IAI America PSEP - Manual (Page 18)

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Output
Process
Input
Process
6. Transference of PIO Signal between Controllers
Please note the following when conducting transference of PIO signal between controllers.
To certainly transfer the signal between controllers with different scan time, it is necessary to
have longer scan time than the one longer than the other controller. To ensure to end the process
safely, it is recommended to have the timer setting more than twice as long as the longer scan
time at least.
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PLC
(e.g. scan time is 20msec)
Also, if one tries to read the signal that is being re-written by the other, the signal may be read wrongly.
Make sure to read the signal after the rewriting is complete. (It is recommended to have more than 2
scan periods to wait.) Make sure not to have the output side to change the output until the other side
completes the reading. Also, a setting is made on the input area not to receive the signal less than a
certain time to prevent a wrong reading of noise. This duration also needs to be considered.
This controller
(scan time 1msec)
As shown in the diagram, the input and output
timings of two devices that have different scan
time do not match, of course, when transferring
a signal.
There is no guarantee that PLC would read the
signal as soon as this controller signal turns on.
In such a case, make the setting to read the
signal after a certain time that is longer than the
longer scan time to ensure the reading process
to succeed on the PLC side.
It is the same in the case this controller side
reads the signal.
In such a case, it is recommended to ensure 2
to 4 times of the scan time for the timer setting
margin.
It is risky to have the setting below the scan
time since the timer is also processed in the
scan process.
In the diagram, PLC can only read the input
once in 20msec even though this controller
output once in 1msec.
Because PLC only conducts output process
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same output status for that while.