7KF02, measured mV does not match with standard mV VS deg C table

Hello Smithyy;

I recommend you start by downloading the following manual; chapters 4,5, and 6 are very useful when you attempt to connect an analog module:

SIMATIC S7-300 S7-300 Module data

First check that your module (6ES7331-7KF02-0AB0) is setup properly: is the range module (on the side of the card) for your group of channels on "A" (required for RTDs) and is the module parametrized correctly in HWConfig? See page 312 (section 6.7) of the manual.

Then validate your connections: are you using 2-wire, 3-wire or 4-wire connections for your RTD sensor, as shown on the same page of the manual? Make sure that Mana pin is connected to a functional ground to eliminate ground loops.

Finally, look at the design of the module (also on the same page): there is an ADC (analog-to-digital converter) chip that retieves the data from the sensor and resolves it to a numerical value. There is no relationship between the value you read and the mV/deg C of the sensor.

The attached document shows the numerical value expected in your PIWxxx register when you read the RTD input value (it comes from the same manual, chapter 5); if you read the value in your program or a VAT table (transfer the value in a MWxx register first) and set it up as a decimal read, you should see a value that is a multiple of 10.0 of the temperature of the sensor.

Hope this helps,

Daniel Chartier