LOGO, AM2 and PT100: measured value on PT100 on AI3 is not the expected value

Hoi Forum!

Thanks for your great Q&A, they helped me a lot... up to now. Therefore a question this time.

I have a LOGO! 12/24 RCEo base module and I wanted to connect a PT100  to this base module, but I figured out I needed a AM2 module to connect the PT100. So I bought the AM2 RTD module. Installation went ok, the power is connected, the module is connected to the LOGO! basemodule and the LED turned green. 

Connected the PT100 as explained in earlier posts. When running (program works like a charm in the simulation)  I monitored through the network and I saw a value -48 been measured on my PT100 on AI3, that was not the value I expected (should be around 20). 

The PT100 was on the IA3 (on the AM2) and gave me this reading. The IA4 was not connected to a sensor, but gave the same reading. The value in the AI4 makes sense to me as the resistance is infinite so it gets the lowest possible reading.

I measured the resistance of the PT100, which was 115 Ohm, quite nice I guess. So that was ok.

Even doing a shortcircuit of the connected PT100 (I think that should measure 200) did not make a difference.

I experimented with moving all AI ports to the AM2, no result, moved the A1 and A2 back, no difference.

I displayed all 8 AI ports, but the two used in the program gave -48 (with only one sensor connected) , the rest -50. 

Does anyone have a clue?

Thanks in advance, 

Ben

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Split from logo8 and AM2 RTD.

Hi ixo65, 

Thanks for your response and the welcome!

I've got 3-wire PT100's, on the attached picture you see two connected on the AM2.

Ow man, what a bummer...but hey, it solved my problem probably.  Much clearer picture attached,  it proves you're right. 

Thanks for this quick analysis!  Kudo's from me :-) 

Cheers 

Ben