MODBUS Commands With a CRC checksum - Sample Script: ModbusCRC.pts

MODBUS commands with a CRC checksum - Sample Script: ModbusCrcScript.pts

The Docklight script ModbusCrcScript.pts and the accompanying project file ModbusCrcTest.ptp demonstrate how to use a Docklight script to automatically calculate the CRC value required to send a valid MODBUS RTU frame.

The project file uses the communication settings listed below, according to the MODBUS implementation class "Basic".

Communication Mode	Send/Receive
Send/Receive on comm. channel	COM1
COM Port Settings	9600 Baud, Even parity, 8 Data Bits, 1 Stop Bit

Getting started

•	Open the project file ModbusCrcTest.ptp (menu Open Project ...) and the script file ModbusCrcScript.pts (menu Open Script ...). The files are located in the ScriptSamples folder, e.g. at C:\Program Files\FuH\Docklight Scripting V1.9\ScriptSamples.

•	Connect the PC's COM port to your MODBUS network. Open the Project Settings... dialog and make sure you have selected the correct COM Port for Send/Receive on comm. channel.

•	Press the Run Script button in the toolbar.

•	Use the Send button to read input register no. 1 from a slave.

•	Enter a slave number in the Send Sequence Parameter dialog, e.g. "01" for addressing slave no. 1.

Docklight now executes the DL_OnSend() procedure which calculates the actual CRC value and overwrites the last two Send Sequence bytes with the CRC low and high bytes.

After sending "Read Input Register" commands to slaves 1 - 4, the communication window could look like this:

8/29/2008 18:45:23.193 [TX] - 01 04 00 00 00 01 31 CA

8/29/2008 18:45:23.342 [RX] - 01 04 02 FF FF B8 80

8/29/2008 18:45:33.145 [TX] - 02 04 00 00 00 01 31 F9

8/29/2008 18:45:33.292 [RX] - 02 04 02 27 10 E7 0C

8/29/2008 18:45:43.237 [TX] - 03 04 00 00 00 01 30 28

8/29/2008 18:45:43.392 [RX] - 03 04 02 00 00 C0 F0

8/29/2008 18:45:58.724 [TX] - 04 04 00 00 00 01 31 9F

8/29/2008 18:45:58.870 [RX] - 04 04 02 04 00 77 F0

The [RX] channel shows the responses from the MODBUS slaves:

slave 1 responded value "-1",

slave 2 responded "10000",

slave 3 responded "0" and

slave 4 responded "1024".

NOTE: If you are using the Docklight MODBUS example on a RS485 bus, you need to check if your RS485 hardware correctly switches between transmit and receive state. You might need to use Docklight's RS485 Transceiver Control feature.

Further Information

•	See Generating Checksums for Send Sequences for more general information on implementing checksum calculations.

•	The CRC calculation is made according to the specifications for MODBUS serial line transmission (RTU mode). Docklight's checksum function supports a "CRC-MODBUS" model for this purpose. See the documentation for DL.CalcChecksum for details.

•	If you do not have any MODBUS slave devices available, you can use a software simulator. See http://www.modbus.org, "Modbus Resources", "MODBUS Serial RTU Simulator". This simulator was used to produce the sample data shown above.