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5.2 Read Holding Register – Function Code 03
5.2.1 Read Holding Register Query
The Read Holding Register Query obtains the contents of up to eight holding registers in the
addressed slave.
Note. The data start register must contain the offset address of the first register to be
accessed, e.g. to start at register 1 the data start register must contain 00 (00H).
Broadcast mode is not supported by Function Code 03.
Example. Read two holding registers from slave (01) starting at holding address 01 (process
variable input).
A
d
d
e r
s
s
F
u
n
t c
0
1
0
3
5.2.2 Read Holding Register Response
The addressed slave responds with its address and function code, followed by the information field.
The information field contains one byte describing the quantity of data bytes to be returned. Two
bytes are used to return each register requested, the first byte containing the high order bits and the
second the low order bits.
Example
PV input (two registers) – 270
PV decimal places – 1
A
d
d
e r
s
s
F
u
n
t c
o i
0
1
0
3
R
e
i g
t s
r e
O
o i
n
H
g i
h
0
0
H
o
d l
n i
B
y
e t
n
C
o
u
t n
H
g i
0
4
0
A
f
s f
t e
N
. o
f o
L
o
w
H
g i
h
0
0
0
0
g
R
e
i g
t s
r e
0
1
H
o
h
L
o
w
8
E
5 MODBUS FUNCTIONS...
R
e
g
s i
e t
s r
E
r r
L
o
w
0
2
C
d l
n i
g
R
e
i g
t s
r e
0
2
H
g i
h
L
o
w
0
0
0
1
r o
C
h
e
c
k
F
e i
d l
(
C
R
C
1
) 6
4
0
B
E
r r
r o
C
h
e
c
k
F
e i
d l
(
C
R
C
1
) 6
D
3
C
C
11
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