Unless AprilAire made a recent change to their protocol the AprilAire 8800 Thermostats are still compatible with the ELK M1 System. If you would like to send a copy of the data captured during the Putty Terminal session we will be glad to review. Send to techsupport@elkproducts.com . If you’d like to send us one of your M1XSP’s and one AprilAire 8800 Thermostat we will be glad to test as well.