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From: touch@dsl.cis.upenn.edu (Joe Touch)
Newsgroups: comp.sys.mac.hardware
Subject: mac serial questions - more information
Summary: serial line draws too much current
Message-ID: <26682@netnews.upenn.edu>
Date: 3 Jul 90 22:18:42 GMT
Sender: news@netnews.upenn.edu
Reply-To: touch@dsl.cis.upenn.edu (Joe Touch)
Organization: University of Pennsylvania
Lines: 50

I'm trying to have the MAC talk to itself - an RS232 loopback,
connecting send to receive.  I was having problems interfacing the
MAC to something else RS232, so I'm trying to back up and figure
out what's what.

First, the MAC RS232 works fine connected to a modem.  Several
different modems, in fact, and other types of serial lines.
But the MAC won't talk to itself.

I measured the follwing with a breakout box - and a digital meter.
I was not intending to measure the AC signals, just the DC for now.
(i.e. I'm not typing when I measure...)

The MAC RS422 has been wired as an RS232 (and works) as follows:
	RS422	RS232
	
	GND  	GND
	RD+  	GND

	RD-	RD
	TD-	TD

MAC DC open circuit  (unconnected)
	send 	-4.7 V
	receive	.427 V (should be nominally zero, but isn't)

MODEM DC open circuit
	send	-9.2 V
	receive	.07 V (nominally zero)

MODEM loopback (send to receive)
	-8.62 V	/ 1.4mA	(current equivalent to 6.2K Ohm terminator)

MAC loopback
	-3.14 V / 21mA (current is way too high - like a 150 Ohm term.)

MAC-MODEM
	mac-to-modem	-4.57 V / 0.6mA (like a 7.6K Ohm term)
	modem-to-mac	-2.22 V / 13.7mA (current is way too high - 162 Ohm)

I'm only trying the loopback here.  The MAC has been happy for
quite a while with only send, receive, and ground out to a modem,
and RD+ connected to ground to accomodate the RS232 single-ended signal,
as all wiring diagrams for the MAC specify.

Any help you could provide would be appreciated.

Thanks.

Joe