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From: csg@pyramid.pyramid.com (Carl S. Gutekunst)
Newsgroups: comp.sys.sequent
Subject: Re: naive question about tty ports
Message-ID: <141434@pyramid.pyramid.com>
Date: 17 Jan 91 20:20:36 GMT
References: <14@mcglin.wpg.com> <50708@sequent.UUCP>
Organization: Pyramid Technology Corp., Mountain View, CA
Lines: 25

As you probably already guessed, the number of wires on a tty connection is a
religious issue, with some technical considerations.

To make the usual ASCII terminal work at all, you need at least 2, 3, and 7.
On most UNIX boxes, you also need to drive the incoming carrier detect signal.
Most vendors, including Sequent, wire their serial ports as a DTE, so this
means pin 8. While you can fake this out at the Sequent end, you don't want
to; the DCE sensing not only ensures that turned off terminals will be logged
out, it also keeps the computer from echoing garbage to itself on the line.

A few terminals, many modems, and virtually all printers implement some kind
of hardware flow control, although its virtually always optional. You'll need
more wires for those. For intelligent modems you have to figure at least 4
pairs (8 wires). Sequent's Systech terminal mixes don't handle hardware flow
control very well, though, so this is not much of an issue in the short term;
but plan for down the road. Actually, for modems I would generally advise to
stick within RS-232 specs: no more than 50', unless you are using a special
low-capacitance cable, in which case you can go to 200' or so.

I would *not* recommend connecting pin 1; it creates funny ground problems if
you aren't absolutely sure of what you are doing. If using shielded wire, then
connect the shield to the connector shell, not to pin 1. (If you aren't using
metal-shell connectors, then their's no point in using shielded cable.)

<csg>