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,

[richard]

would it not be easier to

if Payload[10] -( Payload[8] + Payload[9]) > 0 then goto badckecksum

or conversely

Payload[10] =Payload[8] + Payload[9] to create chksum

[Zebryk]

Right. That's simple.
Problem is when they add up greater than 256 becoming a WORD.

[richard]

it can't become a word if the var is declare as a byte it just rolls over .
Payload var byte[11]

btw the transmitter its not a fineoffset weatherstation ?

[Zebryk]

It can't be that simple?
What if there were more than 2 Bytes like 10?

No it's not a weatherstation, more like a entry door counter.

[richard]

a byte can hold a value between 0 and 255
255 + 1=0
255+10=9
255+255+255+1= 254

[Zebryk]

Richard,

Wow!
That is so clean and easy.
(I do have a tendency of making things complicated.)

Thank you very much!

Jay Zebryk
Southbridge, Massachusetts