You are very observant. :) I have tested with request queries with the same results. The protocol is listed in CCP under AUX -> D-Box - > Motion Controller. The code to change to master volume to -15 is
\x02WVOLM\x1E-15\x03 in ASCII. The response for success is
\x03. (Shown in screen cap). To query the current volume (these seem to be missing in the URC DB) is
\x02RVOLM\x03. At -15 the response will be
-15.0\x03 .
When I was testing just query commands, I kept it simple. I created an If/then statement that sent the query, had a delay (tried 1-30 seconds) then would go to one page if it read the expected results (based on the test results in CCP) or an alternate page if it did not see the expected results. Sometimes it worked, sometimes it did not.
When I test these queries in CCP, the Test Results take a while because CCP starts with USB, then sends the Serial Command. Test Results come back quick after the Serial Commands run. I also did a direct connection with a laptop to the D-Box Controller and tested with Hyperterminal. The results were very quick.