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Is it possible to use the two thermocouple inputs set to ohms with a load cell? I recently discovered they want to hook up two 5000 lb load cells with 350 ohm bridge and 2mv/Volt at rated capacity. Do I have to order a load cell card or can the two thermocouple inputs somehow be used? I have a V350-350-RA22 PLC. My other analog inputs are already used.

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It is possible to use the PT100 inputs to directly read ohms though it is not recommended. The input range is only from 1-320 ohms and will only have 12 bit resolution. You will need to set the jumpers and wire the load cells according to the diagrams within the V350-35-RA22 installation guide. The resolution will be in units of 0.1 ohms and there is a conversion time of 300 ms minimum per channel. Each analog input will require a minimum of 100 ms so if using two analog and two RTD that will be a minimum of 100 + 100 + 300 + 300 = 800 ms minimum conversion time.

It will work to measure ohms but will not be able to use the I/O LC-1 & LC-3 features that include:

  • VisiLogic loadcell function blocks(automatic scaling, net min and max, tare, zero, gross weight, setpoints, and filters)

  • A quicker minimum conversion time (min 6.75 ms if using 2x I/O LC-1's or min 675 ms for 2 loadcells with the I/O LC-3)

  • Accurate resloution (I/O LC-1&LC-3 are 24 bit resolution).

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