Overview of Capabilities

Modbus communication is via a client–server technique, in which only one device (the client) can initiate transactions (called ‘queries’). The other devices (the servers) respond by supplying the requested data to the client, or by taking the action requested in the query. Predix EdgeOS with the Modbus protocol adapter functions as a client device. Typical servers include programmable controllers, RTUs, DCS, I/O systems, data concentrators, flow computers and various instrumentation. The client can address individual servers, or can initiate a broadcast message to all servers. Servers return a message (called a ‘response’) to queries that are addressed to them individually.

The Modbus protocol establishes the format for the client’s query by placing into it the device (or broadcast) address, a function code defining the requested action, any data to be sent, and an error–checking field. The server’s response message is also constructed using Modbus protocol. It contains fields confirming the action taken, any data to be returned, and an error–checking field. If an error occurred in receipt of the message, or if the server is unable to perform the requested action, the server will construct an error message and send it as its response.