Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!mailrus!umix!uunet!pcrat!rick
From: rick@pcrat.UUCP (Rick Richardson)
Newsgroups: comp.dcom.modems
Subject: EIA Flow Control
Summary: Terminal, Computer makers...Wake Up!
Message-ID: <470@pcrat.UUCP>
Date: 20 Feb 88 05:15:01 GMT
References: <8802180550.AA09815@ucbvax.Berkeley.EDU>
Reply-To: rick@pcrat.UUCP (Rick Richardson)
Organization: PC Research, Inc., Tinton Falls, NJ
Lines: 55


	[ Regarding null modem design ]

In article <8802180550.AA09815@ucbvax.Berkeley.EDU> RAF@NIHCU.BITNET ("Roger Fajman") writes:
>Crossing pins 4 and 5 (Request To Send and Clear To Send,
>respectively) makes no sense to me.

As has been pointed out, there are many, many ways to make a null modem.
The cross of 4 and 5 is 'best' when you have devices which implement
the so called "EIA Flow Control" (aka hardware flow control).  Here,
CTS controls the flow from DTE to DCE, RTS controls the flow from
DCE to DTE.  In this scheme, there is no longer any correlation between
the assertion of RTS and the reply with CTS.

Some manufacturers use DTR to do essentially what the RTS signal
does in the above explanation.  This, to me, is ugly since you
lose a useful indication (that there is a live DTE out there).
The lossage of the RTS-CTS line turnaround protocol, on the other
hand, is minor.

	[Triggers my Flow Control Button in general...]

At any rate, the need for "EIA Flow Control" is evident in these
days of packet switched data, be it between packetized modems,
or ISDN TE's.  If you don't have EIA flow control, you'll find
it just about impossible to run an interactive session over
one of these facilities, and then switch in mid-session to
a file transfer using XMODEM, YMODEM, or any other protocol
which uses all 8 bits.  Without EIA flow control, your only
option is "no flow control" for the whole session -- and that
means probable data lossage.  If you've never seen how
frustrating this can be, just do this:  get yourself an aging
VT100, log in to UNIX and turn off Xon/Xoff flow control.
Take out all delays in the termcap/terminfo entry, and try
scrolling around in a file with a screen editor.  Now, even
if you have a spiffy PC that can run 40 megabaud without flow
control talking to a VAX 88Million, you may still have to contend
with a network between you and the VAX that can't (instantaneously)
keep up.  You get the same effect.

	[ Now blast the DTE folk for ignoring DCE needs ]

The unfortunate situation is that, while many DCE manufacturers
have seen fit to provide for EIA Flow Control, the DTE side
of the house has a lot of catching up to do.  And until they
do, you may find yourself saying:

	"Well, I have a ('Blazer, ISDN line, etc.) that
	can move data at (19.2K, 38.4K, etc.), but I only
	run it at 9.6"

-- 
	Rick Richardson, President, PC Research, Inc.
(201) 542-3734 (voice, nights)   OR   (201) 834-1378 (voice, days)
		uunet!pcrat!rick