please see embedded comments below: Dick -----Original Message----- From: Dwight Elvey <elvey(a)hal.com> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Wednesday, June 02, 1999 5:18 PM Subject: Re[2]: More Bringing up a CPM I can assure you that you won't run into much CMOS circuitry in the old IMSAI and Altair stuff. You're right, of course, if your instrument can't be programmed to handle different thresholds then you could be persuaded you see something you really don't, or perhaps be swindled out of seeing what's really there. I remember looking at some new CMOS circuitry about 15 years ago when the high-speed CMOS stuff was still new. It appeared, for all the world, as though the outputs preceded the inputs of a series of gates by quite a difference. I had set my 2467 display to the lower threshold of a cmos schmidt trigger and displayed a couple of other signals with respect to that, including the clock which triggered a sequence. It appeared that the 'Q' of a flipflop preceded the clock by quite a margin, perhaps 3-4 ns. We quickly concluded that a sufficiently long chain would easily allow us to read tomorrow's stock quotations today, thereby making us all rich. - - - it didn't work that way! The way I get around the random sampling error in observing a crystal driven microprocessor circuit is that I use a little PLL to multiply the clock on the board by some constant, say 4, 10, 16 or some such, and sample at that rate as opposed to what the LA wants to use. That makes similarly long pulses look similarly long as opposed to suffering from the temporal distortion introduced by the asynchronism between the LA sampling clock and the circuit on the board under observation. Of course, like a 'scope or signal generator, you have to know how to drive them, just as you must know how to interpret the results. The LA often points up where to hook the 'scope. If it cuts my search through 60-70 signals down to half a dozen, it's made itself worthwhile. I find the LA really handy for MCU-driven cirtuitry, since I can use the LA to take a picture and then spiff it up and print it in the O&M manual. What's more, with easily simulated FPLD's, it's really handy to compare the simulation with what the LA sees. It's often quite heartening to see what you thought you ought to see after working on one of those babies for a couple of weeks. In any case, I've troubleshot dozens of DRAM circuits with either the little front-end sampling mux or a real LA, and have found them to be invaluable in finding component failures, incorrenct jumpering, wrong delay lines, etc. What's more, if I didn't make the mistake, I can prove it. If I did, I can prove I fixed it. That's worth quite a bit of setup. As far as frequency is concerned, the only problem I've encountered which doesn't quickly show up on a LA display is metastability. As fast as things run these days, and with the widespread use of fully synchronous circuitry, almost everything goes into a device through a two-bit shift register stage to mitigate metastability. I find that if I sample at twice or quadruple the clock which drives the pipeline registers, I can catch everything I need. Those old circuits like the IMSAI and ALTAIR CPU circuits relied quite a bit on circuits' propagation delays to build events. Races were common, and that's why folks used to tack 20-400 pf caps all over those boards in hopes of making them work better. Like I wrote before, setup and interpretation is slow, painstaking, and laborious. It's often wrong the first time. Unless you have half-a-dozen similar boards to work on, it's hardly worth the trouble.