Interface Cable

Correct operation of V.25bis requires raising and turning DTR on-and-off. For example, the only way to hang up a call in V.25bis mode is to drop DTR. V.25bis requires that the following circuits be wired in the cable between the DTE and modem (not including grounds):

Pin    Name                                                   

2      Transmit data                                          

3      Receive data                                           

15     Transmit clock (not needed for async V.25bis)          

17     Receive clock (not needed for async V.25bis)           

20     Data terminal ready                                    

6      Data set ready                                         

4      Request to send                                        

5      Clear to send                                          

8      Carrier detect                                         

22     Ring indication

Note: A user with a 3-wire interface (i.e., a cable lacking DTR) may have problems entering the following command sequence:

ATS94=1 (or 2 or 3) AT&W ATZ

Assuming the cable forces DTR high all the time, the user is now "stuck"in V.25bis mode. The modem no longer accepts AT commands. To recover he must cycle power off/on.

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