17 When C-Kermit Dials My V.32bis (or V.34) Modem, I Get the Error 'Can't Change Speed to 14400 (or 28800)'

Dialing is covered in detail in Using C-Kermit, second edition, and the problem listed in the title of this section should occur only rarely in C-Kermit 6.0 (it was quite common in earlier versions).

To recapitulate very briefly: older modems, like the Hayes 1200 and 2400, that did not do error correction or compression, but that could negotiate their modulation speed, would report the modulation speed upon successful connection, and change their interface speed to match. Thus, the communication software would also have to change its own interface speed, or else the user would see only garbage.

Modern modems have two different speeds: the interface speed and the modulation speed. The interface speed can be kept constant even though the modulation speed changes. Or not, depending on how the modem is configured.

C-Kermit versions prior to 6.0 had no way of knowing whether your modem is set up to lock its interface speed, or to change it to match the modulation speed, and therefore no way of knowing whether to believe the "CONNECT 28800" (or whatever) message. By default, for compatibility with the huge installed base of older modems, it did believe, and therefore changed its interface speed according to the CONNECT message.

But if your modem's interface speed is locked (which it SHOULD be if it is an error-correcting, data-compressing modem), you must tell Kermit NOT to change its interface speed by giving it the command:

SET DIAL SPEED-MATCHING OFF

Now to complicate matters, some of the newer modulations report speeds that are not commonly supported by the host operating system, such as 14400 and 28800. Hence the message "Can't change speed to 14400" (or 28800). But even if these speeds were supported, you would not want Kermit changing to them if the modem's interface speed was locked. You would still see only garbage, but you would not get the "Can't change speed" message.

C-Kermit 6.0, by contrast, has a much more comprehensive modem database, and automatically chooses the appropriate SPEED-MATCHING and other parameters when you choose your modem type. Therefore, when you choose a high-speed modem type, one that is capable of speed buffering, C-Kermit automatically set DIAL SPEED-MATCHING to OFF; whereas if you choose (say) the Hayes 2400 modem type, it will set it ON. You can override these automatic choices by giving explicit SET MODEM and/or SET DIAL commands after your SET MODEM TYPE command.

See "Using C-Kermit" for additional detail.