101501-565 Rev C
Page 36 of 58
Dim CSb3 As Integer
Dim CSb$
Dim X
X = 0
For i = 1 To (Len(outputString)) 'Starting with the CMD character
X = X + Asc(Mid(outputString, i, 1)) 'adds ascii values together
Next i
CSb1 = 256 - X
CSb2 = 127 And (CSb1) 'Twos Complement
CSb3 = 64 Or (CSb2) 'OR 0x40
CSb$ = Chr(Val("&H" & (Hex(CSb3))))
ProcessOutputString = Chr(2) & outputString & CSb$ & Chr(3)
End Function
Here is an example of an actual Checksum calculation for command 10
(Program kV setpoint)
The original message with a placeholder for the checksum is
<STX>10,4095,<CSUM><ETX>
First, you add up all the characters starting with the ‘1’ in the command
number, to the comma before the checksum with their ASCII values (in
hexadecimal):
0x31 + 0x30 + 0x2C + 0x34 + 0x30 + 0x39 + 0x35 + 0x2C = 0x18B
Next, you then take the two’s complement of that number by negating it,
by subtracting it from 0x100 (decimal 256), and only retain the lowest 7
bits by bitwise ANDing the results with 0x7F. :
NOTE: This combines the steps of getting the twos complement,
truncating the result to 8 bits and clearing the 8
th
bit.
(0x100 – 0x18B) & 0x7F = 0x75
Finally, bitwise OR the result with 0x40:
0x75 | 0x40 = 0x75
The checksum byte is 0x75 (Decimal 117, ASCII: u)
