Description of a suprisingly simple hardware modification to the OSBORNE-1
to supply software controlled Data Terminal Ready to a modem connected to
the RS-232 Serial Port. It is assumed that those interested in applying the
mod knows how to field strip the OS-1 and remove the motherboard and also
has an understanding of how to tack solder connections to integrated circuits.
If you don't have these basic skills, try and find a helper you trust that
has the knowledge and can guide you.
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After removing motherboard, locate R-29 (about 2 inches or so behind RS-232
connector). It can be further identified by a land running from the resistor
to pin 6 of the RS-232 connector. Remove R-29. Cut and dress approximately
2 inch wire. Tack solder one end of the wire to pin 5 of the ACIA and the other
end to the hole NEAREST, repeat NEAREST, the RS-232 connector where R-29 was
removed. DO NOT use the hole closest to the ACIA as that is +5 volts. Thats all
there is to it. This will give DTR on pins 5,6 & 8 of the RS-232 connector as
long as RTS is high (which it is when you are using the RS-232 port). If you 
were using +5 volts from any of these pins for anything else (can't think of
what), you're out of luck on one or the other as the Osborne schematics do not
agree with the motherboard. If you need +5 volts for another use (??), an
alternative is to leave R-29 in, cut the land on the reverse side of the
motherboard between pins 5 & 6, or 6 & 8, and jumper the ACIA to pin 5 or 8 of
the RS-232 connector. Then configure your modem cable to supply DTR according
to the method selected.
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Programming considerations: Write 6xH (x is don't care) to the MODEM Status
port (2A00H) to drop DTR to the modem. Then write 56H or 55H to set 300 or 1200
baud again. You may have to include a routine to reset your modem after setting
DTR again as I had trouble with my Prometheus getting the baud rate dinged up,
but that is a known problem with some smart modems.
                                                                              
The routines to send ATZ to reset Hayes compatible modems can be borrowed from
the Smartmodem overlays by copying the SMSEND routine.
                                                                             
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Don't know if you need this mod, but my BYE and MEX certainly worked MUCH
better after I made it.  I have to admit, I studied the mod a long time before
attempting it since it was so simple I wondered why OCC did not incorporate
DTR to begin with. Good luck and happy hacking.
                                                                    
                                               Larry G. Bell
