Function Code 03H - GE 350 Communications Manual

GE
Digital Energy
SR350 Feeder Protection System
Chapter 9: MODBUS Functions

Function Code 03H

350 FEEDER PROTECTION SYSTEM – COMMUNICATIONS GUIDE
MODBUS Functions
Modbus implementation: Read Holding Registers
350 implementation: Read Setpoints
For the 350 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 350 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:
Address
006B
006C
006D
The master/slave packets have the following format:
Table 9-1: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 03H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC
Data
022B
0000
0064
BYTES EXAMPLE
1 11
1 03
2 00 6B
2 00 03
2 76 87
DESCRIPTION
message for slave 17
read registers
data starting at
006B
3 registers = 6 bytes
total
CRC error code
9–1