Hi,

I am writing a slave application, which reads the Master's request (03H function), and passes a 4 Byte float followed by 2 Byte CRC.

so for instance - when the master sends me this<pre>
01 03 00 6D 00 02 55 D6

My Slave Replies with
01 03 04 42 c8 00 00 B5 6F (sending the value 100.0)</pre>
But my master polling software gives me CRC error. But I basically use the same algorithm to calculate CRC as i have used when I was sending 2 Byte integer values followed by the checksum.

I am rather new to all these things, and I am sure I am missing something. So please comment and share your thoughts as to why I keep getting this error.

BTW, I am using Modbus Poll as my master. Also I mapped the 32Bit float value across two holding registers. Not sure if that's the right thing to do.

Anyways, thanks for reading. I appreciate any help I can get.

 

Hello,

We have a free MODBUS slave simulator on our website. It can display the bytes received/sent. Use it to view the request from the master, the response from the slave and it might help you solve the issue.

http://www.hmisys.com/downloads/PeakHMISlaveSimulatorInstall.exe

Good luck,

Mark
http://www.peakhmi.com/

 

1) >CRC
The Modbus over serial line V1.02 spec says the RTU frame has the CRC low byte first, then the high byte. (page 13)

Lammertbies calculates B56F for 01030442c80000 in CRC-16 too.
http://www.lammertbies.nl/comm/info/crc-calculation.html

For B56F, which is low, which is high?

2) > I mapped the 32Bit float value across two holding registers.

Yes, that's the way it's done.

 

Your CRC calculation is correct, but you just need to swap the order of the two bytes. The CRC for your data is 6F B5.