I have regarding VMB4RY Protocol - edition 1 (available for download in 4 CHANNEL RELAY MODULE page info vellemanprojects.com/be/en/product/view/?id=363140 ) .
When I compare these specification with your demo project I see that High priority and Low priority bytes have different values in both references.
Demo: High: 0xF8 Low: 0xFB
Protocol document: High: 0x00 Low: 0x11
This make me some confusion, likewise STX and ETX bytes.
If possible I would like that you explains me how it’s works.