The XTB-XM appears to work fine with a new $6 USB to serial port converter:
https://www.ebay.com/itm/182297261602?ssThe smartphone "tap" also seems to be more responsive. Whenever the screen responds to the tap, the XTB-XM toggles the appliance module. When the screen does not respond to the tap, no command is issued to the XTB-XM, so the communication link between the two seems to be working fine.
Converting the XTB-232 to the XTB-XM requires a firmware swap (new PIC microcontroller), changing 3 resistors, and possibly 2 opto-isolators to accommodate the baud rate increasing from 4800 to 19,200.
New XTB-XM's will be custom assembled for $10 more than the standard XTB-232.
Now there does appear to be a problem with my cellphone communicating with the G2. I was able to add one X10 appliance module a week ago, which is what I have been using for testing. Today while I can control that X10 module just fine, the app says there is no X10 interface when I try to add another X10 module. That is confusing because it is sending X10 commands through the XTB-XM.
Jeff