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neil-swart avatar image
neil-swart asked

Multiplus ii GX Modbus TCP Illegal Function ERROR

Hi

I have a Multiplus ii GX, and have been using the Modbus TCP function for a few months now without any issues. However a little over a week ago my read requests have been failing and I get a

Illegal Function ERROR. Now I know my unit ids and registers have stayed the same on the request side because I didn't update any code, and the Multiplus was not set to do auto updates. it's on v2.60.

Any ideas on why this would just stop working?

Thanks

Multiplus-IIModbus TCP
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wkirby avatar image wkirby ♦♦ commented ·
Can you tell us which requests are failing? Unit ID and register?
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neil-swart avatar image neil-swart wkirby ♦♦ commented ·
All of them to be honest, but one example is ID 100 Register 817
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1 Answer
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Mike Dorsett answered ·

Illegal function Error on Modbus is usually caused by request function 3 instead of 4 or trying to write with the wrong function code. It may be possible that an update to the Modbus server has removed some flexibility - some servers give identical responses to read functions 3 & 4, some will give a function error as technically these sets of registers should be in different address spaces.

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