You mean a way to calc a checksum for four bytes?

Here's an easy one, start with the hex value 3F, and XOR it with all four bytes on the way out:

Checksum = 3F

Checksum = Checksum ^ byte_1
Checksum = Checksum ^ byte_2
Checksum = Checksum ^ byte_3
Checksum = Checksum ^ byte_4

Send out via radio byte_1, byte_2, byte_3, Byte_4, Checksum.


Now on the way in the receive device:


Receive byte_1, byte_2, byte_3, Byte_4, Checksum.

Checksum_Check = 3F
Checksum_Check = Checksum_Check ^ byte_1
Checksum_Check = Checksum_Check ^ byte_2
Checksum_Check = Checksum_Check ^ byte_3
Checksum_Check = Checksum_Check ^ byte_4

IF Checksum_Check = Checksum THEN
Data packet was valid
ENDIF


In your program you'd use an array for the data bytes and wrap that up in a FOR...NEXT loop
So to send:

Checksum = 3F
FOR index = 0 TO 4
send byte[index]
Checksum = Checksum ^ byte[index]
NEXT index
send Checksum


Cheers, Art.