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

[KnudsenMJ at aol.com]

In a message dated 8/16/04 7:45:04 AM Eastern Daylight Time, wb8tyw at qsl.net 
writes:

> > 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.

Yes, this feeds the other computer or terminal's DSR and DTR to Coco's CD, 
which is probably the simplest and best, since DSR and DTR will always be on 
when that other device is powered up and ready to run.

>  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 may still need DSR fed back to pin 8, if that's an input.
And while connecting two outputs together means you've made a mistake, 
according to the official RS232 specs, RS232 driver gates are supposed to tolerate 
being crossed to as much as -15V or +15V, and certainly other RS232 signals.

It won't work, but nothing should get the smoke let out of it.  

>  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 DTR 
>  signal.  See CCIT specifications.

Yes, feed the modem's DSR back into its own DTR,  since the Coco doesn't 
supply this signal.

>  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.

Actually, since a modem provides a true CD signal, that is what should go to 
CD of the Coco.  But see below.

>  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.

Strictly speaking, BASIC should require CD only when receiving data from the 
modem.  But since these are full-duplex modems, the assumption must be that 
there is incoming carrier while we are sending data, too.  BASIC seems to use CD 
as a DSR.  That's probably OK.

In fact, the whole RS232 handshaking scheme turned out to be more complex 
than the real world really needed, and in one sense Radio Shack should be 
commended for paring it down to its bare essentials.  Though we mortals can get 
confused as to just what to throw out.  --Mike K.