BACnet Compatible AHU Fan Safety or General Purpose Logic Board

August 6, 2019
Tech Tuesday

Almost a year ago, I wrote a blog about our General-Purpose Logic Boards (RIBMNLB-6/-4/-2). These devices are typically used for AHU Fan Safety and Alarming, and they come with two, four or six inputs and outputs. The inputs accept AHU sensor outputs, and the unit monitors those inputs to determine if an alarm condition exists. This information is used to shut down the on-board, master relay, which would be controlling the AHU fan. The outputs follow the state of the respective inputs and are used for monitoring purposes back at a controller.

Being able to monitor the state of the sensor inputs back at the controller makes tracking down a fault and fixing it much easier, but it requires two, four, or six controller inputs. For this, and other reasons, we developed a BACnet® version of the RIBMNLB-6, which we call RIBMNWLB-7-BC. It can perform all the functions of the RIBMNLB-6, but it is more configurable, has BACnet® MS/TP incorporated and requires little to no controller I/O. It will also continue to work stand-alone, if the network goes down.

The RIBMNWLB-7-BC has seven Binary Inputs (BIs) and two Binary Outputs (BOs). It comes from the factory configured for use in AHU Fan Safety and Alarming applications, which means that BI1 – BI6 must be in a good state for the master relay (BO1) to be energized. Those six inputs can be configured for Normally Open (N/O) or Normally Closed (N/C) sensors, depending on what is written to the polarity property of the respective BI. By default, the Fan Safety function requires N/C sensors, like the RIBMNLB-6, and each input can be overridden using local DIP switches, if not all are required. There are also local DIP switches that can be used to latch any of the six BIs used for the Fan Safety function. If any of the inputs are latched and there is a fault on that input, the master relay can only be energized again by clearing the fault AND clearing the latch. The latch can be cleared by means of a local pushbutton, power-cycling the device or writing a one to the Present_Value property of Binary Value 1 (BV1).

Unlike the RIBMNLB-6, the RIBMNWLB-7-BC has a second relay on-board. This relay is, by default, bound to the seventh Binary Input (BI7). If the input to BI7 is closed (depending on polarity setting) the second relay (BO2) will be energized. A proprietary property of BO2 allows for this binding to be disabled if desired. If binding and the Fan Safety function are disabled, the RIBMNWLB-7-BC functions as a standard BACnet® RIB with seven Binary Inputs and two Binary Outputs. Just like all our BACnet® RIBs, the Binary Outputs can be overridden locally, and the MS/TP address and baud rate are set locally, all via DIP switches.

For more information on the RIBMNWLB-7-BC, visit our website. The previous model name is linked to the product page, where you can find the Datasheet, PIC Statement and Bulletin. You can also contact technical support for questions regarding this and any other product we manufacture.


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About the Author

Henry Smith is a design engineer at Functional Devices, Inc. He has a BS 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 him or her meet a deadline.