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5 MODBUS FUNCTIONS
This section shows typical examples of Modbus function codes 01, 02, 03, 04, 05, 06, 08, 15, 16 and 65.
5.1 Read Coil Status – Function Code 01
5.1.1 Read Coil Status Query
This function allows the user to obtain the ON/OFF status of logic coils within the addressed slave only. Broadcast mode is not
supported with this function code. In addition to the slave address and function fields, the message requires the initial coil offset
address to be read (starting address) and the number of locations to be interrogated.
Note. The coil offset address is the coil number minus one, e.g. to start at coil 21 the data start value must be set to 20
(14H).
– a read coil status request to read 12 coils from slave (01) starting at coil 21 (alarm A status) is shown below.
Example
A
d
d
e r
s
s
F
u
n
t c
o i
0
1
0
1
5.1.2 Read Coil Status Response
The data is packed one bit for each coil (1 = active, 0 = inactive). The response includes the slave address, function code, quantity
of data characters, the data characters and error checking. The low order bit of the first character contains the first addressed coil
and the remainder follow. For coil quantities that are not even multiples of eight, the last characters are filled in with zeros at high
order end.
– the response to the read coil status query shows the following:
Example
Alarm A status
– active
Alarm B status
– acknowledged or inactive
Alarm C status
– active
Alarm D status
– acknowledged or inactive
Alarms E to M status – acknowledged or inactive.
A
d
d
e r
s
s
F
u
n
t c
0
1
0
5.2 Read Input Status – Function Code 02
5.2.1 Read Input Status Query
This function allows the user to obtain the ON/OFF status of logic inputs within the addressed slave. Broadcast mode is not
supported with this function code. In addition to the slave address and function fields, the message requires the initial input offset
address to be read (starting address) and the number of locations to be interrogated.
Note. The input offset address is the input number –1, e.g. to start at input 01 the data start value must be set to 00.
– a read input status request to read 16 inputs from slave (01) starting at input 01 (module A digital input 1) is shown
Example
below.
A
d
d
e r
s
s
F
u
n
t c
o i
0
1
0
2
C
o
l i
S
a t
t r
C
o
n
O
f
s f
t e
H
g i
h
O
f
s f
0
0
o i
n
B
y
e t
C
o
u
t n
1
0
2
n I
p
t u
S
a t
t r
n I
p
t u
n
O
f
s f
t e
H
g i
h
O
f
s f
0
0
l i
S
a t
t r
N
u
m
b
r e
f o
t e
L
o
w
C
i o
s l
H
g i
h
1
4
0
0
D
a
a t
C
o
l i
S
a t
u t
s
D
a
a t
C
o
2
1
o t
2
8
2
9
0
5
0
S
a t
t r
N
u
m
b
r e
f o
t e
L
o
w
n I
p
u
s t
H
g i
h
0
0
0
0
N
u
m
b
r e
f o
E
r r
r o
C
h
e
c
C
i o
s l
L
o
w
0
C
7
C
l i
S
a t
u t
s
E
r r
r o
C
h
e
c
k
F
o t
3
6
0
B
A
N
u
m
b
r e
f o
E
r r
r o
C
h
e
c
n I
p
u
s t
L
o
w
1
0
7
9
k
F
e i
d l
(
C
R
C
1 -
) 6
0
B
e i
d l
(
C
R
C
1 -
) 6
A
C
k
F
e i
d l
(
C
R
C
1 -
) 6
C
6
9
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