Think this might work with my elk?

I assume you want to talk to a Honeywell Enviracom system? Otherwise I don't know what you would do with it.

But I am afraid the checksum calculation is beyond the capabilities of the M1.
HVAC control is generally handled with a dedicated software program in the ELK M1XSP serial port expander. We are working on a new interface to the Carrier Infinity Series System, but I don't think the Honeywell has made the list.
I was wanting humidifier control and dual fuel control and home automation, I couldn't tell by specs of the common m1xsp compatible tstats if I could do every thing the Honeywell Thermostat could do.