CompuServe Messages

Lu, with electronics, things are never simple. Given that any 8520 consists of two seperate circuits sharing some signals, such as the clock, you have the possibility of having only one-half or the totality of a problem. Actually, it is even worse than that. Since some signals on a particular half of an 8520 are split between devices, a single input or output can change the output only with respect to that device. It is only when you swap the 8520's that you have a chance of catching the particular input or output that was affected. Basically, from an electronics technician's viewpoint, the 8520's are a troubleshooting nightmare. If I hadn't spent several years tracking similiar problems in the remoting gear for the Precision Approach Radar, I wouldn't even want to address this one. Too many signals shared too many ways. The best we can do is to swap the 8520's and see if the problem moves. If it does, then test to see where the problem moved to. Usually that will tell you, within a pin or two, where the problem is. Not exactly useful, since you will have to replace the 8520 anyway, but it does tell us that the problem was within the 8520 in the first place. Ummm, one more item. I've seen quite a few cases of half-dead circuits on IC's. Given a certain set of inputs, the output would be screwed up, but if you gave the chip any other set of inputs, it worked fine. Usually this is a case of true IC failure. Basically, the chip is easily confused as to what it is supposed to do. This only happens about 10% of the time. Most failures are on the input or output side, at the chip lead level, thankfully. With electronics, dead or alive is not enough. You have to ask, exactly WHAT is dead or alive. Life was much simpler in the DTL and TTL days <sigh>. "You do trust me, don't you? Of course you do." — "To Play the King" (BBC)