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From: touch@gradient.cis.upenn.edu (Joseph D. Touch)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Mac plus serial port problem
Keywords: plus, modem, serial
Message-ID: <44291@netnews.upenn.edu>
Date: 7 Jun 91 02:59:18 GMT
References: <1991Jun6.171508.29333@cbnewsk.att.com>
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Reply-To: touch@gradient.cis.upenn.edu (Joseph D. Touch)
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I have seen it - try replacing the rs422 receiver chip on the
motherboard.  The problem is that bad wiring/bad devices can partially
fry it, making it draw too much current.  The printer works on that
port since you're sending to it, not receiving from it.  The modem
works on the other port because that input to the chip isn't fried.

If you want to verify this, check the current across the send and
receive wires; in each case it should be about 1.5 milliamps,
definitely much under 10.   Mine read over 10 when it was broken.
RS422 and RS232 devices are both voltage driven, and their drivers (on
the modem) can't handle high currents.

        Joe

********** WARNING *************************************
THIS IS NOT FOR BEGINNERS - if you aren't familiar with chip
replacement, get someone to do this for you.  And it definitely
voids your warranty.  Do this at your own risk.
********** WARNING *************************************

PS F.Y.I. the chip is a 2632, in my Mac it was a 26LS32 and I replaced
it successfully with a 26C32 CMOS version.  I removed the old chip by
first cutting it off the board (snip the legs near the body of the
chip with diagonal cutters or nail clippers(!)), then carefully
removing each leg from the board individually with a soldering iron.
It destroyes the chip you're removing, but it's a lot easier than
trying to desolder all 16 pins at once!  Be very careful, it's a
multilayer board, and you need a low-wattage iron or you'll damage it.
Then remove the solder from the holes (solder wick, or a desoldering
tool).  Insert a socket (it's a low speed part, so it'll be OK), and
solder it in.  Plug the new chip in (careful to install it in the same
orientation as the old chip).