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Problem Running RTD & MODBUS together

Hi,

I'm running a program to read and record values from an PT-100 RTD and a MODBUS device. The problem is, the resistance value of the PT-100 isn't read properly by the dataTaker. If I run the PT-100 program alone, there is no issue about the reading. But once the MODBUS device is connected to the sensor port, and the program runs together, the resistance values of the PT-100 increases by more than 100 ohms. Eg, the actual temperature is 40'C, and resistance of around 115 ohm, but the datataker logs a temperature of 400'C, and resistance of 235 ohm.

I've tried to scale the value of the PT-100, but once the MODBUS device is shut down, the temperature values I get are very small, sometimes going to negative values, when it should be around 20'C.

Has anyone faced a similar problem before?

Hi, I'm running a program to read and record values from an PT-100 RTD and a MODBUS device. The problem is, the resistance value of the PT-100 isn't read properly by the dataTaker. If I run the PT-100 program alone, there is no issue about the reading. But once the MODBUS device is connected to the sensor port, and the program runs together, the resistance values of the PT-100 increases by more than 100 ohms. Eg, the actual temperature is 40'C, and resistance of around 115 ohm, but the datataker logs a temperature of 400'C, and resistance of 235 ohm. I've tried to scale the value of the PT-100, but once the MODBUS device is shut down, the temperature values I get are very small, sometimes going to negative values, when it should be around 20'C. Has anyone faced a similar problem before?

Hi arivan,

That should not happen, you may have ground loop in your system.
Could you describe your connection in details?

Best regards,
Rudy Gunawan

Hi arivan, That should not happen, you may have ground loop in your system. Could you describe your connection in details? Best regards, Rudy Gunawan

Hi Rudy,

I am using the sensor port for the RS485 connection, with twisted-pair cable connected to the TX Z & RTS Y and the shield cable connected to GND. As for the PT-100, it is a 3-wire RTD connected to the Channel 1.

How am I to find the ground loop?

Thank you.

Hi Rudy, I am using the sensor port for the RS485 connection, with twisted-pair cable connected to the TX Z & RTS Y and the shield cable connected to GND. As for the PT-100, it is a 3-wire RTD connected to the Channel 1. How am I to find the ground loop? Thank you.

Hi arivan,

How did you connect the ground of your dataTaker and your MODBUS sensor?
Is there any sensor involved?

When you said shield cable connected to GND, did you mean D GND?
RS485 sensors usually have three wires and not using cable shield as you GND.
Shield should be connected to your enclosure grounding point.

Best regards,
Rudy Gunawan

Hi arivan, How did you connect the ground of your dataTaker and your MODBUS sensor? Is there any sensor involved? When you said shield cable connected to GND, did you mean D GND? RS485 sensors usually have three wires and not using cable shield as you GND. Shield should be connected to your enclosure grounding point. Best regards, Rudy Gunawan

Hi Rudy,

Sorry for the late reply. We found out there was a grounding loop in the RS485 connections. I used an additional 3rd wire connected to the D GND, and the shield only at the enclosure's ground.

The problem seemed to have stopped, although the PT-100 still seems to read abit higher than usual. For instance, it returns a value of 90"C, when the actual value should be 65'C.

Hi Rudy, Sorry for the late reply. We found out there was a grounding loop in the RS485 connections. I used an additional 3rd wire connected to the D GND, and the shield only at the enclosure's ground. The problem seemed to have stopped, although the PT-100 still seems to read abit higher than usual. For instance, it returns a value of 90"C, when the actual value should be 65'C.

Hi arivan,

Could you check your PT-100 resistance at 65 degC?

There could be external disturbance on resistance reading, could you also check PT-100 reading when all sensors (including RS-485 sensor) are disconnected?

Best regards,
Rudy Gunawan

Hi arivan, Could you check your PT-100 resistance at 65 degC? There could be external disturbance on resistance reading, could you also check PT-100 reading when all sensors (including RS-485 sensor) are disconnected? Best regards, Rudy Gunawan
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