Thanks for the great help, Andrew. I made the changes you suggested, ran i2cdetect, and still got nothing.
It's time for a confession. As I did the first "smoke test" and powered up the RasPi with the two boards, I wasn't sure how the software worked, but rambled through your AB Electronics website, blundered into i2cdetect, and tried it with no results. I had no idea if i2cdetect told me anything useful, so continued pursuing other software.
I then started checking my wiring, and found that the voltage was 1.5 volts instead of the expected 5 volts. I suspected that the power supply socket I had soldered to one of the boards was installed wrong, removed it, and wired it to the far end of my power bus. This again gave the wrong voltage. I started working my way down the power bus, and discovered to my horror that the wires to the IO Pi Plus boards were reversed. I swapped them and finally got the correct 5 volt power on the power bus. I crossed my fingers, hoping that there was no lasting damage, and started chasing the software angle through this series of posts.
I suspect that I have fried both boards, Am I correct? In that case, I'll need to buy two more boards. I have re-wired the power bus with color-coded wire. (The boards aren't repairable, right?) I worry that I might also have damaged all the other boards, but I can't tell until I get the IO Pi Plus boards running.
So, I've learned a couple of things: (1) use my new low-power soldering gun, and be more careful, especially on the fragile address pads (2) Color-code not just the power bus wiring but all wiring, and double check it with a voltmeter berore connecting it to any of the boards, especially the IO Pi Plus boards (3) don't mount the external power supply socket on either IO Pi Plus board (4) get the software running and check out the IO Pi Plus boards before wiring up the other boards. (5) wire up the boards one-at-a-time, checking as you go.
Thanks for sticking with me through this series of blunders. (Spoiler alert: it's not over yet!)
Computer geek since 1964. Upgraded to FORTRAN66 back in the day. Downhill ever since.