IDEC FC6A-C16K1CE Communications Manual page 139

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