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From: jhc@mtune.ATT.COM (Jonathan Clark)
Newsgroups: comp.dcom.modems,news.sysadmin
Subject: Re: Hardware Protocol
Message-ID: <1156@mtune.ATT.COM>
Date: Mon, 24-Aug-87 22:32:35 EDT
Article-I.D.: mtune.1156
Posted: Mon Aug 24 22:32:35 1987
Date-Received: Wed, 26-Aug-87 01:22:58 EDT
References: <192@caeco.UUCP> <2849@phri.UUCP> <1103@laidbak.UUCP> <1172@geac.UUCP> <419@piaget.UUCP> <374@pcrat.UUCP> <305@pvab.UUCP>
Reply-To: jhc@mtune.UUCP (Jonathan Clark)
Followup-To: comp.dcom.modems
Organization: AT&T ISL Middletown NJ USA
Lines: 21
Keywords: RS-232 CTS flow-control
Xref: mnetor comp.dcom.modems:866 news.sysadmin:341

In article <305@pvab.UUCP> robert@pvab.UUCP (Robert Claeson) writes:
>Aren't there pins named "Secondary DCD, CTS, RTS" and some others at pin
>12, 13, and 19? Can't they be used to provide a "real" two-way flow control
>without violating the RS232 standard?

No. One of the (demonstrably) little-known facts about the full RS-232
interface is that there are *two* independant data channels in there,
called Primary and Secondary (would you believe). The Secondary channel
has a full set of control pins (those which don't refer to the physical
state of the modem, logically enough [sic]). Actually there is or was
at least one modem which implemented a tertiary channel!

Anyway, the whole point of this is that while RTS/CTS full-duplex flow
control does violate the letter of the RS-232 standard, it is a de facto
standard because it is too useful to give up. Personally I'll take any
sort of out-of-band flow control, I almost don't care what it is.
-- 
Jonathan Clark
[NAC,attmail]!mtune!jhc

An Englishman never enjoys himself except for some noble purpose.