MODBUS

Conceived in the 1980's by Gould for use with their Modicon programmable logic controllers, it provides a method of handling communications between a "Master" and up to 247 "Slave" devices.

The Modbus protocol defines how the master and slaves establish contact, how the sender and receiver are identified, how messages are exchanged in an orderly manner and how errors are detected. The minimum unit of communication, or transaction, can be a single query and its associated response or a "broadcast" message (sent to address 0).

Although many of the characteristics of Modbus are fixed, some are user selectable; such as the choice of transmission medium and baud rate. These user selectable parameters are set at each station at time of system configuration. The Modbus protocol provides message frames (or "envelopes") in which messages can be sent to addressed devices. Each message contains information relating to the address of the intended receiver, what the receiver is to do, data needed to perform the action, and a means of checking for errors.

When the message reaches the intended destination, the Slave device reads the message and, provided that no errors have occurred, performs the requested task and returns a message to the Master. The returned message contains the slave address, the action performed, data acquired as a result of the action, and a means of checking for errors. No response is transmitted if the received message is a "broadcast"

Common installations of Modbus use a two wire twisted pair cable operating with RS485 electrical signals. There are 2 transmission modes; ASCII (7 bit hexadecimal data) or RTU (8 bit binary data) For implementation on Status products, RTU is due to its superior data transfer rate.

MODBUS TCP/IP

TCP/IP is the set of protocols that are used over various physical layers to run over the internet. An extension of Modbus, Modbus TCP/IP basically embeds a Modbus frame into a TCP frame in a simple manner.

This is a connection-oriented transaction that means every query expects a response. This query/response technique fits well with the master/slave nature of Modbus, adding to the deterministic advantage that Switched Ethernet offers industrial users.

Modbus TCP/IP is simply Modbus within a TCP 'wrapper'. It is therefore extremely simple for existing Modbus devices to communicate over Modbus TCP/IP. To do this a gateway device is required to convert Modbus protocol to Modbus TCP/IP. Status Instruments will offer such gateways. The Gateway acts as shown below.

Back