Basler Electric BE1-11 Modbus Protocol - Manual (Page 7)

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General Information
This document describes the Modbus
™ communications protocol employed by BE1-11 Protection
Systems and how to exchange information with these protection systems over a Modbus network. The
BE1-11 communicates 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 (BE1-11) responds to the query. When a
Modbus master communicates with a slave, information is provided or requested by the master.
Information residing in the BE1-11 is grouped categorically as follows:
•
Global Parameters
•
Control Parameters (Select Before Operate)
•
Setting Parameters
•
Report Parameters
•
Metering Parameters
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
Select Before Operate (SBO) functions are used to change active settings groups and control outputs.
There are four settings groups in the BE1-11, one of which may be selected as active using SBO
commands.
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
network and will always generate a response to the query unless certain error conditions occur. The
BE1-11 is designed to communicate on the Modbus network only as a slave device.
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.
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 BE1-11 maps all available data into the Modicon 984 holding register address space and 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
•
Function 08 (08 hex), subfunction 04 - diagnostics: force listen only mode
•
Function 16 (10 hex) - preset multiple registers
BE1-11 General Information