits by the error control protocol and data compression techniques such as V.42bis. When the interface is run at high speed, data loss due to modem buffer overrun is quite likely to occur if flow control is absent or not functioning properly.

Hayes modems support the industry standard "RTS/CTS" hardware flow control (invoked by the &K3 command) and "XON/XOFF" character-based flow control (&K4) schemes. However, in some situations, it is not possible to use either of these. For example, hardware flow control cannot be used if the DTE serial port does not support the RTS and/or CTS hardware signals, if corresponding conductors are not present in the interface cable, or if intermediate equipment does not properly transfer the RTS and CTS signals. XON/XOFF flow control cannot be used if the XON and XOFF characters appear in the user data or in the control information associated with the file transfer protocol in use, since this would interfere with the use of these characters for flow control, resulting in failure of data transfer and possibly locking up the interface.

Hayes addressed these problems in the design of V-series, OPTIMA and ACCURA EC modems. Desiring to provide a complete solution for all users, Hayes provided a third flow control scheme, which accommodates systems which cannot use RTS/CTS but which must transfer data containing the XON and XOFF characters. This scheme, known as Transparent XON/XOFF Local Flow Control (invoked by the &K5 command), uses only the Transmit Data and Receive Data circuits, yet provides for the transfer of all 256 possible 8-bit characters over the interface without interfering with the flow control scheme.

### D.9.1 Summary

Transparent XON/XOFF Flow Control functions on the interface between the local DTE (computer or terminal) and local DCE (modem). When the data is placed on the link between the modems, the original characters have been restored. Neither the remote DTE nor DCE are aware, or need to be aware, of the fact that Transparent XON/XOFF Flow Control is in use.

Transparent flow control can be viewed as a layered process, organized as follows. Note that this process is duplicated in the reverse direction (operates in both directions simultaneously).

### D.9.2 Transparentization

Transparent XON/XOFF Flow Control operates by scanning the data stream for XON ($11), XOFF ($13), and DLE ($10) characters ("$" indicates hexadecimal), with either 0 or 1 in the high-order bit position. When one is found, a DLE with same parity character is inserted ahead of it. The original character is then "transparentized" by exclusive-ORing the character with the value $21 (hexadecimal 21, decimal 33, binary 00100001), which preserves the parity bit value. XON and XOFF characters are thus "hidden" in the data stream.

### D.9.3 Flow Control

Once the XON and XOFF characters have been transparentized (hidden), the DTE and DCE are free to insert XON and XOFF characters for flow control purposes at any point in the data stream. This includes the possibility of inserting XONs and XOFFs between a DLE and the following transparentized character. XON and XOFF characters in the user data will not interfere with the flow control scheme. The operation of this XON/XOFF flow control process is identical to traditional XON/XOFF flow control.

### D.9.4 De-Transparentization

In the de-transparentization process, the receiving device scans for DLEs. When one is found, it is discarded, and the following character is once again exclusive-ORed with the value $21, recovering the original character. Note that XONs and XOFFs that appear between the DLE and the following transparentized character are used for flow control purposes and do not affect the detransparentization process.

### D.9.5 Examples

The following discussion is written in terms of the actions performed by the computer software. The modem performs identical actions on the reverse directions of transmission. This text is not meant to constrain implementation or to imply that better implementations are not possible, but serves simply as an example.

#### D.9.5.1 Transmitter Example

Each character in the user data stream is individual examined. It it is any of the six characters $10, $11, $13, $90, $91, or $93, it is exclusive-ORed with the value $21, and prefixed by a DLE character ($10). This can also be viewed as a replacement function according to the following table:

    Char      Replaced By
    ------------------------------------------------------------------------
    $10       $10 $31
    $11       $10 $30
    $13       $10 $32
    $90       $10 $B1
    $91       $10 $B0
    $93       $10 $B2
    ------------------------------------------------------------------------

The characters are transmitted using the parity setting then in force on the DTE-DCE interface; the XOR with $21 preserves the correct value of the parity bit.

If the software wants the modem to suspend delivering data, it inserts an XOFF ($13 at current parity setting) in the transmitted data stream at any arbitrary point, which may be between a DLE ($10) and the following transparentized character. The XOFF should be issued BEFORE the buffer is completely full, since there may be a lag of several characters before the modem is able to react to the XOFF and suspend delivery. To resume delivery of data, the software inserts an XON ($11 at current parity setting) at any arbitrary point. Characters inserted for flow control purposes are NOT passed through the transparency algorithm defined above.

#### D.9.5.2 Receiver Example

Each character received from the modem is individually examined. A "transparency sequence in progress" flag (TSIP-FLAG) is maintained as part of a simple state machine. The initial value of the TSIP-FLAG is OFF. The following tests and actions should be undertaken in the order listed.

