knudsen@ihlpl.ATT.COM (Knudsen) (10/21/88)
OK, so far there have been only a couple short articles this time around on hacking the Modem Pak. Let me add some more stuff from memory; last year this was discussed somewhere. The Modem Pak, now on sale for $30 at RadShack, contains a 6551 ACIA/UART (same as in the RS232 Pak) plus a single-chip 300 Baud modem. The general belief is that ordinary TTL logic links the two chips together. To use the Pak as an RS232 Pak with an external modem (like you want 1200 Baud, doncha?), the TTL signals out of the ACIA chip (Transmit Data, Request to Send, Data Term Ready) must be converted to RS232 higher-voltage levels. Likewise, incoming signals from the outside like Receive Data, Carrier Detect, Clear to Send, Data Set Ready must be converted from RS232 levels to TTL. The conversions are strictly voltage-to-voltage, and there are common TI and Motorola chips to do them. Separate chips are needed for each direction. You need -5 or -12 in addition to +5, and +12 should be used if you have it (as in an MPI). *Possibly* the Modem Pak generates these voltages itself. And you should mount a female DB-25-pin connector on the thing when you're done. Else solder a cable to your modified Pak with a male DB25 at the far end to go into your external modem. You may get away with running TTL ouputs from the ACIA chip into some RS232 modems, but never do the reverse unless you enjoy replacing big DIPs a lot. Remove the modem chip, or at least disconnect it from any of the ACIA's inputs. MIDI conversion is easier, and takes less hardware. More details, if needed, in another article. -- Mike Knudsen Bell Labs(AT&T) att!ihlpl!knudsen "Lawyers are like handguns and nuclear bombs. Nobody likes them, but the other guy's got one, so I better get one too."