Modbus Functions; Function Code 03H; Function Code 04H - GE 345 Communications Manual

CHAPTER 1: COMMUNICATIONS GUIDE

MODBUS Functions

Function Code 03H

Function Code 04H

345 TRANSFORMER PROTECTION SYSTEM – COMMUNICATIONS GUIDE
Modbus implementation: Read Holding Registers
345 implementation: Read Setpoints
For the 345 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 345 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 33: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 03H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA 1 (see definition above)
DATA 2 (see definition above)
DATA 3 (see definition above)
CRC
Modbus Implementation: Read Input Registers
345 implementation: Read Actual Values
Data
022B
0000
0064
BYTES EXAMPLE
1 11
1 03
2 00 6B
2 00 03
2 76 87
BYTES EXAMPLE
1 11
1 03
1 06
2 02 2B
2 00 00
2 00 64
2 54 83
MODBUS FUNCTIONS
DESCRIPTION
message for slave 17
read registers
data starting at
006B
3 registers = 6 bytes
total
CRC error code
DESCRIPTION
message from slave
17
read registers
3 registers = 6 bytes
value in address
006B
value in address
006C
value in address
006D
CRC error code
1–171