Modules included in a UPS system can communicate over a Controller Area Network using messages having separate fields within an arbitration field. In particular, the separate fields can be separate priority fields, separate module identifier fields, and separate source and destination routing fields. The separate priority field can allow identical message types having different priority to be transmitted within the UPS system. For example, in a UPS system according to the invention, a rectifier off command can have greater priority than a rectifier reset command. The separate module identifier fields can uniquely identify the location of a module in the UPS system which transmitted the message. For example, the module identifier can be provided by customized wiring in a wiring harness connected to a respective communications interface circuit. The separate destination routing fields can be used to transmit messages to and from bridges in the UPS system. The message can also include separate data fields that organize the data into data categories and subcategories.