Basler Electric BE1-700 Modbus Protocol - Manual (Page 11)

9376700991 Rev D BE1-700 Modbus™ Protocol 1-1
SECTION 1 • GENERAL INFORMATION
Introduction
This document describes the Modbus™ communications protocol employed by BE1-700 relays and how
to exchange information with BE1-700 relays over a Modbus network. The BE1-700 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 (BE1-700) responds to the query. When a
Modbus master communicates with a slave, information is provided or requested by the master.
Information residing in the BE1-700 is grouped categorically as follows:
• Session Parameters
• 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 two settings groups in the BE1-700, 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 and will
always generate a response to the query unless certain error conditions occur. The BE1-700 is designed
to communicate on the Modbus network only as a slave device.
Message Structure
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.
Device Address Field
Although Modbus protocol limits a device address from 1 - 247, a BE1-700 can be assigned a device
address in the range of 1 - 65534. The address is user-selectable at installation and can be altered during
real-time operation.
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.
Function Code Field
The BE1-700 maps all available data into the Modicon 984 holding register address space (4XXXX) and
supports the following function codes:
• Function 03 (03 hex) - read holding registers
• Function 06 (06 hex) - preset single register (write single holding 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, non-broadcast and broadcast
The query data block contains additional information needed by the slave to perform the requested
function. The response data block contains data collected by the slave for the queried function. An error
Data Block Field