FotS

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
===Modbus CRC uses the bit by bit method===

===Introduction:===
This CRC (Cyclic Redundancy Check) is for the Modbus RTU protocol as used in RS232 and RS485 communications. The CRC is a 16 bit word that is calculated from all of the data in a message. It cannot correct any data corruption but it will indicate if a message has been corrupted in some way. This program uses the bit-by-bit method to calculate the CRC.

{{tag>Modbus CRC}}
===PLEASE NOTE:===
Although this is a Modbus CRC it is in no way restricted to be only used for the Modbus protocol. Any message structure consisting of a stream of 8-bit data bytes can utilise it. For example:
Msg$ = “The quick brown fox jumps over the lazy dog”

===Acknowledgements:===
This function was written by two members of The Back Shed forum:

**TassyJim** wrote the MMBasic version of the function.
**Phil23** converted it to a string function to make it more user friendly.

I made a few tweaks to improve speed.
Bill McKinley

An example a Modbus message (in Hex) could look like:
<code>
Slave Address             18
Function Code             03
Start Address (Hi)        0B
Start Address (Lo)        B9
Number of points (Hi)     00
Number of points (Lo)     01
CRC (Lo)                  55
CRC (Hi)                  C2
</code>
Note that the High/Low order of bytes in the CRC is the reverse of that used for the data. I don’t know why.

In MMBasic without the CRC, that message would look like:

<code>TxMsg$ = CHR$(&H18)+CHR$(&H03)+CHR$(&H0B)+CHR$(&HB9)+CHR$(&H00)+CHR$(&H01)</code>

Add the CRC to the message by:

<code>TxMsg$ = TxMsg$ + CrcStr(TxMsg$)</code>

Performing a CRC check on a message that already includes a valid CRC will produce a 'CRC' of &h000 so:
Check the received message (eg RxMsg) by:

<code>IF CrcStr(RxMsg$) = CHR$(0) + CHR$(0) THEN ' CRC is OK
' Process the response
ELSE
' Bad CRC - do nothing - don't respond
END IF</code>

===Assumptions:===
None

===The Function:===
<code>
FUNCTION CrcStr(a$) AS STRING
  LOCAL ErrorWord% = &HFFFF, n, j, LSB AS INTEGER
  FOR n = 1 TO LEN(A$)
    ByteVal = ASC(MID$(a$, n, 1))
    ErrorWord% = (ErrorWord% AND &HFFFF) XOR ASC(MID$(a$, n, 1))
    FOR j = 1 TO 8
      LSB = ErrorWord% AND &H0001
      IF LSB = 1 THEN ErrorWord% = ErrorWord% - 1
      ErrorWord% = ErrorWord% / 2
      IF LSB = 1 THEN ErrorWord% = ErrorWord% XOR &HA001
    NEXT j
  NEXT n
  CrcStr = CHR$(ErrorWord% AND &HFF) + CHR$(ErrorWord% >> 8)
END FUNCTION
</code>