robtillaart:
have a look at this (not tested)
That looks great, with only one caveat: the protocol seems to expect the checksum to be sent in the form of an one-byte value rather than as a decimal ascii string. That seems pretty strange since the checksum is not guaranteed to be a printable character, but that's how it was described. So I think the checksum calculation needs to return a byte (not an int) and the output code needs to do a serial write (not a print).