(A note to Chris Foley regarding the circuit he proposed on 10/10/04.)
I now have the circuit breadboarded and working pretty well. Since we last talked I added another pair of 555 ICs between the 4017 and the optoisolators so that I can control the output pulse width. This makes my tester a little more versatile. I got that working with a little help from other people in the group (thanks, guys!).
As you might have noticed in related threads I started, I also had to switch CLK and CLKEN from the way you had shown it, and ground RESET. Now it works, and I can adjust both speed and pulse width.
The remaining question is regarding output pulse shape. If I connect a 10k resistor between +12 and the optoisolator collector, and ground the emitter, I can see the optoisolator switch from open to conducting. When it's open the emitter is at +12 and when it conducts this voltage drops to 0 almost instantaneously. However, when it opens again the rise back up to +12 is an exponential, taking about 2-3 milliseconds. Is this to be expected, or am I overlooking something.
Ed