PMMI  MM-103 Modem

This was the first modem that PMMI came out with. It was widely used and introduced many of us to the concept of a telephone modem.

The MM-103 interfaced to the S-100 bus through 4 IO ports.  The computer baud rate was 61 up to 600 baud. The board had a UART LSI chip to convert the bus serial data to serial format for the modem chip (a Motorola MC6860L).  There was auto-answer capability in hardware. The board could be utilized to trigger an interrupt line but this was not often used.

A telephone line "coupler" was required by the phone company to use the modem (a different time/era).  It was supplied by PMMI.

Simple software examples were provided in the manual, but as I said above there were a number of excellent public domain CP/M based programs early and widely used with the board. 

A fairly detailed manual was supplied (but without a schematic). It can be obtained here.
  

Note on Bell 103 Protocol.
Bell 103 - Asynchronous data transmission, full-duplex operation over 2-wire dialup or leased lines; 300-bps data rate. Ideal for the "low demand" user who exchanges files infrequently with another PC user or an on-line bulletin board. Comparable to ITU V.21. This modulation is well suited for bad phone lines – as the communication guys use to say "103 modems will work on barb wire".

Other PMMI  S-100 Boards
MM-103 Modem    Bell 212A Modem     VT1 Modem