GE DGP Series Instruction Manual page 197

9 COMMUNICATIONS
9.2 MODBUS COMMUNICATIONS
This section describes the Modicon Modbus RTU communication protocol used by the DGP Digital Generator
Protection Relay.
The device serial communication parameters, such as baud rate and DGP Unit ID, are set via the keypad. If
the DGP baud rate differs from the Modbus server baud rate, the device will not communicate to the server.
The Unit ID also must be set properly to avoid conflict with other devices connected on the same network.
Even though the setting range allows 16 bit values for Unit ID, the relay should be programmed for a Unit ID
ranging from 1 to 127.
The DGP implements a subset of protocols defined by the Modicon Modbus RTU protocol standard. Multiple
DGP relays can be configured as slaves to a single Modbus master through the RS485 port (using an RS485
to RS232 converter). The DGP is always a slave – it cannot be programmed as a master. Even though the
Modbus protocol is available in Modbus RTU, Modbus ASCII and Modbus Plus protocols, only the Modbus
RTU protocol is supported by the DGP.
One data frame of asynchronous transmission from the DGP is defaulted to 1 start bit, 8 data bits, no parity
bits, and 1 stop bit at 9600 baud. The baud rate, parity bits and number of stop bits can be changed through
the DGP keypad. This setting cannot be changed through the Modbus COM port.
A complete request/response sequence consists of the following bytes.
Modbus Request Transmission:
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
DATA
REGISTER CODE
CRC Hi
CRC Lo
Slave Response Transmission:
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA
CRC Hi
CRC Lo
a) SLAVE ADDRESS
This is the first byte of every transmission. It represents the Unit ID of the device programmed through the DGP
keypad. In the master request, the slave address represents the address of the slave the message is intended
for. In the slave response it is the address of the slave that is responding to the master request. Slave address
0 is reserved for broadcast transmissions by the master as specified by the Modbus protocol. The DGP does
not support the broadcast transmissions. The DGP will respond only if the slave address specified by the mas-
ter request matches its Unit ID; otherwise the DGP relay will not respond.
GE Power Management
Bytes
1
1
2
variable number depending on function code
2 bytes
High byte of CRC
Low byte of CRC
Bytes
1
1
variable depending on number of registers
variable number depending on the function
High Byte of CRC
Low byte of CRC
DGP Digital Generator Protection System
9.2 MODBUS COMMUNICATIONS
9.2.1 INTRODUCTION
9.2.2 DATA FRAME FORMAT & DATA RATE
9.2.3 DATA PACKET FORMAT
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