Thanks that was very helpful. I'll double check the A/D lines and all other pins on the 80C85 but as I recall they were all toggling nicely in the 0-5V range. I'm having to relearn how 8080'ish systems work. It is one of the first processors I learned on (well Z80 on a TS 1000) and then I got into the 6502 and really went to town.
I'll also try to determine if the AD buffers / mux are functioning properly. Unless I misread the schematic M1, M2 and M21 are the primary culprits. I also just noticed that the AL/E line runs though a buffer so I need to check that signal on both sides of the buffer as well. Thanks again, Jeff From: M100 <[email protected]> On Behalf Of Fugu ME100 Sent: Tuesday, May 1, 2018 5:16 AM To: [email protected] Subject: Re: [M100] SPAM-LOW: Re: New member - question on 'half' alive Model 100 On boot up one of the first things the M100 does is to configure the PIO (81C55) in order to scan the keyboard for the ctrl-break. The PIO and LCD all use port addresses, there should be a lot of activity on the IO/M pin on a normal boot! Indeed within about 10 instructions, for a normal boot, the IO/M pin should be toggling quite a lot :) However if the address or data lines are failing then it might just execute code that does not include any IO/M based operations. For example, if the mux is broken then the reset vector might never be loaded so it could just execute random code. I tend to feel the 80C85 is probably OK (the fact the IO/M line toggles on reset is a good sign), as is the ROM. Most likely there is a buffer issue either a failed line, very weak drive or even a simple dry joint. The parts are quite old and some might just fail due to age. If you can I would check the address and data lines are all being driven with full voltage swings and none are stuck or just hovering around. This is kind of hard to do easily but might save you the headache of replacing a 40pin CPU package. You might want to check the +5V is OK on all the chips just in case the leaking caps did etch through a power copper trace.
