Weird Checksum

[Ken R.]

Greetings,

I need to calc a checksum for a block of 16 data points represented as hex
values. Each block of data starts with 21 00 and ends with 03. Each data
point is a 16 bit value with a range of 00 00 - 7f ff (0 - 32,767)
The checksum can not exceed ff. I have tried several crc calcs with no luck.
Any ideas?

I have included a few examples.

STX = [21][00]
Data points 1 - 8
[00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
Data points 9 - 16
[00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
EOT = [03]
ChkSum = [22]

[21][00]
[00][00][7f][ff][00][00][00][00][00][00][00][00][00][00][00][00]
[00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
[03]
[a2]

[21][00]
[00][00][7f][ff][7f][ff][00][00][00][00][00][00][00][00][00][00]
[00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
[03]
[22]

[21][00]
[00][00][7f][ff][00][64][00][00][00][00][00][00][00][00][00][00]
[00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
[03]
[c6]

[21][00]
[00][00][7f][ff][00][66][00][00][00][00][00][00][00][00][00][00]
[00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
[03]
[c4]


Thanks for any help.

Ken

[Paul Marshall]

On 20 Jul 2000 18:22:25 -0700, "Ken R." <Krac@acommsw.com> wrote:

»I need to calc a checksum for a block of 16 data points represented as hex
»values. Each block of data starts with 21 00 and ends with 03. Each data
»point is a 16 bit value with a range of 00 00 - 7f ff (0 - 32,767)
»The checksum can not exceed ff. I have tried several crc calcs with no luck.
»Any ideas?

(snip)

From your examples, it looks like the checksum is obtained by
simply XOR-ing all 35 bytes of a block together.

--
Paul Marshall
pmarshal@vulcraft-al.com

[Ken R.]

Hi Paul,

>From your examples, it looks like the checksum is obtained by
>simply XOR-ing all 35 bytes of a block together.


I tried the XOR but the result needs to be a value between 00 and ff. I tried
a mod 255 on the result. The closest I got has taking the 2 right side characters
from the Hex result.

Ken

[Paul Marshall]

On 24 Jul 2000 17:07:43 -0700, "Ken R." <krac@acommsw.com> wrote:

»>From your examples, it looks like the checksum is obtained by
»>simply XOR-ing all 35 bytes of a block together.
»
»
»I tried the XOR but the result needs to be a value between 00 and ff. I tried
»a mod 255 on the result. The closest I got has taking the 2 right side characters
»from the Hex result.

Hi Ken,

A byte is in the range [00, FF]. XOR-ing two bytes together
results in one byte, still in the range [00, FF]. You should
never get a result over 255 (= FF).

Try this:

==========
Public Function XORByteArray(bt() As Byte) As Byte
Dim btBuf As Byte
Dim lngK As Long

For lngK = LBound(bt) To UBound(bt)
btBuf = btBuf Xor bt(lngK)
Next
XORByteArray = btBuf
End Function

Public Sub test()
Dim bt(0 To 34) As Byte
'[21] [00]
'[00][00][7f][ff][7f][ff][00][00][00][00][00][00][00][00][00][00]
'[00][00][00][00][00][00][00][00][00][00][00][00][00][00][00][00]
'[03]
'[22] <---checksum
bt(0) = &H21
bt(4) = &H7F
bt(5) = &HFF
bt(6) = &H7F
bt(7) = &HFF
bt(34) = &H3
Debug.Print "[" & Hex$(XORByteArray(bt)) & "]"
End Sub
==========

--
Paul Marshall
pmarshal@vulcraft-al.com