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1.4:
Function
Codes
A packet’s Function Code tells the addressed Slave what action to perform. EPM devices support the
following Modbus Function Codes:
1.4.1: Function Code 03—Read Holding Registers
Q
This function allows a Master station to read one or more parameter values (data registers) from an
EPM Slave. The data registers are 16-bit (two byte) values transmitted in “Big Endian” format:
high-ordered byte first, low-ordered byte second.
Q
The Master device sends a packet defining a start register for the Slave and the number of registers
to read. The Slave responds with a packet containing the requested parameter values within the range
specified in the request.
Q
In the following example, a Master device requests an EPM Slave at address 01H to transmit two
values beginning at Register 00001. The Slave replies with values 3031H and 3037H from Registers
00001 and 00002.
Hex Dec
03H 3
06H 6
1610H
Read Holding Registers
Preset Single Register
Preset Multiple Registers
Function Code
Description
Table 1.1: Function Codes
Slave Address
Function Code
Data Starting Address-Hi
Data Starting Address-Lo
Number of Registers-Hi
Number of Registers-Lo
CRC-Lo
CRC-Hi
01H Slave Address 01H
03H Function Code 03H
00H
00H
00H
02H
C4H
0BH
Byte Count 04H
30H
31H
30H
37H
F1H
2AHCRC-Hi
CRC-Lo
Data 2-Lo
Data 2-Hi
Data 1-Lo
Data 1-Hi
Slave PacketMaster Packet
Table 1.2: Function Code 03 Example