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DECS-250
Modbus™ Communication
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.
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.
Although Modbus protocol limits a device address from 1 - 247. The address is user-selectable at
installation and can be altered during real-time operation.
Function Code Field
The function code field in the query message defines the action to be taken by the addressed slave. This
field is echoed in the response message and is altered by setting the most significant bit (MSB) of the
field to 1 if the response is an error response. This field is 1 byte in length.
The DECS-250 maps all available data into the Modicon 984 holding register address space supports the
following function codes:
•
Function 03 (03 hex) - read holding registers
•
Function 06 (06 hex) - preset single register
•
Function 08 (08 hex), subfunction 00 - diagnostics: return query data
•
Function 08 (08 hex), subfunction 01 - diagnostics: restart communications option
Содержание DECS-250
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Страница 56: ...40 9440300990 Rev J Contact Inputs and Outputs DECS 250 ...
Страница 84: ...68 9440300990 Rev J Limiters DECS 250 ...
Страница 108: ...92 9440300990 Rev J Power System Stabilizer DECS 250 Figure 85 PSS Function Blocks and Software Switches ...
Страница 114: ...98 9440300990 Rev J Power System Stabilizer DECS 250 Figure 96 PSS Parameter Settings ...
Страница 124: ...108 9440300990 Rev J Stability Tuning DECS 250 ...
Страница 126: ...110 9440300990 Rev J Mounting DECS 250 Figure 103 Overall and Projection Mounting Dimensions ...
Страница 127: ...9440300990 Rev J 111 DECS 250 Mounting Figure 104 DECS 250 Escutcheon Plate Dimensions ...
Страница 128: ...112 9440300990 Rev J Mounting DECS 250 Figure 105 Panel Cutting and Drilling Dimensions for DECS 250 Panel Mounting ...
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Страница 210: ...194 9440300990 Rev J CAN Communication DECS 250 ...
Страница 304: ...288 9440300990 Rev J Maintenance DECS 250 ...
Страница 316: ...300 9440300990 Rev J Specifications DECS 250 ...
Страница 334: ...318 9440300990 Rev J Analog Expansion Module DECS 250 ...
Страница 341: ...9440300990 Rev J 325 DECS 250 Contact Expansion Module Figure 188 CEM 2020 Input Contact and Output Contact Terminals ...
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