Has anyone ever made a specific list of 74 series chips that this cannot successfully diagnose? Or does it depend on the circuit in which they're placed (what the inputs / outputs are tied to)? I seem to remember being told once that it can't always successfully diagnose certain logic chips. I don't have enough field experience to know when i use it if the results I'm getting indicate a faulty IC or just an IC it can't reliably diagnose. In fact, so far i don't think it's been of any use to me whatsoever. Obviously i'm ignoring those with more than 16 pins! More specifically, i was looking at some 74LS157's the other day that are part of a 2 to 1 multiplexer prior to some ram ICs. The one that had the WE lines as two of the outputs was showing a fault on both of them, but replacing it made no difference, i thus assumed the comparator was giving a false positive (or negative!). Was this right, or should i really continue to suspect that IC? It's definately in the part of the schematics where my problem lies. Please excuse if it's a bit of a dumb question to some, but i'm merely trying to familiarise myself better with some of the tools that i've picked up over the course of the last few years. Thanks, Martin. If you ever want to (un)subscribe yourself with TTL, you can send mail to: <Majordomo@flippers.com> with the following command in the body of your email message: (un)subscribe techtoolslist or from another email account, besides xxx@yyy.com: (un)subscribe techtoolslist xxx@yyy.com