I worked in “low speed modems” when they weren’t considered low speed. In my experience low speed modems were used primarily in two places; Point-of-Sale terminals and cable/satellite set-top-boxes. These are devices that needed only to transmit <1K byte or so of data. An FSK modem (V.23 FSK, V.22 300, Bell 103) can connect to the phone line in 200-400ms. A 1200B/S modem (Bell 212, V.22) can connect in 600-1200ms. In “old days” vendors were charged by the “message unit” of 6 seconds to their call centers, so connecting, transferring data and hang-up timing was important if you were doing millions of connections. Now they just want to manage the call volume.

Low Speed modems have great signal to noise performance. Modems transmit analog “symbols” comprised of 1 to many (thousands) of bits / symbol. Generally speaking, the simpler the symbol (fewer bits/symbol) the better the signal to noise performance. A 75B/S V.23 needs perhaps a 6db SNR to work, 1200B/S V.22 needs perhaps 10db SNR, 2400 V.22bis about 16db SNR and on.

In the general case another factor is (on chip) filtering. Starting at 2400B/S V.22bis an adaptive equalizer is used. It must be “trained” to the line which makes connecting longer and possibly more difficult. This gets layered in with error correction, data compression echo canceling all of which contribute in different ways to the SNR performance and connectability. A V.90/92 56K modem needs perhaps 30db? SNR and 10-20 seconds to connect.

All modems should support async and should talk to the 8032 UART just fine. FSK modems are only async. 1200B/S and higher can be synchronous (using tx and rx clocks) as they use synchronous data pumps (not V.23 or Bell 202 1200B/S)

Silicon Systems designed a series of low cost single chip modems in the ‘80’s specifically designed for the 8032. Surprisingly, they still exist, see: "www.teridian.com" and look for the “K-Series” modems. If you’re new to modems I would try and stick to the 1200B/S versions. They should have a ton of support code and they have a modem design guide.


Stephan Buck