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Path: utzoo!linus!security!genrad!decvax!minow
From: minow@decvax.UUCP (Martin Minow)
Newsgroups: net.dcom
Subject: In defense of XOFF/XON flow control
Message-ID: <257@decvax.UUCP>
Date: Thu, 3-Nov-83 20:03:19 EST
Article-I.D.: decvax.257
Posted: Thu Nov  3 20:03:19 1983
Date-Received: Sun, 6-Nov-83 19:14:41 EST
Organization: DEC UNIX Engineering Group
Lines: 35

I should like to warn the gentle reader against using any of
the RS232 signals (CD, DTR, etc.) for flow control handshaking.
There are just too many places where this will not work, including
remote modems, port multiplexors (such as Gandalf and Micom),
20 Ma. current loop links, and pretty much anything with an
operating system that you can't dig into to "fix" to your
requirements.  Also, it will not work in the future when local
area networks will carry most terminal traffic.

The fact that operating systems do not generally understand these
signals -- and are not going to in the future -- will cause problems
when you attempt to sell your product to a wider audience.

While the XOFF/XON protocol isn't perfect, it does work reasonably
well in a variety of environments and is not too difficult to
implement.

A contributor suggested that buffer limitations are a factor in
rejecting XOFF/XON.  This does not seem to be the case in my
work.  We watched large amounts of data passed into a DECtalk
text to speech terminal over a 9600 Baud terminal line and seldom
saw more than one or two characters transmitted from the host
after DECtalk sent an XOFF.  (DECtalk's effective data rate is
around 300 Baud, so it sent XOFF's often.)   By the way, our
input buffer size is (I think) around 512 characters, and we send
XOFF if there are more than 256 characters pending.  This gives
the host 250 msec. (at 9600 Baud) to stop.   We send XON when the
buffer is 3/4 empty.  This works well.  RAM chips are cheap.

An historical note:  XOFF and XON were originally used to control
the paper tape reader reader run relay on ASR 33 teletypes.
XON turned the relay on; XOFF turned it off.

Martin Minow
decvax!minow