TJACOBS@cc.utah.edu (07/15/90)
My SID-II works fine on other macs including my cx at work, but it doesn't work properly on my cx at home. The sound is getting scrambled in such a way that it looks like the sound ribbon has been cut length wise and put back together in the wrong order!!! I can see the sound wave but it is discontinous. The wave will go off the top and continue back on the bottom. Really bizarr. Now I'm wondering if this is a hardware problem with a very early cx. I have some vague recollection of there being a problem with the really early cx's of which my home cs is. The serial ports seem to work just fine for printing and modeming (as it is right now) but perhaps it doesn't work well when it is externally clocked as I think the SID is. Any ideas? Tony Jacobs Center for Engineering Design University of Utah
mikec@wheaties.ai.mit.edu (Mike E. Ciholas) (07/16/90)
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.