Arcane modem configuration (was Re: Modems on Ultrix)

[Greg Noel]

In article <584 at pyramid.UUCP> csg at pyramid.UUCP (Carl S. Gutekunst) writes:
>Unfortunately, a lot of terminals use DTR for their flow control, and RTS as
>'power on' (including all Wyse and Liberty terminals).

For this, and other kinds of braindamage, I resort to specialized short cables
(about two inches long, like a sex converter) that makes the necessary changes.
In this case, a simple one that swapped DTR and RTS would suffice.

(Actually, my null modem cables are the same length -- any cable I have that's
over a foot long is straight pin-to-pin with at least the nine standard signals
being carried.  That way I never have to test a cable to see what it does -- if
it's long, it's straight; if not, it had better be labled with what it does.)

>Note that using RTS and CTS for full-duplex flow control isn't technically
>correct either, but that's a lot better than using DTR....

Truth.  (Enter lecture mode for those that don't know the history.)  RTS and
CTS are intended for half-duplex flow control -- that's where they get their
names.  (A half-duplex line is one where only one side is sending data at a
time and if data is to flow in both directions, the two ends must alternate.
A line where data flows in only one direction is a simplex link, and a line
where data can flow in both directions at the same time is a full-duplex link.)
If somebody on a half-duplex connection wants to talk, they raise Request To
Send to indicate this.  When the other end is ready to accept data, it raises
Clear To Send.  On a full-duplex line one doesn't have to worry about line
turnarounds, but the flow-control/handshaking tradition remains, so the signals
are often adapted for out-of-band flow control.  (Hey!  OK, I'll stop now --
give me back my soapbox!)

>No! NEVER jumper #1 and #7 together!! I realize a lot of equipment does this,
>but just because the machine is wrong it doesn't mean the mistake should be
>duplicated in the cable. .......

I should have been clearer.  Carl is correct, and I would only jumper them
together if I had verified that both ends made this mistake and the equipment
absolutely needed to be grounded.  Often the cable shield can be used instead
for this pin instead of an actual wire, but again, circumstances vary, and
what is reasonable under one circumstance is not reasonable under another.  If
you are not knowledgable in this area, get help.  (I consider myself only semi-
knowledgable in this area, enough to know the options and potential problems,
but not enough to know the correct solutions, so I \always/ get an expert to
look at my wiring plans before I commit.)
-- 
-- Greg Noel, NCR Rancho Bernardo    Greg at ncr-sd.UUCP or Greg at nosc.ARPA