Sshqgl[ $ 5hprwh $fwlydwlrq, Calculating the crc – Grass Valley XSwitch Feb 01 2005 User Manual

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Calculating the CRC

Given: The XSWITCH information for the fields:

and the basic program to calculate the CRC:

BASIC CRC PROGRAM VER. 2.0 USING VISUAL BASIC 3.0

Static P(200) As Integer

ICRC = 0

POLY = &H8408

Dstring$ = Input_Text$

‘ This is the input string.

D = 0

i = 1

P(0) = 0

For J = 1 To Len(Dstring$)

c$ = Mid$(Dstring$, J, 1)

Casc = Asc(c$)

‘ Finds decimal value contained in character’s ASCII value.

D = D * 16 + Casc

‘ Hex value calculated.

P(0) = P(0) + 1

P(i) = D

i = i + 1

D = 0

Next J

‘ After calculating input string’s characters to hex value,

CRC = ICRC

Calculate the CRC.

For i = 1 To P(0)

D = P(i)

For J = 1 To 8

XFLG = (CRC Xor D) And 1

CRC = Int(CRC / 2) And &H7FFF

If XFLG Then CRC = CRC Xor POLY

D = Int(D / 2)

Next J

Next i

....................... CrCHexValue$ = Hex$(CRC)

‘ This is ENTIRE CRC string.

dummylength = Len(CrCHexValue$)

LSB = Val(“&H” + Right$(CrCHexValue$, 2)) ‘ Convert hex value to decimal value.

MSB = 0

If dummylength = 3 Then MSB = Val(“&H” + Left$(CrCHexValue$, 1))

If dummylength > 3 Then MSB = Val(“&H” + Mid$(CrCHexValue$, dummylength - 3, 2))

Packet$ = Dstring$ + Chr$(LSB) + Chr$(MSB) ‘ This is the packet string.

End

Where the input string is made up of single characters to represent the field’s decimal value in the character’s own
ASCII value AND the actual command string for the XSWITCH.

DEST

SRC

LEN

C/C

DATA1

…

DATAn

CRCL

CRCH