Digital Energy

Multilin

Multilink ML1200

Managed Field Switch

Chapter 18: Modbus Protocol

18.1Modbus Configuration

18.1.1 Overview

Modicon programmable controllers as well as other PLCs can communicate with each other and other devices over a variety of networks. The common language used by all Modicon controllers is the Modbus protocol. This protocol defines a message structure that controllers recognize and use regardless of the networks over which they communicate. It describes the process a controller uses to request access to another device, how it will respond to requests from the other devices, and how errors will be detected and reported. It establishes a common format for the layout and contents of message fields. The Modbus protocol thus operates at the layer 7 of the OSI 7 layer stack. Additional information on Modbus can be found at http://www.modbus.org and other related sites.

RFC 1122 Requirements for Internet Hosts - Communication Layers defines how Modbus packets can be carried over a TCP/IP transport and how Modicon controllers or other PLC devices can communicate over a TCP/IP network. To facilitate this communications, the MultiLink ML1200 Managed Field Switch allows Modbus connectivity.

As per this RFC, Modbus communications take place on TCP port 502. Please make sure the network security devices do not block port 502. If port 502 is blocked, which is the normal case with many firewall and other security devices, the communications between two Modbus devices over a TCP/IP network will not succeed.

18.1.2 Command Line Interface Settings

The following command-line interface settings are available:

modbus <enabledisable> modbus port=<portdefault> modbus device=<devicedefault> show modbus

MULTILINK ML1200 MANAGED FIELD SWITCH – INSTRUCTION MANUAL

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GE Multilink ML1200 Managed Field Switch Modbus Protocol, Modbus Configuration, Command Line Interface Settings