Despatch Protocol Plus Modbus Communications Manuale Utente Scaricare il pdf (Pagina 52)

Chromalox Instruments and Controls

A-51643 Rev. 6 10/06/03 46

'can be calculated before the TxMessage is sent, when RxMessage contains

'the expected message length, then reception is complete.

'Check for valid message CRC:

RxCount = Len(RxMessage)

CRCValue = GetCRC(Mid(RxMessage, 1, RxCount - 2))

'compare result with received CRC:

If (CLng(Asc(Mid(RxMessage, RxCount - 1, 1))) + _

(CLng(Asc(Mid(RxMessage, RxCount, 1))) * 256)) = CRCValue Then

'CRC result matches OK (process data in RxMessage as needed)

Text1 = "OK "

'display values of message's data bytes:

For i = 1 To RxCount

Text1 = Text1 & "<" & Asc(Mid(RxMessage, i, 1)) & ">"

Next i

Else

'bad CRC result!

End If

End Select

End Sub

With the PC connected to a Protocol

Plus controller on COM1, and the controller is set for Address 1,

Baud Rate 38400, No Parity, clicking the command button produces the following display:

The data shows a good reply, with data values of 93, 113, and 0 for Holding Registers 16, 17, and 18,

respectively.

1 2 ... 47 48 49 50 51 52 53

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Despatch Protocol Plus Modbus Communications Manuale Utente Pagina 52