Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: RS232 Flow Control
Message-ID: <1990Nov16.180612.21317@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <1990Nov10.013730.28838@julius.cs.uiuc.edu> <1990Nov13.184339.4756@zoo.toronto.edu> <10276@uhccux.uhcc.Hawaii.Edu> <josef.658656371@peun11>
Date: Fri, 16 Nov 90 18:06:12 GMT

In article <josef.658656371@peun11> josef@nixdorf.de (Moellers) writes:
>   "Circuit 106 - Ready for sending [this refers to pin 5]
>    Direction: From DCE
>    Signals on this circuit indicate whether the DCE is prepared to
>    accept data signals for transmission on the data channel ...
>   "
>   IMHO this indicates that CTS is indeed meant to allow for hardware
>   flow control.

Sort of kind of.  The trouble is that RTS is not symmetrical with it.
The way it works is that you raise Request To Send to indicate that you
wish to send data, and the modem replies with Clear To Send to indicate
that you should go ahead.  This is meant for half-duplex modems that
have to know when they should be talking and when they should be silent.
The reason for the feedback path is that it can take them a noticeable
fraction of a second to establish outgoing carrier.

"DCE" is standardese for "modem".  The standards don't envision the
possibility of directly connecting two computers (etc.) at all; they
were written strictly in terms of computer/terminal-to-modem hookups.
And the notion of flow control never appears in them at all.

Actually, I believe RS-232D (yes, RS-232C is obsolete) explicitly identifies
an optional symmetrical-flow-control role for RTS and CTS.  However, almost
nobody is aware of RS-232D yet, and nobody really implements RS-232D, any
more than they really implemented RS-232C.  The general answer is still
"you have to find out what your particular device wants".
-- 
"I don't *want* to be normal!"         | Henry Spencer at U of Toronto Zoology
"Not to worry."                        |  henry@zoo.toronto.edu   utzoo!henry
