143
FX
3U
/FX
3UC
Series Programmable Controllers
Programming Manual - Basic & Applied Instruction Edition
6 What to Understand before Programming
6.3 I/O Processing and Response Delay
1
In
troduct
io
n
2
Ov
e
rv
ie
w
3
In
st
ru
ct
ion
Li
st
4
D
evi
ce
s
in
De
ta
il
5
Speci
fied t
h
e
D
evi
ce &
C
onst
a
nt
6
Bef
o
re
Pr
ogr
a
m
m
ing
7
Bas
ic
In
st
ru
cti
o
n
8
FNC0
0
-FNC0
9
P
ro
g
ra
m F
lo
w
9
FNC1
0
-FNC1
9
M
ov
e
&
C
om
par
e
10
FNC
20-
FNC
2
9
Ar
ith.
&
Logi
c
O
per
at
io
n
6.3
I/O Processing and Response Delay
1. Operation timing of I/O relays and response delay
FX PLCs execute the I/O processing by repeating
the process (1) to process (3).
Accordingly, the control executed by PLCs contains
not only the drive time of input filters and output
devices but also the response delay caused by the
operation cycle.
Acquiring the latest I/O information
For acquiring the latest input information or
immediately outputting the operation result in the
middle of the operation cycle shown above, the I/O
refresh instruction is available.
2. Short pulses cannot be received.
The ON duration and OFF duration of inputs in PLCs require longer time than "PLC cycle time + Input filter
response delay".
When the response delay of the input filter "10 ms" is considered and the cycle time is supposed as "10 ms",
the ON duration and OFF duration should be at least 20 ms respectively.
Accordingly, PLCs cannot handle input pulses at 25 Hz (1000 / (20 + 20) = 25) or more. However, the
situation can be improved by special functions and applied instructions of PLCs.
Convenient functions for improvement
By using the following functions, PLCs
can receive pulses shorter than the
operation cycle:
• High speed counter function
• Input interrupt function
• Pulse catch function
• Input filter value adjustment function
[Input
processing]
Input image
memory is read
[Output processing]
Result is
transferred to
output latch
memory
The ON/OFF status of input
terminals is received at one time.
Input image is read,
and operation is
executed according
to program.
Output
devices
are driven
Batch I/O method
(refresh method)
(1)
(3)
Scan time
(operation
cycle)
[Program processing]
Image memory of
each device is
updated
(2)
Program
processing
Program
processing
Program
processing
Program
processing
ON
ON
OFF
OFF
"Input ON" cannot
be received
This "input ON" can be received
This "input OFF"
cannot be received
Input processing
( Time)
Output processing
Operation cycle
