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
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
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