Path: utzoo!attcan!uunet!snorkelwacker!ai-lab!mikec
From: mikec@wheaties.ai.mit.edu (Mike E. Ciholas)
Newsgroups: comp.sys.mac.hardware
Subject: Re: SID & early cx WEIRDNESS !
Message-ID: <9431@wheat-chex.ai.mit.edu>
Date: 15 Jul 90 18:21:54 GMT
References: <77778@cc.utah.edu>
Reply-To: mikec@wheat-chex.ai.mit.edu (Mike E. Ciholas)
Organization: CEDAR Technologies
Lines: 40

In article <77778@cc.utah.edu> TJACOBS@cc.utah.edu writes:
>My SID-II works fine on other macs including my cx at work, but it doesn't
>work properly on my cx at home.  [stuff deleted about garbled sound, etc.]
>
>Any ideas?
>
>Tony Jacobs

This problem took me a LONG time to find.  The Solution: cut pin 7 on the din 8
connector.  Looking from the rear of the connector, pin 7 is the top center pin
and can be easily cut from the rear.

The problem is this:

On Mac II, SE, and on up, pin 7 on the din 8 was connected to the data carrier
detect line on the SCC chip.  This causes interrupts to occur whenever the
signal transitions from high to low or vice versa.  The input to the Mac goes
to a differential line receiver which compares the input signal with ground.
On the SID-II circuit board, I grounded pin 7, just trying to be safe.  Bad
move.  Now any noise on the line causes tons of interrupts freezing the
machine.

But there's more!  Since the differential receivers have some few millivolts of
offset (that is, they aren't perfect), some machines aren't affected at all!
I, of course, had the misfortune of testing on only machines that worked!  So
the SID got shipped that way.  Then I heard people say it doesn't work.  Well,
I eventually found a machine it didn't work on, and with some effort, found
the problem.  Up until now, machines either locked, or worked.  It sounds
like your problem is border line, occasionally working, sometimes not causing
garbled sound messages.

Hope this helps.

Mike Ciholas

mikec@ai.mit.edu

PS: I will be sending a note to all SID owners advising them of this flaw and
the simple fix.  Also, capacitor C29 is labeled backwards on the circuit board
so everybody probably put it in backwards.  It still works that way, though.