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From: tim@ora.UUCP (Tim O'Reilly)
Newsgroups: comp.unix.questions
Subject: Re: DSR
Message-ID: <650@ora.UUCP>
Date: Wed, 26-Aug-87 12:21:47 EDT
Article-I.D.: ora.650
Posted: Wed Aug 26 12:21:47 1987
Date-Received: Sat, 29-Aug-87 06:19:55 EDT
References: <8998@brl-adm.ARPA>
Organization: O'Reilly & Associates, Inc., Newton, MA
Lines: 62
Summary: RTS/CTS should also be connected

In article <8998@brl-adm.ARPA>, art@acc.arpa writes:
> 
> For asynch RS-232 cables,  I recommend the following:
> 
> 
> 	SD   2 -------------\ /----------------- 2   SD
> 			     x
> 	RD   3 -------------/ \----------------- 3   RD
> 
> 	RTS  4 ---+			    +--- 4   RTS
> 		  |			    |
> 	CTS  5 ---+			    +--- 5   CTS
> 
> 	DSR  6 ---+			    +--- 6   DSR
> 		  |			    |
> 	SG   7 ---|-------------------------|--- 7   SG
> 		  |			    |
> 	DCD  8 ---o---------\ /-------------o--- 8   DCD
> 			     x
> 	DTR 20 -------------/ \----------------- 20  DTR
> 
> 
> Art Berggreen
> art@acc.arpa
> 
> ------
Note that the cable shown here is a null modem cable for connecting tw
DTE (Data Terminal Equipment) interfaces.  DTE-DCE (i.e. terminal to
modem) connections would not cross lines 2 and 3, and 8 and 20.

There are a variety of ways to do cable workarounds for DSR/DCD/DTR;
you'll probably get people recommending other versions of this cable
that also work.  Most hinge on the fact that carrier detect is a more crucial
indication of connection than DSR, and so most UNIX systems monitor
DCD instead of DCR.  

However, I do want to point out one drawback of the cable shown here,
which has nothing to do with DSR/DCD/DTR:

A cable with CTS and RTS jumpered together will work fine for normal
terminal operation (people don't type all that fast)  but if you try
to upload text from a PC using a cable like this, the receiving
system will tend to lose characters.  

While CTS/RTS were originally designed to turn
around a half-duplex modem connection, nowadays, they are often used
for hardware flow control.   If there is any chance your cable will
have to carry data from the terminal to the host at computer speeds
rather than at human speeds, you should be sure to include
RTS and CTS.  For the null-modem cable shown above, the
two lines should be crossed.  For a straight through cable, they
should (obviously) be straight through. 

For UUCP, creating a cable like this is not absolutely necessary,
since UUCP will retry packets to make sure everything gets through.
However, you are increasing system overhead and decreasing performance
for the sake of two wires.
-- 
Tim O'Reilly (617) 527-4210
O'Reilly & Associates, Inc., Publishers of Nutshell Handbooks
981 Chestnut Street, Newton, MA 02164
UUCP:	seismo!uunet!ora!tim      ARPA:   tim@ora.uu.net