Thanks for responding. Yes, I have an AVR-988 and its serial documentation is very similar to what you linked to. I don't think I have seen any receivers that differ much from that, but with the addition of HEOS, I'm a little concerned.

I'm controlling my AVR-988 via IR, but I do happen to know that they have made changes to that protocol as I recently needed to program a controller for a Denon X3700H in a museum installation that replaced an earlier model. There were no open RS232 ports available so I needed to stay with IR. The older model matched the IR of my Denon. The new model did not. So that becomes part of my concern that they have made some overall changes.

I suspect the lack of documentation comes with the fact that we are dealing with the consumer division. My experience has been primarily with Denon Pro.