Control Network Newsletter

Simple to Use Modbus to BACnet Gateway

April 2012 – BACnet has become the dominant building automation protocol but Modbus devices used for boiler control, variable speed drives, and metering applications are frequently found on jobs requiring integration to a BACnet system. To make Modbus serial devices appear as individual BACnet devices, Contemporary Controls has introduced the BASgatewayLX Modbus to BACnet Gateway designed to make gateway commissioning as simple as possible.

The BASgatewayLX has one 10/100Mbps BACnet/IP Ethernet port and one opto-isolated master Modbus serial port supporting either Modbus RTU or Modbus ASCII slave devices. Up to 30 Modbus slave devices can be accessed at baud rates as high as 115.2 kbaud. A total of 1000 Modbus registers can be scanned. Using the concept of virtual BACnet routing each uniquely addressed Modbus slave device appears as an individual BACnet device with a unique BACnet device instance assignment. Within this BACnet device object are a collection of BACnet objects that relate to Modbus registers. What is needed is a device profile for each Modbus type device which allows for the mapping of Modbus registers to BACnet object instances. Contemporary Controls maintains a library of common device profiles with the more popular profiles shipped with the BASgatewayLX. Energy meter device profiles from Veris, Northern Design and Consolidated Controls are part of the library and more are frequently added.

A device profile lists all available Modbus registers for a particular Modbus device type along with recommended BACnet object instance designations. A checkbox alongside a register exists to indicate that this point is to be scanned. Approximately 25 registers in each device profile are marked for scanning but the user can add or delete from this list. The edited device profile is then loaded into one of the 30 possible virtual BACnet device slots forming a scan list. Each device slot scan list can be the same or different depending upon the Modbus device type or the amount of points needed to be scanned. The only restriction is the 30 slot limit and the maximum total point count of 1000.

If the desired device profile is not available, Contemporary Controls will provide it upon request. Custom device profiles can be uploaded to the BASgatewayLX. Resident device profiles can be edited to form scan lists using web pages on the unit. No special tools other than a web browser are needed. A resident Modbus utility program allows for the manual testing of a Modbus point which is handy during checkout.

The BASgatewayLX Modbus to BACnet Gateway is part of the company's BASautomation line of BACnet products. It is DIN-rail mounted and is powered from a 24 VAC/VDC source. For more information, visit the BASgatewayLX product page.