[Coco] Re: archive posted - PC<->CoCo RS232 info

[John E. Malmberg]

Robert Emery wrote:
> 
> Anyway, it looks like the CD line at the CoCo end is the one left hanging. I
> have another diagram for connecting to a modem which shows CD connected to pins
> 20, 6 and 8 of a db25.

If that is intended for a null modem connection to a terminal or another 
computer, then that is the correct wiring.

If that is intended for a connection to an external modem, then the 
wiring is incorrect, and only pin 6 (DSR) of the modem should be 
connected to the CD pin of the COCO, otherwise you have multiple outputs 
shorted together, and that could be bad for the modem.

The modem will likely require pin20 also be connected to pin6 as it is 
not supposed to allow an outgoing connection with out it seeing a DTE 
signal.  See CCIT specifications.

It may be possible to OR the CD and DSR signals together with some 
isolation hardware, but for the purpose of the COCO, the DSR line is 
probably good enough.

The BASIC ROM definitely requires the CD line to be asserted, or it will 
not output more than one character, and then the I/O will hang waiting 
for it.

-John
wb8tyw(at)qsl.net
Personal Opinion Only