NI Modbus CRC creation

[Sid1991]

I have used Ni modbus library. I want to send wrong TCP/IP modbus "CRC" to a device from LabVIEW, for testing some functionality of the device.

Could you please guide me?

How can i change the CRC value in the modbus library?

[pincpanter]

There are at least three Modbus libraries available. Only one of them (the old NI Modbus.llb) could be modified by the user, the other are password protected. Which one are you using? (hint: post one vi of yours or a picture of the BD showing the Modbus functions).

[Sid1991]

Is this library support NI Labview 2019?

Could you please share me the library file?

I have attached picture of block diagram.

[RavensFan]

That purple library is supported in LabVIEW 2019, but its subVI's are locked once you get to a certain level, the level you'd need to play with in order to fake another CRC.

Here is a link to a library I've used and is fully open.  You can dig all the way down the subVI hierarchy, find the subVI where the CRC is calculated, make a copy and apply some code to allow you to fake a bad CRC.

https://www.ni.com/example/29756/en/

[pincpanter]

Consider that the interface of the library RavensFan suggested is different from the one you are using now.

[Mark_Yedinak]

If this is for testing purposes only another way to accomplish this would be to capture the data that gets sent to the device and simply use a raw TCP or serial connectiuon to the device and send the raw data. Using that you can manipulate any field within the packet.

[Sid1991]

Okay will check.

thank you

[K C]

Hi,

Normal ModBus/TCP does not use the ModBus CRC. The data is transmitted without the CRC.

If the receiver is a converter from TCP to serial than that device will add the CRC

Kees

[Sid1991]

Yes, Thank you .