See this discussion... https://proforums.harman.com/amx/discussion/138762/matrix-audio-mrc-8
S-series RS232 is attached. I am not confident the MRC keypads will work with the S-series controllers because they have historically been matched to the controller. I began using the product line starting with the MRC series and have no experience with the older S-series. MRC keypads worked with MRC but not MI. MI came at the point of AMX acquisition and MI keyads worked with Tango. I have used MRC, Mi, and my current solution is Tango.
The serial protocol shared by Chris above (for the S series) is carried through all the way to the Tango. We've supported each with the same command set.
Two notes of import: