Modbus Function Codes; Supported Function Codes; Read Actual Values Or Settings (Function Code 03/04H - GE B30 Instruction Manual

B.2 MODBUS FUNCTION CODES
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
B
HEX DEC
03 3
04 4
05 5
06 6
10 16
This function code allows the master to read one or more consecutive data registers (actual values or settings) from a relay.
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 only support one of function codes 03h and 04h. The UR interpretation 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 packetsin Modbus RTU. Modbus TCP/IP ADUs have a MBAP
instead of slave address, and CRC is in another stack layer. 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 values 40, 300,
and 0 from registers 4050h, 4051h, and 4052h, respectively.
Table B–4: 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
B-4
MODBUS DEFINITION
Read holding registers
Read holding registers
Force single coil
Preset single register
Preset multiple registers
B.2.2 READ ACTUAL VALUES OR SETTINGS (FUNCTION CODE 03/04H)
EXAMPLE (HEX)
11
04
40
50
00
03
A7
4A
B30 Bus Differential System
B.2.1 SUPPORTED FUNCTION CODES
GE MULTILIN DEFINITION
Read actual values or settings
Read actual values or settings
Execute operation
Store single setting
Store multiple settings
SLAVE RESPONSE
PACKET FORMAT
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA #1 - high
DATA #1 - low
DATA #2 - high
DATA #2 - low
DATA #3 - high
DATA #3 - low
CRC - low
CRC - high
APPENDIX B
EXAMPLE (HEX)
11
04
06
00
28
01
2C
00
00
0D
60
GE Multilin