If the character received is $13 or $93 (XOFF with either parity), it is interpreted (regardless of the setting of the TSIP-FLAG) as a request from the modem for the DTE to suspend transmission of data to the modem. The receiver portion of the software must communicate this request to the transmitter portion of the software in a timely manner, since the modem has a limited amount of buffer space to allow for additional characters to be received after it sends XOFF (about 64 characters maximum). The XOFF character is discarded from the received data stream. The TSIP-FLAG is not changed.

If the character received is $11 or $91 (XON with either parity), it is interpreted (regardless of the setting of the TSIP-FLAG) as a request from the modem for the DTE to resume transmission of data to the modem, if any is available. The XON character is discarded from the received data stream. The TSIP-FLAG is not changed.

If the character is neither an XON or and XOFF, the following steps are performed:

  If the TSIP-FLAG is ON, the received character is exclusive-ORed with $21, then delivered to the higher layer for further processing (e.g., placed in a receive data buffer). The TSIP-FLAG is then turned OFF.
  If the TSIP-FLAG is OFF and the character received is $10 or $90 (DLE with either parity), the beginning of a transparency sequence is indicated. The DLE is discarded from the received data stream, and the TSIP-FLAG is turned ON.
  If the TSIP-FLAG is OFF and the character received is neither a $10 nor $90, the character is a normal untransparentized user data character which should be delivered to the higher layer for further processing (e.g., placed in a receive data buffer). The TSIP-FLAG remains OFF (unchanged).

### D.9.6 Conclusion

When properly implemented, Transparent XON/XOFF Local Flow Control permits a fully-functional flow-controlled interface even when only three conductors are present (Transmit Data, Receive Data, and Signal Ground). Versatile software may use the &T19 cable test feature of Hayes modems to determine whether or not RTS/CTS flow control can be used, and, if RTS/CTS is unavailable, automatically select &K5 operation to use Transparent XON/XOFF flow control, and allow the user to continue to transfer data using any protocol or data contents without concern for possible interference with the XON/XOFF flow control scheme.

## D.10 General Tips and Techniques

The following are tips and techniques that may help in the exchange of information between the software controller and the modem command processor.

- Commands in the command line should be ordered starting with the safest and ending with the most risky. Risk is defined as the potential to generate an ERROR, causing the remainder of the command line to be ignored.
- Any command that may return ERROR should be anticipated. This or other unexpected results can be ignored unless the command is critical (configuration or call placement).
- Send I0 or I4 at 1200 bps, which is supported by the majority of modem products. A modem reset (&F or Z) should be performed at 1200 bps before sending the identification commands.
- Setup processing can be speeded by sending all but the last D or S0 command at the highest DTE rate supported by the modem. The last command must be sent at the speed at which the connection should be made (except V-series, OPTIMA and ACCURA EC products which specify this with S37).
- Any dependency on proper cabling can be eliminated by avoiding techniques that depend on RS-232 signals:
    Have the software scan for result codes, rather than depending on the condition of the CD line.
    Transparent flow control should be used with V-series, OPTIMA and ACCURA EC products rather than with RTS/CTS signals.
    The escape sequence with guard time process and H command should be used to hangup instead of terminate a connection by dropping DTR.
    Any unexpected RING result codes may indicate the last command may not have been processed correctly. The command should be- issued.

### Sample Controller/Modem Exchange

    Clock     Controller                    Speed      Modem
                                            (1200)
                                             bps)
    ------------------------------------------------------------------------
    00000     ATZ<CR>
    00034                                              ATZ<CR>
    00068                                             (one second to do
                                                       reset)
    01068                                             0<CR> (V0 stored as
                                                       default)
    01084     (delay additional 600ms)
    01684     ATEQV1S0=0S12=10S4=3HI<CR>
    01884                                             ATEQV1S0=0S12=10S4=3HI
                                                      <CR> (echo)
    02084                                             <CR><LF>960<CR><LF>
    02142                                             <CR><LF>OK<CR><LF>
                                             (19200
                                              bps)
    02192     ATM0X4L1S12=10S2=1&Q5W1S36
              =7S37=9&K5<CR>
    02206                                             <CR><LF>OK<CR><LF>
    02209     ATDT9W14045551212<CR>
    38000                                             <CR><LF>CARRIER
                                                      2400<CR><LF>
    45000                                             <CR><LF>PROTOCOL:
                                                      NONE<CR><LF>
    45010                                             <CR><LF>CONNECT
                                                      2400<CR><LF>
                                            (2400
                                            bps)

    (Connection Established)
    -----------------------------------------------------------------------
                                            (2400
                                             bps)
    00000     (delay 300ms, need 200, add
              100 for safety)
    00300     (escape char is ^A, S2=1)
    00313                                              (waits 200ms)
    00513                                              <CR><LF>OK<CR><LF>
    00538     ATHE1S2=43S12=50W0&Q0<CR>
    00630                                              <CR><LF>OK<CR><LF>
    ------------------------------------------------------------------------