Simple 1 Byte Checksum Method

[Zebryk]

Hello,

I need to encode/decode a simple 8Bit Checksum based on a transmitter which has no documentation.

Apparently, it is just using the last 2 Bytes of a 10 Byte packet to calculate it.
(Figured THAT out by dumb luck.)

Using a Logic Analyzer, it appears it is the simple sum of these two bytes.
Payload[8] + Payload[9] = CheckSum
5 + 2 = 7
245 + 2 = 247
53 + 1 = 54
54 + 1 = 55
21 + 2 = 23
etc, etc,

I have not been able to capture what happens over 255 but assume it must roll-over somehow?

Browsing all the "CheckSum" threads here, I think the code may be something like the following:
----------------------------------
Payload VAR BYTE[11]
(Payload elements...)
CheckWord VAR WORD
CheckSum VAR BYTE


CheckWord = Payload[8] + Payload[9]
CheckSum = CheckWord.byte0 ^ $FF '<--- This?
'CheckSum = CheckWord // 256 '<--- Or That?
-----------------------------------


Any ideas?

Thanks,