Despite looking at the wiring diagrams at least a dozen times before I made the wire, and at least a dozen more times after it didn't work, I had the transmit wires of the two devices connected together as well as the receive wires of the two devices.

It's apparently a case of reading it wrong the first time, then seeing what I expect to see on the page rather than what is actually there every other time. As a programmer who has been bitten by this too many times to count, you'd think I'd be immune to it by now. Fortunately, it was a quickj find with the oscilloscope. When I hooked it up, I could see the signal being sent on the wires from the HDS I expected to be listening for a signal, then was stunned when I went back to that diagram yet another time and realized how stupid I had been.