4-20ma Transmitter and SM1231 overshoot or big offset with shunt resistor

Hi everyone,

Newbie here,

I'm trying to get my SM1231 AI8 and AI4 attached to a S7-1200 CPU 1214C DC/DC/DC  to work properly with my transmitters that are 2 wires 4-20ma (some are for temperature and some for pressure but get the same problem with both). 

First I tried wiring them as Siemens says: see first attachment

I was getting an error warning and a big overshoot: on a -50 c to 150c scale reading was 184 and ambient temp was 17c sensor being in ambient air. Sensor is a Pt1000 rdt wired to head mount transmitter with a two wire 4-20ma output. When looping with same power supply but with a multimeter everything was right on 9.38ma = 17.25c with proper scaling

- every unused inputs are shorted and set at 0-20ma 

I've read about using a shunt resistor, the thread mentioned using a 500 ohms resistor, which I've done I got a linear reading but with a big offset !

Is it normal to have to use a resistor in such a simple circuit? Should I counter the offset in the program or calculate a proper shunt resistance ? Should I look somewhere else for electrical issue ( Which i've been going over and over ) - Voltage is stable, power suppy dedicated to PLC ( I've tried looping with an external 24v supply) 

Thank you 

You are using the wrong connection diagram for a two wire 4-20mA transmitter. You should be using the diagram in the attachment.

You do not need an external resistor if you configure the inputs as current inputs. Also you can select the 4-20mA range instead of the 0-20mA range. This may make your scaling easier.

I recommend that you study the S7-1200 System Manual especially chapters 4, 5, 6, and 7.

It is available here: 

https://support.industry.siemens.com/cs/us/en/view/109764129 

 Sorry it was late and saved the wrong image from web browser, I am wired exactly as the image you've attached, I'll modify my original post

LarryAm

You are using the wrong connection diagram for a two wire 4-20mA transmitter. You should be using the diagram in the attachment.

 That's good.

If you are configured as 0-20mA current, then the value read directly from the module should be 12967 ((27648/20mA)*9.38mA). If the configuration is 4-20mA then the value read from the module should be 9297 ((27648/(20mA-4mA))*(9.38mA-4mA)).

Larry

Sageorvis

 Sorry it was late and saved the wrong image from web browser, I am wired exactly as the image you've attached, I'll modify my original post

LarryAm

You are using the wrong connection diagram for a two wire 4-20mA transmitter. You should be using the diagram in the attachment.

@huggy_d1: Agreed. Short-circuiting a device connected to the analog input is a bad idea.

That is not what is said by Sageorvis. He is connecting the (+) and (-) pins of the unused analog input.

EDIT
Siemens recommendations attached.