9440300990 28-1
DECS-250 Modbus
®
Communication
28 • Modbus
®
Communication
Introduction
This document describes the Modbus
®
communications protocol employed by DECS-250 systems and
how to exchange information with DECS-250 systems over a Modbus network. DECS-250 systems
communicate by emulating a subset of the Modicon 984 Programmable Controller.
This product contains one or more nonvolatile memory devices. Nonvolatile
memory is used to store information (such as settings) that needs to be
preserved when the product is power-cycled or otherwise restarted.
Established nonvolatile memory technologies have a physical limit on the
number of times they can be erased and written. In this product, the limit is
100,000 erase/write cycles. During product application, consideration should
be given to communications, logic, and other factors that may cause
frequent/repeated writes of settings or other information that is retained by the
product. Applications that result in such frequent/repeated writes may reduce
the useable product life and result in loss of information and/or product
Modbus communications use a master-slave technique in which only the master can initiate a transaction.
This transaction is called a query. When appropriate, a slave (DECS-250) responds to the query. When a
Modbus master communicates with a slave, information is provided or requested by the master.
Information residing in the DECS-250 is grouped categorically as follows:
• General
• Binary Points
• Metering
• Limiters
• Setpoints
• Global Settings
• Relay Settings
• Protection Settings
• Gains
• Legacy Modbus
All supported data can be read as specified in the Register Table. Abbreviations are used in the Register
Table to indicate the register type. Register types are:
• Read/Write = RW
• Read Only = R
When a slave receives a query, the slave responds by either supplying the requested data to the master
or performing the requested action. A slave device never initiates communications on the Modbus and will
always generate a response to the query unless certain error conditions occur. The DECS-250 is
designed to communicate on the Modbus network only as slave devices.
Refer to the Communication chapter for Modbus communication setup and the Terminals and Connectors
chapter for wiring.
Message Structure
Device Address Field
The device address field contains the unique Modbus address of the slave being queried. The addressed
slave repeats the address in the device address field of the response message. This field is 1 byte.
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