Normal. Moxa MGate MB3180/MB3280/MB3480 Series, MGate MB3170/MB3270 Series

A.

Modbus Overview

Introduction

Modbus is one of the most popular automation protocols in the world. It supports both serial and Ethernet devices. Many industrial devices, such as PLCs, DCSs, HMIs, instruments, meters, motors, and drivers, use

Modbus as their communication standard.

Devices Are Either Clients/Masters or

Servers/Slaves

All Modbus devices are classified as either a client/master or a server/slave. Clients/Masters initiate all communication with servers/slaves and do not communicate to other clients/masters. Servers/Slaves are completely passive and communicate only by sending a response to a client's/master’s request.

Server/Slaves Are Identified by ID

Each Modbus slave in a system is assigned a unique ID between 1 and 247. Whenever a client/master makes a request, the request must include the ID of the intended recipient. Client/Master devices themselves have no ID.

0

Broadcast address

1 to 247

Slave individual address

248 to 255

Reserved

Communication Is by Request and Response

All Modbus communication is by request and response. A client/master sends a request, and a server/slave sends a response. The client/master will wait for the server/slave’s response before sending the next request. For broadcast commands, no response is expected. This three scenarios are explained below:

Normal

The client/master sends a request to the server/slave. The server/slave sends a response with the requested information.

MGate MB3000 Modbus Gateway User Manual 85