Operation Timing Of I/O Relays - Mitsubishi Electric MELSEC-Q Structured Programming Manual

FXCPU Structured Programming Manual
[Device & Common]
2.2.3

Operation timing of I/O relays

The PLC executes sequence control by repeatedly executing the following processing procedure.
In this batch I/O method, not only are there driving times of input filters and output devices but also response
delays caused by operation cycles.
Input processing
1)
Read-in
X000
X001
X002
2) Read-out
X000
Y000
Auxiliary relay
Repeated
operation
[The time
required for a
cyclic operation
is called
operation cycle
(scan time).]
The above method is called the batch I/O method (or refresh method).
40
Input
image
memory
Program processing
Y000
3) Write
4) Read
Device
-out
M 0
image
5) Write
memory
Output processing
6) Output
Output
latch
memory
1) 2) 3)...6) indicate the processing order.
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 PLC is executing
the program, 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 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 sequence from the step 0,
and then writes the operation result to the image memory
at each 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
Y000
the latch memory. This is the actual output of the PLC.
Y001
External output contacts inside the PLC operate after the
response delay time of the output devices.
Y002
2 Devices in Detail
2.2 Input/Output Relays [X and Y]