User`s manual

FX3U Series Programmable Controllers

User’s Manual - MODBUS Serial Communication Edition

7 MODBUS Standard Commands

7.6 Read Holding Registers (Command Code: 0x03)

Reads the values of one or more holding registers.

1) Request message format (Master → Slave)

2) Response message format (Slave → Master)

(When completed normally)

(When completed with an error)

*2. Exception and error codes are stored in special data registers and special auxiliary relays in the case

of error completion. Refer to Chapter 6 for storage location, confirmation methods, and other detailed

contents.

Data

(L) (L)

Function code

Function

code

(03

Head holding register

number

(0000

H to FFFFH)

Read points

(0001H to 007DH)

(H)

Data

. . .

(L) (L)

Function code

Function

code

(03

Number of

read bytes

m = n x 2

Device data

(Number of read bytes n x 2)

*1 For example, if n = 4, the number of bytes is

calculated as 4 x 2 = 8 bytes

(H) (H)

Data

Function code

Function

code

(83

Exception

code *