Fieldbus and Networking in Process Automation

9 Modbus and Modbus Plus

9.1 Introduction

MODBUS is a serial communication protocol initially developed by AEG- Modicon. It was initially designed to operate with programmable logic controllers (PLCs). It is an application layer messaging protocol, operating at layer 7 of the Open Systems Interconnection (OSI) protocol, and provides clientserver communication between devices connected on different types of networks. The MODBUS protocol layers are shown in Figure 9.1, with the OSI protocol layers shown beside it. It defines a method of accessing and controlling a device by another irrespective of the type of physical network involved.

For MODBUS, no interface is required, as is the case with many other buses. A user has the option of choosing ...