GE DGP Series Instruction Manual page 199

9 COMMUNICATIONS
9.3 MODBUS FUNCTIONS
a) DESCRIPTION
Reads the binary contents holding/input registers (actual values) in the slave. They can be set point registers
or any of the information reports, such as actual values.
b) QUERY
The query message specifies the starting register and the number of registers to be read.
Field:
SLAVE ADDRESS
FUNCTION CODE
Starting address
Num points
CRC
c) RESPONSE
The register data in the response is packed as two bytes per register. For each register the first byte contains
the higher order byte and the second contains the lower order byte.
Example of typical response message
Field:
SLAVE ADDRESS
FUNCTION CODE
Byte Count
Data 1
......
Data n
CRC
GE Power Management
9.3.1 FUNCTION CODE 03/04: READING HOLDING/INPUT REGISTERS
Bytes Example (hex):
1 11 Message for slave 17
1 03/04 Read registers
2 Starting address of the register. High byte first and then the Low Byte
2 Number of registers to read. High Byte first then Low byte
2 CRC Calculated by master. High byte first. Low byte next
Bytes Example (hex):
1 11 Message from slave 17
1 03/04 Read registers
1 Number of registers to read.
2 High Byte first then Low byte
2 High Byte first then Low byte
2 CRC calculated by slave. High byte first, Low byte next
DGP Digital Generator Protection System
9.3 MODBUS FUNCTIONS
9
9-
7