Appendix D V.25bis Operation | Multi-Tech Systems MT2834BL Series

Appendix D - V.25bis Operation

Chapter 4 described a set of commands which let the modem dial, hang-up, and be configured for various applications. However, these commands, the AT command set, are only functional when the DTE transmits data asynchronously. That is, they cannot be used with synchronous equipment such as that found in IBM's Binary Synchronous Communications (BSC) and Synchronous Data Link Control (SDLC)* environments.

The ITU V.25bis commands provide you with an alternate set of commands and responses to those described in Chapter 5 for applications in which the DTE is synchronous. V.25bis commands support dialing functions in asynchronous or synchronous mode according to the recommendations of the ITU; however, the AT commands are understood to reign in the asynchronous world, while the V.25bis commands are relegated to the synchronous world of datacomm. If you need to establish a datacomm link over Public Switched Telephone Network (PSTN) dial-up services, and your equipment outputs a synchronous data stream,

V.25bis commands are essential. V.25bis mode AT commands do not include any modem configuration commands. You can execute normal AT commands in V.25bis mode for modem configuration.

Delayed and Forbidden Numbers

V.25bis provides the facility to delay failed call retry attempts by putting numbers that failed to connect on a special Delayed Number list. Subsequent dialing of these numbers will be delayed (time specified by a country regulation) and an appropriate message displayed. If the number is retried more than the maximum allowed number of times (number is also specified by country regulation), it is placed on the Forbidden Numbers list, and no further retries will be allowed. When the Forbidden Numbers list is full, no dialing is allowed and a CFIFF indication is given. The modem in AT command mode will respond with NO CARRIER. If country regulations require that the Forbidden Numbers list be checked in AT mode, then NO CARRIER will be the response to a dial attempt in AT mode. Numbers will also be put on the Forbidden Numbers list if the Delayed Numbers list is full and a new number fails for the first time. In that case, the new number will be added to the Delayed Number list and the oldest existing number added to the Forbidden Numbers list. Numbers are removed from the Forbidden Numbers list by after a certain time has past (also by country regulation).

Some country regulations have numbers remain on the Forbidden Numbers list permanently. The Delayed Number and Forbidden Numbers lists are eight numbers long (20 characters each).

V.25bis Operation

Operation in V.25bis mode is similar to AT command mode in that certain DIP-Switch functions are important to its operation. V.25bis does not include any speed detection for asynchronous mode, so when you are giving commands, you must stay in your initial speed (i.e., if you change your terminal speed while entering an AT command, you will get no responses). In synchronous mode, the modem supplies the clock, so the synchronous terminal “knows” the speed.

You must be in V.25bis mode for the commands described here to function. Most AT commands will also function, except those associated with dialing such as ATD, ATN, ATO, and ATU. To get into V.25bis mode, you type AT$V1. At this point your modem does no more speed or parity detection (things associated with asynchronous operation). To get out of V.25bis mode and back into AT command mode, enter AT$VO. The AT$V2 command allows you to run one V.25bis command from AT command mode without

leaving AT command mode.

There is no command to select between asynchronous and synchronous V.25bis operation. The position of DIP-Switch #12 selects between modes. For synchronous mode, DIP-Switch #12 must be UP, and the AT command mode enabled.

Another asynchronous mode concern is the problem of connecting at a different speed than the speed at which the serial port is set. If your modem port speed is different from the serial port speed, you must either:

1)Enable speed conversion and have flow control on, or

2)Enable connect responses (with the ATX1 command) and change the serial baud rate after receiving a connect message.

If the $VD1 command is entered when in Auto-Answer mode, the modem answers immediately upon receiving the first ring.

There is no disconnect message (NO CARRIER) if a normal connection is made.

Multi-Tech Systems MT2834BL Series Appendix D V.25bis Operation, Delayed and Forbidden Numbers