RTD Connection to Siemens S7-300 PLC

[Brian McDonald]

I am trying to use the Siemens S7-300 PLC system to monitor temperature changes using a two wire 100ohm platnium RTD. I am using a SM331 AI 8x12bit Analog input card, model # 331-7KF01-OABO. I am having trouble getting the PLC to read any value from the RTD, I am pretty sure that I have the connection made properly but I am completely stumped. If anyone can help please let me know.

 

[dchartier]

Hello Brian;
I hope I am not pointing out the obvious, but I have no idea what experience you have with Siemens' analog modules. To setup an analog input module for a PT100, there are 3 steps to follow:
1)in HardwareConfig, you must set the parameters for RTD inputs for your PT100 channel (as seen in the picture I have attached); I presume you have already done this.
2)you can see that letters (A) and (B) appear under the parameters of the input channels (actually, (C) and (D) are also available). These letters indicate the position required for the range modules that exist on the outside of the SM331; you must physically remove the module from the rack and look at the 4 square blocks that can be found on the side of it. Each side of the square is marked with letters A to D, and you must position them according to the parameters you have set in the HardwareConfig.
3)correct wiring for the RTD is now required.
Hope this helps,
Daniel Chartier

 

[Tom Jenkins]

I don't know about Siemens specifically, but most RTD input cards are set up for 3-wire (or occasionally 4-wire) RTDs. You indicate you have a 2-wire RTD. In that case, you may have to jumper between two of the RTD input card terminals so it can pretend the third wire is there.

This will affect accuracy, but probably not much. The purpose of hte third wire is to compensate for the resistance of the RTD leads. If you have short runs of generaously sized wire the error probably won't be significant. If you have a critical application you should check into the lead resistance and calculate how much error it may induce.