Possible Checksum Calculation

[Chet T16]

Hi guys,

I'm going nuts here trying to figure out how the last byte in a packet is being calculated, the final column in the below hex value table.

I have the rest of the values figured out.

A4	AA	C0	01	E8	00	FF	FF	42	65
A4	AB	C0	01	E8	00	FF	FF	42	64
A4	AC	C0	01	E8	00	FF	FF	41	60
A4	B7	C0	01	E8	00	FF	FF	42	78
A4	B8	C0	01	E8	00	FF	FF	45	70
A4	B9	C0	01	E8	00	FF	FF	45	71
A4	BA	C0	01	E8	04	FF	FF	4A	79
A4	BB	C0	01	E8	02	FF	FF	48	7C
A4	BC	C0	01	E8	01	FF	FF	4B	7B
A4	BD	C0	01	E8	02	FF	FF	4C	7E
A4	BE	C0	01	E8	04	FF	FF	4E	79
A4	BF	C0	01	E8	04	FF	FF	46	70
A4	C0	C0	01	E8	04	FF	FF	89	C0
A4	C1	C0	01	E8	04	FF	FF	89	C1
A4	C2	C0	01	E8	02	FF	FF	48	05
A4	C3	C0	01	E8	02	FF	FF	87	CB
A4	C4	C0	01	E8	02	FF	FF	87	CC
A4	C5	C0	01	E8	00	FF	FF	42	0A
A4	C6	C0	01	E8	00	FF	FF	82	C9
A4	C7	C3	01	E8	00	FF	FF	82	CB
A4	C8	C0	01	E8	00	FF	FF	82	C7
A4	C9	C0	01	E8	00	FF	FF	82	C6
A4	CA	C0	01	E8	00	FF	FF	42	05
A4	CB	C0	01	E8	00	FF	FF	82	C4
A4	CC	C0	01	E8	00	FF	FF	82	C3
A4	CD	C0	01	E8	00	FF	FF	41	01
A4	CE	C0	01	E8	00	FF	FF	45	06
A4	67	C0	01	E8	02	FF	FF	4C	A4
A4	67	C0	01	E8	00	FF	FF	41	BC
A4	68	C0	01	E8	00	FF	FF	41	A4

Byte two is a sequential number and when all the others stay the same the "checksum" generally decreases by 1, which may tell me something! I've been trying all sorts of things like addition and bit flipping but not getting anywhere.

Pointers very much appreciated.

[HKJ]

Have you tried XOR

[stmdude]

Appears to be 8-bit XOR.

Try this tool: https://www.scadacore.com/tools/programming-calculators/online-checksum-calculator/

[WillHuang]

The checksum is calculated as follows:

Checksum = 0xFF

For each byte in packet excluding the last byte:
Checksum ^= byte

Finally, checksum ^= 0xFF.

This should yield the last byte in the column

[Peabody]

It's two's complement.  Add the bytes, XOR the least significant byte of the sum with 0xFF, then add 1.

[hlavac]

it seems to be simple xor of the bytes. if you xor them all you get 00.
to calculate, xor all the bytes except the checksum and put the result in checksum

[Chet T16]

You guys are good, I knew someone would know..just not what seems to be everyone except me 

It is indeed an XOR, I ran some code to check incoming packets and they all matched.

Thanks!