Repurpose a BACnet™ Object

Recently, I was on a technical support call with someone using one of our BACnet™ products. They had an interesting question. They had an RIBTW24B-BCAO, which has the following BACnet™ objects:

  • Binary Output (BO1) – Relay Output
  • Binary Input (BI1) – Dry Contact Input
  • Analog Output (AO1) – 0-5V, 0-10V, or 4-20mA
  • Analog Input (AI1) – 0-5V, 0-10V, or Thermistor

When purchasing this device, they had not intended to use the Analog Input. Their application was needing the Binary Output, Binary Input, and Analog Output. However, they decided it would be nice to verify run status on the motor they were controlling with the relay. For this, they had one of our current sensors monitoring the hot feed to the motor. However, they were short one Binary Input to get this status to their controller. They asked if they could somehow configure the unused Analog Input to accept the output of the current switch and effectively use it as a Binary Input. My first thought was that this was possible. I asked the customer if I could do some checking and testing before I said yes.

After checking the product schematic and doing verification on our BACnet™ test network, I was able to respond to the customer. I told them that it was possible and relatively easy to configure the device for their need. All our BACnet™ products that offer an Analog Input can be configured in this manner.

It is as simple as setting the device up the same way it would be to use a thermistor on the Analog Input. The bulletins for the products above have detailed steps to configure them for use with a thermistor. Once the setup is finished, a dry contact output can be connected to the Analog Input and used like a Binary Input.

The steps to follow for the RIBTW24B-BCAO are on page 3 of the product Bulletin (B1756). Steps for the other three models are on page 4 of their product Bulletin (B1243). For all products, the setup is the same:

  1. Install the THERM SELECT jumper.
  2. Install either J2 or J3 (it does not matter which one).
  3. Open SW1
  4. Close SW2
  5. Connect the dry contact output to Terminals AI and THERM +5V.

There are other steps to follow for connecting a thermistor that will be skipped for this application. Those steps just enable the product to properly scale the reading to Celsius or Fahrenheit.

In the customer’s application, they were using a current switch as the dry contact input. For them, whenever the motor started running and drew more current than the threshold set on the current switch, its contact would close, and they would read 1023 on Analog Input, Present Value. Whenever the motor stopped and the current switch opened, they would then read a zero on the Analog Input, Present Value. The Analog Input is not a Boolean object like the Binary Input, so they planned to do some work on the front-end wire sheet so it would make sense for the user.

If you have a unique or complex application and need some help, send us an email or give us a call. We are here to help and love to hear about the interesting ways our customers use our products!

Henry Smith
Henry Smith

Henry Smith is the Product Design Manager at Functional Devices, Inc. He has a BS in EET obtained from Purdue University in 2014 and a lifelong interest in electronics. As an engineer at Functional Devices, he gets to provide Technical Support to our customers, from distributors to specifying engineers and installers.

Henry enjoys providing tech support, as it allows our company to assist at every level of our product’s lifecycle and exposes us to interesting and unique applications. While not every technical question is unique, even answering a simple question or providing someone with the information in a timely manner can go a long way to helping them meet a deadline.

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