Controller Area Network to Modbus network bridge tointerface gas detection units with Building ManagementSystems
Matthew Quinton · 2015
Building Management Systems (BMS') are computer systems designed to control systems inside buildings or other facilities. While BMS' are common, there is no one size fits all approach. Controller Area Network (CAN) is a communication protocol sometimes used within BMS'. Modbus is a very common industrial communications protocol. The two protocols are not directly compatible and need to be 'bridged’ to communicate with each other. Gas Detection Australia (GDA) design and manufacture gas detection equipment. They have a current and ongoing need to interface Modbus enabled equipment with CAN enabled equipment in client BMS'. This project is sponsored with the aim of producing a network bridge to translate between the two protocols. The specific Modbus variation implemented is Modbus ASCII master. The design was based around the PIC 18F87K22 microprocessor. This was chosen to remain consistent with other GDA products. The communication interfaces were designed using integrated circuits that closely mimic the software development tools. This was a deliberate choice made to make software development simpler and to make it easier to translate source code to the finished product. A testing method was also created to allow the assessment of bridge performance. Testing demonstrated proof of concept using the development board. Separate testing of RS-485 hardware suggests that the full hardware specification is valid. Stress tests were carried out and determined that the bridge could be expected to be capable of responding to four CAN messages per second. The testing was limited by issues relating to the inconsistent operation of the CAN interface.