IDEC FT1A Series User Manual page 277

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). One 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). One through 64 consecutive
words can be read out.
Communication Frame
Request from Modbus Master
Slave No.
xxh
ACK Reply from Modbus Slave
Slave No.
xxh
NAK Reply from Modbus 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
RTU Mode
•
Request from Modbus Master
ACK Reply from Modbus Slave
NAK Reply from Modbus 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)
S AXIS P
MART
No. of Words
xxxxh
First High Byte
xxh
/L U ' M FT9Y-B1378
RO ITE SER S ANUAL
11: M
ODBUS
First Low Byte
xxh
C
OMMUNICATION
Last Low Byte
xxh
11-13