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From: brian@sdcsvax.UUCP
Newsgroups: net.dcom
Subject: hardware flow-control on a VAXen
Message-ID: <1747@sdcsvax.UUCP>
Date: Sun, 4-May-86 07:23:28 EDT
Article-I.D.: sdcsvax.1747
Posted: Sun May  4 07:23:28 1986
Date-Received: Tue, 6-May-86 05:34:45 EDT
Organization: UCSD wombat breeding society
Lines: 33
Keywords: cs-11

Recently, whilst installing our Ungerman-Bass NIUs on one of the VAXen
here at UCSD, I discovered that certain models of comm line multiplexors
can do hardware flow control.  In particular, the Emulex CS-11 (which
does a pretty good job of looking like a DH11) has a switch for each
line located in the multiplexor panel that when turned on causes the
UART transmitter to pay attention to Clear-To-Send.  The manual for the
CS-11 is very clear on this, but I'd never looked.  (Stupid, hah!)

This hardware flow control is completely invisible to the software.
What it does is when CTS is not asserted, it makes the transmitter stop
at the end of the current character - so you get essentially
instantaneous flow control.  The software (at least, most drivers I've
poked at) don't look at the CTS bit in the status register, so they just
seem to think that the current character is taking an extraordinarly
long time to send.  For most applications, this works extremely well.

If you have had problems with getting output stopped to avoid
overrunning buffers in some peripheral (like lan boxes, terminals,
buffered modems, etc) because the VAXen sometimes will send as many as
64 characters AFTER you ship it an X-OFF, this may provide an answer.

Moral: check your hardware manual.  It might well be that the particular
comm multiplexor you're using might do the right thing, even though the
software doesn't.

	Brian Kantor	UC San Diego

	decvax\ 	brian@sdcsvax.ucsd.edu
	ihnp4  >---  sdcsvax  --- brian
	ucbvax/		Kantor@Nosc 

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