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Measuring Discrete Inputs Using the BULLET and the TCM

Measuring Discrete Inputs Using the BULLET and the TCM

Having a WirelessHART gateway available provides end users with many potential uses. One of them is in moving discrete inputs via the WirelessHART gateway in a simple and affordable manner. This can be easily accomplished using the unique capabilities of Moore Industries’ TCM Temperature Concentrator Monitor and the BULLET WirelessHART Adapter.

Using the BULLET, 16 inputs can be sent from the TCM via WirelessHART to the gateway. Essentially, what this is doing is converting the contact closures to binary 0s and 1s so they can be read by a HART, MODBUS RTU, MODBUS/TCP master or any host connected to the WirelessHART gateway. By using a parallel resistor and implementing some basic configuration, the TCM can sense a discrete contact input status and scale it as a binary 0 or 1 for the host system.

Here are the steps needed to accomplish this:


For each discrete input, wire the discrete as though it is a 2-wire resistance input to each TCM channel. Install a nominal 1000Ω resistor in the same input terminals. When the discrete is closed the input will read ~0Ω; when the discrete is open the input will read ~1000Ω.


Using the TCM PC Configuration Software, configure the input for 2-wire resistance. Leave the range of at the default setting of 4000Ω.


Since discretes are reported as binary 0s and 1s, custom curves can be used for each discrete input to create either a 0 or a 1.

  • 0/Open Contact = ~1000Ω
  • 1/Closed Contact = ~0Ω

Once this is done, you can connect the TCM to your WirelessHART network using the BULLET or directly wire the TCM to your HART input card. 

This input and configuration scheme is not unique to the TCM: you can measure discrete inputs with any of our modern temperature products with a digital output (such as the THZ3, TIM, TMZ, TPZ and TFZ). This creates a range of multi-channel and single-channel solutions for converting discrete inputs to HART, MODBUS, PROFIBUS PA and FOUNDATION Fieldbus protocols.

Published: May 28, 2015