Modbus Client-Server Principle

Overview

The Modbus RTU protocol exchanges information using a request-reply mechanism between a client and a server. The client-server principle is a model for a communication protocol in which one device (the client) controls one or more other devices (the servers). In a standard Modbus network, there is one client and up to 31 servers.

A detailed description of the Modbus protocol is available at www.modbus.org.

Characteristics of the Client-Server Principle

The client-server principle is characterized as follows:

• Only one client is connected to the network at a time.

• Only the client can initiate communication and send requests to the servers.

• The client can address each server individually using its specific address or all servers simultaneously using address 0.

• The servers can only send replies to the client.

• The servers cannot initiate communication, either to the client or to other servers.

Client-Server Communication Modes

The Modbus RTU protocol can exchange information using the two communication modes:

• request-reply mode

• broadcast mode

Request-Reply Mode

In the request-reply mode, the client addresses a server using the specific address of the server. The server processes the request then replies to the client.

1 Request

2 Process

3 Reply

Broadcast Mode

The client can also address all servers using address 0. This type of exchange is called broadcasting. The servers do not reply to broadcasting messages.

Response Time

The response time Tr is the time needed by a server to respond to a request sent by the client:

Values with the Modbus protocol are:

• Typical value < 10 ms for 90% of the exchanges

• Maximum value is around 700 ms. Hence it is recommended to implement a 1 second time-out after sending a Modbus request.

Data Exchange

The Modbus protocol uses 2 types of data:

• bits

• 16-bit words called registers

VigiPacT relays support both types of data.

Each register has a register number. Each type of data (bit or register) has a 16-bit address.

The messages exchanged with the Modbus protocol contain the address of the data to be processed.

Registers and Addresses

The address of register number n is n-1. The tables detailed in the following parts of this document provide both register numbers (in decimal format) and corresponding addresses (in hexadecimal format). For example, the address of register number 100 is 0x0063 (99).

Frames

All the frames exchanged with the Modbus protocol have a maximum size of 256 bytes and are composed of 4 fields: