Hi Everyone,

I developed a Modbus RTU slave in C (windows environment). Is the CRC format [Lobyte] [Hibyte] both in request and answer frame? For calculating the CRC I used the function that is reported on the standard document (Modbus_over_serial_line_V1_02) and the function should return the CRC in the format [Lo][Hi], but when I try the application with a Modbus master simulator (e.g SimplyModbus 6.3.6 or Modscan32) the CRC's that I receive are always swapped comparing to the one calculated from the function. Now, according to the standard, is the function or the simulator wrong! Thanks everyone!

Marcello

 

The CRC is always sent little endian, low byte first, high byte last.
There is an excel calculator available at
www.simplymodbus.ca/crc.xls

 

Hello,

> Is the CRC format [Lobyte] [Hibyte] both in request and answer frame?

Yes.

Right the CRC is calculated. When transmitted it is lo byte then hi byte.

So, the calculated and the transmitted are backwards.

As a slave just include the two bytes, in the order received and the CRC, if good, will be 0.

>...and the function should return the CRC in the format [Lo][Hi]

return (uchCRCHi << 8 | uchCRCLo) ;

I am confused.

Good luck,
Mark
http://www.peakhmi.com/

 

Thank you for the answer.

>>...and the function should return the CRC in the format [Lo][Hi] return (uchCRCHi << 8 | uchCRCLo) ;

So when I calculate the CRC with the function I have to swap the byte before sending it back to the master?

Thanks
Marcello

 

Hello,

The proof of the pudding is in the eating.

Does it work when you swap the bytes?

Good luck,

Mark
http://www.peakhmi.com/

 

Hello Mark,

yes, it work on both simulator when I swap the byte; on the contrary when I don't swap the byte it doesn't work.

Thanks.
Marcello

 

Here is how we calculate the CRC.
We do not need to swap because there are 2 arrays.
array_crc_low[] and array_crc_high[]

http://pvbrowser.org/pvbrowser/sf/m...Modbus.html#af22f8da932f8c51dc584596f2e7e4d47

http://pvbrowser.org/pvbrowser/sf/manual/rllib/html/classrlModbus.html