SR3 SERIES PROTECTIVE RELAY PLATFORM – COMMUNICATIONS GUIDE 8-1
SR3 Series Protective Relay Platform
Chapter 8: MODBUS Functions
GE
Grid Solutions
MODBUS Functions
Function Code 03H
Modbus implementation: Read Holding Registers
3 Series relay implementation: Read Setpoints
For the 3 Series relay implementation of Modbus, this function code can be used to read
any setpoints (“holding registers”). Holding registers are 16 bit (two byte) values
transmitted high order byte first. Thus all 3 Series relay
Setpoints are sent as two bytes.
The maximum number of registers that can be read in one transmission is 125.
The slave response to this function code is the slave address, function code, a count of the
number of data bytes to follow, the data itself and the CRC. Each data item is sent as a two
byte number with the high order byte sent first.
For example, consider a request for slave 17 to respond with 3 registers starting at address
006B. For this example the register data in these addresses is as follows:
The master/slave packets have the following format:
Table 8-1: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 03H
Address Data
006B 022B
006C 0000
006D 0064
MASTER TRANSMISSION BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 11 message for slave 17
FUNCTION CODE 1 03 read registers
DATA STARTING ADDRESS 2 00 6B data starting at 006B
NUMBER OF SETPOINTS 2 00 03 3 registers = 6 bytes
total
CRC 2 76 87 CRC error code
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