jhs@MITRE-BEDFORD.ARPA (08/17/87)
To recap: Mr. Simon's Panasonic P1091i printer doesn't work correctly when driven by his Atari 1049ST (after having worked just fine with it for a couple of years), but works fine when driven by an IBM PC compatible, using the same cable. That would be fine, but the Atari also drives an IBM printer just fine, thank you, with the same cable. Curiouser and curiouser said Alice! First, let me applaud Mr. Simon's diagnostic approach. Nothing wrong with that, except that it didn't finger the problem. He did determine that the Least Significant Bit (LSB) isn't getting through from the Atari to the Panasonic. What to do next? This is obviously one of those less than obvious problems that drive the service shops mad and cause the customers to be absolutely CERTAIN they are getting a runaround. Another way to put this is, Aha!, we have one of those truly INTERESTING problems to figure out! (Reminds me of an old Chinese curse...) There are several possible causes of this sort of situation. Basically, I think they fall into two general categories. Maybe somebody else can think up other categories. Category 1: Seemingly identical conditions not identical. E.g. something about the connectors on the various units causes something different to happen when the plug is plugged into one and not to happen with the other, like a pin that shorts in one case and not the other. Category 2: Something is marginal. E.g. a driver device in the Atari has failed, and for some reason the failed driver is unable to drive the Panasonic printer input circuitry, but the IBM printer draws less current, and so it is able to read the weakened signal OK. My hunch is that we have a case of Category 2, and PROBABLY in fact the specific example I have suggested. Look closely at the driver circuit that drives the failed bit, and I suspect you will find that it is bad and that replacing that chip will fix it. Alternatively, the circuitry in the Panasonic printer has developed a short or a near short, and the driver in the Atari isn't quite able to hack it but the PC clone driver is able to do the trick. Inspect the circuit boards of both units closely along the foil(s) leading from the affected signal pin, looking for a solder splash or other possible partial short in the printer, and perhaps for a cold solder joint in the Atari. Wiggle the Atari's driver IC in its socket. Etc. etc. If this doesn't lead to a solution, it might be Category 1. Inspect the connectors on both units and both ends of the cable closely. I hope these thoughts lead you to the solution. If not, keep telling yourself what an INTERESTING problem it is! Good luck! -Johh Sangster / jhs@mitre-bedford.arpa