Calculating CRC for Modbus RTU

[tekart]

Hi, I'm working on a SCADA industrial control system that communicates via Modbus RTU. I'm using a 16F648A with a MAX232 serial driver. The hardware all works fine, but I can't figure out how to calculate the CRC. Apparently Modbus has a specific CRC format, and I found a routine in another forum that claimed to work in PBpro but it isn't.

Here's what I'm using:

CRC=$FFFF ' initialize value
For X = 0 to 11 ' number of bytes to process
CRC = CRC ^ dataCRC[X] ' read through data array
For Z = 1 to 8
IF CRC.Bit0 = 1 Then ' check bits
CRC = $A001 ^ (CRC >> 1)
Else
CRC = CRC >> 1
EndIF
Next Z
Next X
Return

The host is sending:
Tx:0F 03 00 00 00 04 45 27
and my code is returning:
Rx:0F 03 08 13 24 13 88 13 5B 00 01 38 6B
- the last 2 bytes should be the correct CRC but my host simulator is rejecting it.

FYI, the software Modbus host simulator I'm running is Modbus Poll from http://www.modbustools.com/
Nice tool!

Anyone been down this road before?

[Kamikaze47]

do you have the bytes in the correct order?

if not, try changing it to: FOR X=11 TO 0 STEP -1

[Darrel Taylor]

There are only 11 bytes in the packet.
Change it to ...

For X = 0 to 10

Then you should get $0FD2 for the CRC.

[tekart]

I changed the loop to 10 and got a CRC of 0F D2,
but the Modbus Poll simulator still shows a CRC error.

I am processing the bytes in the CRC calculation code in the same order that I am transmitting them - seem reasonable to me.

[Darrel Taylor]

According to this page ...
http://www.lammertbies.nl/comm/info/...lculation.html

0FD2 is the correct CRC for CRC-16 (Modbus) using [0F 03 08 13 24 13 88 13 5B 00 01].
enterd as hex.

Could the simulator be using a different CRC polynomial?

[Darrel Taylor]

Also,

If I enter [0F 03 00 00 00 04] as sent from the host above, it shows 0x2745 for the CRC, which is the same thing the host sent.

So that has to be the right algorithm.

[tekart]

FYI - here's a screen shot of my Modbus Poll simulator screen and setup:



Hope the image shows OK

[tekart]

Also,

If I enter [0F 03 00 00 00 04] as sent from the host above, it shows 0x2745 for the CRC, which is the same thing the host sent.

So that has to be the right algorithm.

So maybe it's my data formatting - or something???

[tekart]

Also,

If I enter [0F 03 00 00 00 04] as sent from the host above, it shows 0x2745 for the CRC, which is the same thing the host sent.

So that has to be the right algorithm.

Hey Darrel,
You gave me the final clue - it was a byte order issue I needed to reply with the bytes reversed: D2 0F. Your result above clued me in!

Thanks much - see screen shot showing no errors.

Guy

[Darrel Taylor]

He He,

You gotta watch out for those "Little Endians" ...
They'll shoot you in the butt with an arrow if your not looking.

Glad I could help,