modem connects as a caller but not as a callee 1

[mordax]

I have 2 landlines and a separate modem connected to each of them monitoring their actions by 2 instances of Hyperterminal. If I dial modem B from modem A and respond to “RING” by ATA, the modems connect after 30 secs or so. In the other direction (dialing A from B) “RING” appears, ATA is obviously accepted, because the modem starts making noises, but the connection is never established. The tones in those 2 cases are different, too. First tone seems the same; the 2nd, in the successful case, is a the usual complicated sequence of tones, in the bad case, it is a single tone of a higher pitch which remains unchanged until it times out or I hit return. Result “NO CARRIER” (for both modems).
The modems: AOPEN modem board (B) and TRUST MD 1200 (external) (A). Is one of the modems defect or badly configured or what could it be?

 

[pansophic]

You probably have a mis-match in the starting connection speed of the two modems. If you reset the two modems to factory defaults by issuing AT&F and try to connect using Hyperterminal on both ends, I suspect that you will not have an issue in connecting.

If they work this way, then you need to be looking at your initialization strings for both ends and your interface speeds for both ends. You will be far better off if you configure the modems to begin speed negotiation at 33.6K rather than 56K, since you can only achieve V.90 rates if one end is digitally attached to the telephone network (there can only be one A/D conversion).

You may also want to look at the fallback rates that each of the modems is configured to support.

In general, when working with modems and embedded devices, the initial negotiation speed is configured to be static at a rate that is near the serial interface speed. So if you are only providing a 9.6K interface from your micro controller then manually configure the serial interface and the line negotiation speeds at 9.6K.


pansophic