Appendix C – TAP Checksum Calculation - 15
Appendix C – TAP Checksum Calculation
Following is sample Visual Basic™ code that creates a string to be sent to the paging system. A subset of the string
has a checksum calculation performed on it, and then the checksum and a control character are appended to form the
string to be sent to the paging system. Refer to the TAP specification at
http://www.pcia.com
, for more details on
the checksum calculation.
'String that checksum is calculated on, of the form:
‘<STX>ID<CR>Message<CR><ETX>
TXString$ = Chr(2) & txtID.Text & Chr(13) & txtMessage.Text & Chr(13) & Chr(3)
'Calculate and append checksum
j = 1
Sum = 0
Do Until j > Len(TXString$)
B$ = Mid(TXString$, j, 1)
D = Asc(B$)
Sum = Sum + D
j = j + 1
Loop
'Create the three characters to be transmitted to represent this checksum.
d3 = 48 + Sum - Int(Sum / 16) * 16
Sum = Int(Sum / 16)
d2 = 48 + Sum - Int(Sum / 16) * 16
Sum = Int(Sum / 16)
d1 = 48 + Sum - Int(Sum / 16) * 16
Check1$ = Chr$(d1)
Check2$ = Chr$(d2)
Check3$ = Chr$(d3)
CheckSum$ = Check1$ & Check2$ & Check3$
'Create complete string to be sent to paging system, of the form:
‘<STX>ID<CR>Message<CR><ETX>Checksum<CR>
TXString$ = TXString$ & CheckSum$ & Chr(13)
