Circuits Which Cannot Be Programmed And Countermeasures; I/O Processing And Response Delay - Mitsubishi FX3G Series Programming Manual

FX /FX /FX /FX
3G 3U 3GC 3UC
Programming Manual - Basic & Applied Instruction Edition
6.2.3

Circuits which cannot be programmed and countermeasures

1. Bridge circuit
A circuit in which the current flows in both directions should be changed as shown in the figure on the right (so that a
circuit without D and a circuit without B are connected in parallel).
A B
E
C
D
2. Coil connection position
• Do not write a contact on the right side of a coil.
• It is recommended to program a coil between contacts first.
The number of steps can be saved when a coil (E) between the contacts A and B is programmed first.
A B
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 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
Series
F
D
C
E
OFF
Output processing
6 What to Understand before Programming
6.3 I/O Processing and Response Delay
C E B
A
A E D
C
A
B D
(1) [Input
processing]
Input image
memory is read
(2) [Program processing]
Scan time
Image memory of
(operation
each device is
cycle)
Batch I/O method
(refresh method)
This "input ON" can be received
"Input ON" cannot
be received
ON ON
Program Program
processing processing
Input processing
( Time)
F
E
C
The ON/OFF status of input
terminals is received at one time.
Input image is read,
and operation is
executed according
to program.
updated
(3)
[Output processing]
Output
Result is
transferred to
devices
output latch
are driven
memory
This "input OFF"
cannot be received
OFF
Program Program
processing processing
Operation cycle
165
1
2
3
4
5
6
7
8
9
10