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From: david@varian.UUCP
Newsgroups: net.info-terms,net.periphs
Subject: Re: Use of ring indicator for autoanswer
Message-ID: <170@varian.UUCP>
Date: Wed, 12-Oct-83 13:54:00 EDT
Article-I.D.: varian.170
Posted: Wed Oct 12 13:54:00 1983
Date-Received: Fri, 14-Oct-83 03:08:56 EDT
References: uiucdcs.3197
Lines: 46

Speaking of the RING indicator, prowling through the manual for the
Able DH/DM multiplexor, I found an obscurely-documented but very useful
use of it:

Section 7.4 (pg. 7-5):
In applications where modem control is not required, the RING line can be
used as a device handshake signal which stops transmission to that device
in a manner that is completely transparent to the software ... A jumper
placement connects the RING line to the clear-to-send input of the UART
... the UART clear-to-send input is removed when the RING signal is placed
at the inactive level. The loss of CTS inhibits the UART from raising
its transmitter-ready flag and thus the next character is not loaded into
the transmit holding register until the CTS signal has been restored by
reactivation of the RING signal.

I was able to use this feature to connect a serial printer that uses
DTR control to UNIX without any software modifications (it seems to me
that mdmbuf in the 4.1BSD tty driver doesn't work; am I right?).  I
connected the jumper (I had to phone Able to find out where they are
- if you look at the Figure 3-1 on pg 3-3, there are 16 pairs of circles
marked "RESERVED FOR FUTURE USE"; each pair corresponds to one of the
16 lines; jumpering pins 28 and 29 together will affect line 0, ...
60 and 61 affect line 15).  I then made a special cable for the computer
end which connected RING (pin 22) to the DTR output of the printer
(which was now CARRIER (pin 8), having gone through a null modem, as
both the printer and the multiplexor are DTE equipment).  I created
a new version of the printer queuing software with all the names changed
(lpr-->lp2, lpd-->lpd2), linked /dev/ttyxx to /dev/lp2, and that was
it.

If anyone has questions about what I did, feel free to call or send me 
mail. I don't know if all Able multiplexors have this feature; we
received ours in February 82, and the manual is dated July 81.  We
also have an older Able DMAX/16, which is a 3 board set (the DH/DM
is a single board), and I haven't found anything in the manual about
such a feature.

	David Brown
	Varian Instruments 2700 Mitchell Dr.  Walnut Creek, Ca. 94598
	(415) 945-2199
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