Modbus Function Codes; Supported Function Codes; Read Actual Values Or Settings (Function Code 03/04H - GE G60 Instruction Manual

APPENDIX B
B.2MODBUS FUNCTION CODES
®
Modbus officially defines function codes from 1 to 127 though only a small subset is generally needed. The relay supports
some of these functions, as summarized in the following table. Subsequent sections describe each function code in detail.
FUNCTION CODE MODBUS DEFINITION
HEX DEC
03 3 Read holding registers
04 4 Read holding registers
05 5 Force single coil
06 6 Preset single register
10 16 Preset multiple registers
This function code allows the master to read one or more consecutive data registers (actual values or settings) from a relay.
Data registers are always 16-bit (two-byte) values transmitted with high order byte first. The maximum number of registers
that can be read in a single packet is 125. See the Modbus Memory Map table for exact details on the data registers.
Since some PLC implementations of Modbus
allows either function code to be used for reading one or more consecutive data registers. The data starting address will
determine the type of data being read. Function codes 03h and 04h are therefore identical.
The following table shows the format of the master and slave packets. The example shows a master device requesting
three register values starting at address 4050h from slave device 11h (17 decimal); the slave device responds with the val-
ues 40, 300, and 0 from registers 4050h, 4051h, and 4052h, respectively.
Table B–3: MASTER AND SLAVE DEVICE PACKET TRANSMISSION EXAMPLE
MASTER TRANSMISSION
PACKET FORMAT
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS - high
DATA STARTING ADDRESS - low
NUMBER OF REGISTERS - high
NUMBER OF REGISTERS - low
CRC - low
CRC - high
GE Multilin
GE MULTILIN DEFINITION
Read actual values or settings
Read actual values or settings
Execute operation
Store single setting
Store multiple settings
B.2.2 READ ACTUAL VALUES OR SETTINGS (FUNCTION CODE 03/04H)
®
only support one of function codes 03h and 04h, the relay interpretation
SLAVE RESPONSE
EXAMPLE (HEX) PACKET FORMAT
11 SLAVE ADDRESS
04 FUNCTION CODE
40 BYTE COUNT
50 DATA #1 - high
00 DATA #1 - low
03 DATA #2 - high
A7 DATA #2 - low
4A DATA #3 - high
DATA #3 - low
CRC - low
CRC - high
G60 Generator Management Relay
B.2 MODBUS FUNCTION CODES
B.2.1 SUPPORTED FUNCTION CODES
EXAMPLE (HEX)
11
04
06
00
28
01
2C
00
00
0D
60
B
B-3