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1.3 Cautions on Creation of Fundamental Programs
16
FXCPU Structured Programming Manual
(Application Functions)
1.3 Cautions on Creation of Fundamental Programs
This section explains cautions on programming.
Refer to the following manual for cautions on structured programs and programming languages:
→ Q/FX Structured Programming Manual (Fundamentals)
Refer to the following programming manual for detailed operations of and cautions on devices and
parameters:
→ FX Structured Programming Manual (Device & Common)
1.3.1 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 processing (1) to processing (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 (REF) 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 "10 ms" of the input filter 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)) or more. However, the situation
can be improved by PLC special functions and applied instructions.
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
ON ON
OFFOFF
"Input ON" cannot
be received.
This "input ON" can be received.
This "input OFF" cannot
be received.
Input processing
( Time)
Output processing
Operation cycle
Program
processing
Program
processing
Program
processing