Basler Electric BE1-851 Modbus Protocol - Manual (Page 9)

9289900992 Rev F BE1-851 Modbus
™ Protocol
1-1
SECTION 1
• GENERAL INFORMATION
INTRODUCTION
This document describes the Modbus
™ communications protocol employed by BE1-851 relays and how
to exchange information with BE1-851 relays over a Modbus
™ network. The BE1-851 communicates by
emulating a subset of the Modicon 984 Programmable Controller.
FUNCTIONAL DESCRIPTION
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 (BE1-851) responds to the
query. When a Modbus
™ master communicates with a slave, information is provided or requested by the
master. Information residing in the BE1-851 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 BE1-851, 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 BE1-851 is
designed to communicate on the Modbus
™ only as a slave device.
A master can query slaves individually or universally. A universal (“broadcast”) query, when allowed,
evokes no response from any slave device. If a query to an individual slave device requests actions
unable to be performed by the slave, the slave response message contains an exception response code
defining the error detected. Exception response codes are quite often enhanced by the information found
in the “Error Details” block of holding registers.
Message Structure
Master initiated queries and BE1-851 responses share the same message structure. Each message is
comprised of four message fields. They are:
• Device
Address (1 byte)
• Function Code
(1 byte)
• Data Block
(n bytes)
• Error Check field
(2 bytes)
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, a BE1-851 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.