John, thank you for your quick reply. When I set Pro Tools to use the HUI protocol, I can see the single-note ping (note C-2, velocity 0) at intervals of 800ms. However, my attempts to echo the ping back to the host using Max are unsuccessful: Pro Tools still complains about not being able to communicate with the HUI.
However, when I set Pro Tools to use the Command 8 protocol, I see a similar single-note ping (note C-2, but here with velocity 127) at intervals of 500ms. When I echo this note back to the host using Max, Pro Tools stops complaining, and everything appears to work smoothly. However, echoing this note back also causes Pro Tools to generate two additional pings every 500ms: one on C#-2 with velocity 76, the other on C#-2 with velocity 12. These new pings puzzle me, as does the fact that got the ping to work with the Command 8 protocol but not the HUI protocol.
Please let me know if you have any more ideas.
Many thanks for your help!
Ben