The Modbus protocol defines a message structure and format used in communication transactions. Modbus devices communicate using a master-slave method, in which only the master device can initiate a communications transaction. There can be only one master device on a Modbus network—in most integrations (modbusAsync, modbusTcp), this is the JACE. All other devices must be Modbus slaves.

However, note that two “slave” Modbus drivers are also available, in which the station can act as a dumb slave (server), using modbusSlave or modbusTcpSlave components. Usage of these drivers is expected to be infrequent. However, basic Modbus principles remain the same.

Before reading about Modbus, please understand that any NiagaraAX Modbus integration uses Modbus proxy points to provide monitoring and control, similar to other NiagaraAX integrations. Therefore, many of the following Modbus topics mention items specific to NiagaraAX components, to help clarify station configuration.

The following topics may contribute to a basic understanding of Modbus: