[Coco] Supercomm: more details

[KnudsenMJ at aol.com]

In a message dated 8/13/04 6:09:25 PM Eastern Daylight Time, james at skwirl.ca 
writes:

> As I see it, DCD should be fed by DTR and RTS... I dunno, it seems that 
>  DTR would be a sensible source for DCD... DCD is more of a 
>  session-based on or off, like DTR and DSR (from what I've seen in the 
>  past) whereas RTS and CTS are more for hardware handshaking...

You have a good point there -- DCD is indeed session-level, like DTR and DSR.
However, many RS232 interfaces are designed to receive only when DCD is on, 
to prevent responding to noise on the line (makes plenty sense with a real 
modem), but I think the practice of feeding RTS to DCD on a Null modem was to make 
sure the receiving end read data only when the sender was actively sending it.

ISTR some interfaces and software drivers are set up to drop the session the 
first time DCD goes down -- if so, you definitely do want DCD fed from DTR, 
not RTS.

Main thing is to feed *something* to DCD so it is high.  Some 25-pin RS232 
interfaces supply +12 and -12 constant voltage just so you have a place to tie 
such things to.  --Mike K.