FC6A S
ERIES
M
ICRO
S
MART
A
LL
-
IN
-O
NE
T
YPE
C
OMMUNICATION
M
ANUAL
FC9Y-B1730
6-13
6: M
ODBUS
C
OMMUNICATION
Function Code 03 (Read Holding Registers) and Function Code 04 (Read Input Registers)
Function code 03 reads word device data of D (data register), T (timer preset value), or C (counter preset value). 1 through 64
consecutive words can be read out.
Function code 04 reads word device data of T (timer current value) or C (counter current value). 1 through 64 consecutive words
can be read out.
Communication Frame
Request from Modbus RTU Master
ACK Reply from Modbus RTU Slave
NAK Reply from Modbus RTU Slave
Communication Example
•
Modbus RTU Mode
Purpose
Read 2 words starting at data register D1710.
D1710
→
(1710 – 0) + 400001 = 401711
Modbus address: 401711
Extract lower 5 digits
→
1711
1711 – 1 = 1710 = 6AEh
Communication frame address: 06AEh
Condition
Slave No. 8
D1710 data: 1234h
D1711 data: 5678h
Request from Modbus RTU Master
08 03 06AE 0002 (CRC)
ACK Reply from Modbus RTU Slave
08 03 04 12 34 56 78 (CRC)
NAK Reply from Modbus RTU Slave
08 83 xx (CRC)
Slave No.
Function Code
Address
No. of Words
xxh
03h / 04h
xxxxh
xxxxh
Slave No.
Function Code
Quantity of Data
First High Byte
First Low Byte
Last Low Byte
xxh
03h / 04h
xxh
xxh
xxh
xxh
Slave No.
Function Code
Error Code
xxh
83h / 84h
xxh