Function Code 03H (Read Setpoints) - GE Multilin 850 Communications Manual

CHAPTER 11: MODBUS PROTOCOL

Function Code 03H (Read Setpoints)

8 SERIES PROTECTIVE RELAY PLATFORM – COMMUNICATIONS GUIDE
Error ID Exception
01 ILLEGAL FUNCTION
02 ILLEGAL DATA ADDRESS
03 ILLEGAL DATA VALUE
Modbus Implementation: Read Holding Registers
8 Series relay Implementation: Read Setpoints
For the 8 Series relay implementation of Modbus, the function code can be used to read
any setpoints ("holding registers"). Holding registers are 16 bit (two byte) values
transmitted high order byte first. As a result all 8 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:
Address
006B
006C
006D
The master/slave packets have the following format:
Table 11-1: Master Packet Format for Function Code 03H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC (low, high)
Table 11-2: Slave Packet Format for Function Code 03H
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA 1
DATA 2
DATA 3
DATA 4
CRC (low, high)
FUNCTION CODE 03H (READ SETPOINTS)
Description
The function code transmitted is not one of the
functions supported by the 8 Series relay.
The address referenced in the data field transmitted
by the master is not an allowable address for the
8 Series relay.
The value referenced in the data field transmitted by
the master is not within range for the selected data
address.
Data
022B
0000
0064
BYTES EXAMPLE
1 FE
1 03
2 4A 8A
2 00 04
2 67 F4
BYTES EXAMPLE
1 FE
1 03
1 08
2 00 00
2 04 B0
2 00 00
2 00 64
2 E7 75
DESCRIPTION
message for slave 254
read registers
data starting at 4A8A
4 registers = 8 bytes total
CRC error code
DESCRIPTION
message from slave 254
read registers
4 registers = 8 bytes total
value in address 4A8A (16-bit)
value in address 4A8B (16-bit)
value in address 4A8C (32-bit)
CRC error code
11-3