Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site cdstar.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!genrad!panda!talcott!wjh12!cdstar!saltiel
From: saltiel@cdstar.UUCP (Jack Saltiel)
Newsgroups: net.dcom,net.periphs
Subject: Re: Definition of "null modem" cable
Message-ID: <156@cdstar.UUCP>
Date: Wed, 28-Aug-85 22:41:31 EDT
Article-I.D.: cdstar.156
Posted: Wed Aug 28 22:41:31 1985
Date-Received: Sat, 31-Aug-85 03:30:47 EDT
References: <195@almsa-1>
Distribution: net
Organization: Cambridge Digital Systems, Inc.
Lines: 57
Xref: watmath net.dcom:1218 net.periphs:834

In article <195@almsa-1>, control@almsa-1 (William Martin) writes:
> What is the exact definition of a "null modem" cable?
> 
> Or is it a general term with no exact definition, except that it implies
> that the 2 & 3 connections are crossed?
> 
That's exactly right. Depends upon what modem functions you are
trying to 'nullify' or imitate. The one that we use that usually
(99%+) succeeds in allowing data to flow is:

-----------------------
    f     2  -------------------\ /------------------  2    f
                                 X
    e     3  -------------------/ \------------------  3    e

    m     4  ----\				 /---  4    m
                 |                               |
    a     5  ----/                               \---  5    a

    l     6  ------\                          /------  6    l
                   |                          |
    e     7  ----------------------------------------  7    e
                   |                          |
	  8  ------+                          +------  8
                   |                          |
	 20  ------/                          \------  20

		  2 = TD Transmit Data
		  3 = RD Receive Data
		  4 = RTS Request to Send
		  5 = CTS Clear to Send
		  6 = DTR Data Terminal Ready
		  7 = Signal Ground
		  8 = CD Carrier Detect
		 20 = DSR Data Set Ready

	Alternate arrangements have 4 & 5 crossed between the two
	end to allow cooperating hardware perform a
	RTS/CTS handshake.

	Other have that and/or DTR on the left tied to DSR & CD on
	the right & visa versa. This allows another kind of
	handshake. If you don't need the handshake, use the one
	drawn above. It will almost always allow data to pass.

	(PS: I may have RD & TD, RTS & CTS, and DTR & DSR
	reversed--this is from memory(at home) but just check an
	EIA chart first. It only matters if you're doing any
	hardware handshaking.)

-- 
					Jack Saltiel
					Cambridge Digital Systems
					{wjh12,talcott}!cdstar!saltiel

	"Here's to plain speaking and clear understanding."
	"I like a man who likes to talk."