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Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!rochester!ken
From: ken@cs.rochester.edu (Ken Yap)
Newsgroups: comp.sys.ibm.pc,comp.periphs,comp.misc
Subject: Re: RS-232 protocol primer (long)
Message-ID: <1989Jun24.005740.19326@cs.rochester.edu>
Date: 24 Jun 89 00:57:40 GMT
References: <1538@mtunb.ATT.COM> <592@megatek.UUCP>
Reply-To: ken@cs.rochester.edu (Ken Yap)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 32

|> Why is the word STANDARD in quotes?  There is, in fact, a REAL LIVE
|> EIA standard, designated RS-232-C and dated 1981.  (Originally 1969,
|> reaffirmed 1981.)  It DOES NOT specify the DB-25 interface connector,
|> or any interface connector at all.  It DOES specify:

One reason the standard is dated is that it only talks about DCEs and
DTEs. Many real world devices are hard to classify either way. Is a
computer a DCE or a DTE. If I connect a terminal to a mainframe, it
should look like a DCE, right? But what if I have a PC and dial out
with a modem? Shouldn't it be a DTE then? What about a printer?  A DTE,
of course. OK, but what about hardware flow control? The RTS/CTS lines
are intended to the modem to tell the terminal when to send. But in
this case it is the printer that should tell the computer when it is
ready to receive. And on it goes.

To add to the confusion, many terminal manufacturers (DEC especially)
use the opposite type of connector from what most other people use. And
yes, the standard doesn't specify a connector so in principle computers
with 9 pin connectors are conforming, if irritating.

The problem is simply that the standard is inadequate for all the uses
it has been pressed into. It's a miracle things work together at all.

All of these problems can be solved with a little detective work. I
recommend the book The RS232 Solution (I think) for anybody who will be
dealing with RS232 interfacing in a big way. You thought a set of
straight through and null modem cables would solve your problems?  Read
the book and learn how cables with too many wires connected can
actually prevent the interface from working.

Sorry I've strayed from the original subject, which is really about
software flow control, but hardware handshaking is a big headache too.