ModbusTCP with Strings

[David_Stone]

Doing the same as this thread here:

Automation Direct ViewMarQ

Reviving an old thread here in hopes someone can help. We finally are trying a Viewmarq with Ignition via Modbus TPC and I’m having a hard time with the addressing and setup. I have an established connection on the gateway and set up a message on the Viewmarq; our first step was to try and write a variable but this isn’t working. We then went back and tried the example above (adopted for our device name) and still no joy. I’d really love to get this working. Tech support at both Inductive and A…

I need to send the following string:
<ID 0><CLR><WIN 0 0 287 31><POS 0 0><LJ><BL N><CS 0><GRN><T>Testing</T>
This needs to be terminated by a carriage return $0D in ascii.

Msg = '<ID 0><CLR><WIN 0 0 287 31><POS 0 0><LJ><BL N><CS 0><GRN><T>Testing2</T>' + chr(13) + chr(13)
length = len(Msg)
system.opc.writeValues('Ignition OPC UA Server',['[Sign]1.HRS11000:'+str(length)],[Msg])

ModbuCapture.pcapng (7.4 KB)

In the Wireshark capture, the characters are reverse byte order, and "Testing2" string does not work, but "Testing" does work. "Testing" is sent with the manufacturers tool running on the same computer as the gateway. "Testing2" is sent with the code above.

Does anyone have any great ideas about what is different in these packets? I see the CC 0D terminating string, but Ignition will not send the packet if I put in a chr(204) + chr(13) as the end of the string.

[pturmel]

@David_Stone

the characters are reverse byte order

There's setting for that.

@David_Stone

['[Sign]1.HRS11000:'+str(length)]

Just use a constant length. Ignition will null-pad.

@David_Stone

will not send the packet if I put in a chr(204) + chr(13) as the end of the string

Characters in jython are not bytes. They are unicode codepoints and will be encoded as bytes. I'm not sure what you need for this, perhaps a byte array.

If you cannot get this work, consider using my driver's system.opc.rawModbus() function.

[Kevin.Herron]

@pturmel

@David_Stone

will not send the packet if I put in a chr(204) + chr(13) as the end of the string

Characters in jython are not bytes. They are unicode codepoints and will be encoded as bytes. I'm not sure what you need for this, perhaps a byte array.

Yeah this bit has me curious. Wondering if there's some unicode roundtrip stuff going on, since this is a String he's writing to after all.