I think that was the problem. I connected pin 6 of the '138 to +5v and connected pin 19 of the '688 to pin 4 of the '138, and it seems to work now!

Thank you everyone for the help. Next time I build one of these things, I think I'll spend more time error-checking before I build it...

Glad to hear that it is working!

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Yes, I'm thinking that if you had no other source of information, the pulses-per-second on your ROM's chip select, or your IRQ input, or perhaps on A8, will tell you something about where code is executing, whether interrupts are occurring as you expect, or what stack activity is going on - all depending of course on how carefully you've constructed your test code.

Alright, you're not giving this a strong recommendation, so in that sense we agree. I guess what bothers me about frequency counting mode is that its specification may vary drastically from one multimeter to another. Even the dumbest meter will probably respond satisfactorily when used for audio and mains-frequency work. In that realm one can assume the wave being measured cycles above and below zero (ie; no DC offset), is periodic and doesn't have a duty cycle approaching 0 or 100%. But with digital waveforms any of those assumptions could prove false.

Some manufacturers will design their products to perform beyond the lowest common denominator and others won't. To know the difference the user would have to have access to the specs and have studied them to understand what they imply. For an experienced person that's perhaps acceptable but I'd certainly be cautious about suggesting frequency counting mode to a novice.

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

OK, I see your concern - you wonder if a frequency readout from a multimeter will or won't be able to count arbitrary edges at say a 1MHz or 2MHz rate. All I can say is that mine seems to work!

Less controversially, perhaps, I'd also recommend choosing a model with an audible continuity test - it's a lot easier to walk your probes around the circuit keeping an ear out for beeps than to keep looking from circuit to display and back.

Logic probes are inexpensive and in my not-totally-unbiased opinion, are superior to other test devices for the sort of digital troubleshooting hobbyists would be likely to do. That isn't to say that a DVM doesn't have a place. However, its ability to detect a very short duration, high-frequency pulse, say, what one might see on the RWB output of the MPU, which usually lasts no more than 1-1/2 clock cycles, is quite limited. Any logic probe worthy of the name will be able to see such a pulse and produce a positive indication of its presence.

After a logic probe, I would reach for a 'scope if I suspected that signal quality was contributing to whatever problem was being observed. However, when I was trying to figure out why instructions such as STA UART,X were failing in POC V 1.0, I only needed my logic probe to come to a determination. I did observe signals on the 'scope after I had decided I knew what was causing the problem, but only for curiosity's sake.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!