74
FX
3U
/FX
3UC
Series Programmable Controllers
Programming Manual - Basic & Applied Instruction Edition
4 Devices in Detail
4.2 I/O Relays [X, Y]
4.2.3
Operation timing of input relays
The PLC executes sequence control by repeatedly executing the following processing procedure. In such
batch I/O method, there are not only driving time of input filters and output devices but also response delays
caused by operation cycles. (Refer to Section 6.3.)
Input processing
Input
t
e
rm
inal
Input
Image
Memory
1) Read-in
X000
X001
X002
Program processing
Device
Image
Memory
Output processing
Out
put
t
e
rm
inal
Output
Latch
Memory
6) Output
Y000
Y001
Y002
X000
M 0
4) Read-
out
Y000
Auxiliary
relay
2) Read-out
Repeated oper
at
io
n
[T
he t
im
e
r
equi
red f
o
r a
cyclic
oper
at
io
n i
s
c
a
lled op
er
at
ion
cycle
(s
c
an t
ime)
.]
Y000
Input processing
Before executing a program, the PLC reads the ON/
OFF status of all input terminals inside the PLC into
the input image memory.
Even if inputs change while the program is
executed, the contents of the input image memory
remain unchanged, but the changes in inputs are
read during the input processing in the next cycle.
Even if an input contact changes from ON to OFF or
from OFF to ON, its ON/OFF status is judged after
the response delay (approximately 10 ms) caused
by the input filter.
(When the input filter of an input terminal is the
digital type, its value can be overwritten by a
sequence program.)
Program processing
The PLC reads the ON/OFF status of each device
from the input image memory and other device
image memories according to the contents of
instructions in the program memory, executes
operations in turn from the step 0, and then writes
the operation result to the image memory at every
time.
Accordingly, the contents of the image memory of
each device change as the program is executed.
The operation of a contact inside an output relay is
determined by the contents of the output image
memory.
Output processing
When execution of all instructions is finished, the
ON/OFF status of the image memory of outputs (Y)
is transferred to the output latch memory. This is
actual output of the PLC.
Contacts for external output inside the PLC operate
after the response delay time of output devices.
The above method is called batch I/O method (or refresh method).
1) 2) 3) .... 6) indicate the processing order.
3) Write
5) Write
