Basler Electric DGC-2020HD Modbus Protocol - Manual (Page 7)

9469300991 Rev A 1
DGC-2020HD Modbus ™ Protocol General Information
General Information
This document describes the Modbus communications protocol employed by DGC-2020HD systems and
how to exchange information with DGC-2020HD systems over a Modbus network. DGC-2020HD 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 (DGC-2020HD) responds to the query.
When a Modbus master communicates with a slave, information is provided or requested by the master.
Information residing in the DGC-2020HD is grouped categorically as follows:
• General
• Global Settings
• Configuration
• Binary Points
• Metering
• Breaker Settings
• Bias Control
• Pulse Outputs
• Bus Condition
• DGC Settings
• Control Settings
• Protection Settings
• Remote Module Settings
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 DGC-2020HD is
designed to communicate on the Modbus network only as a slave device.
Refer to the Instruction Manual for Digital Genset Controller DGC-2020HD (Basler Publication
9469300990) for Modbus communication setup and 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 DGC-2020HD 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