S
There seems to be a bit of a debate about sequential versus non sequential I/O scans.
I thought I would step in and explain existing practice here.
In the majority of modern PLC systems the application program sees a unchanging view of the inputs as they were at the start of the scan, and
sees the outputs as they have been modified by the application program.
That is an output that was off at the start of the scan, but turned on during the scan will appear to be on to instruction later in the scan.
This is accomplished by having an I/O update task that runs in parallel with the logic solving engine. At the beginning of a scan the logic solving engine makes it's own copy of the I/O image table, and uses this for the remainder of the scan. Thus you get the dual benefit of a consistent view of the inputs for the application program, and allowing input scanning, and the setting of outputs to run in parallel, thus not delaying the logic solving engine.
I think we should adopt this technique.
--
Stan Brown [email protected] 843-745-3154
Westvaco
Charleston SC.
--
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I thought I would step in and explain existing practice here.
In the majority of modern PLC systems the application program sees a unchanging view of the inputs as they were at the start of the scan, and
sees the outputs as they have been modified by the application program.
That is an output that was off at the start of the scan, but turned on during the scan will appear to be on to instruction later in the scan.
This is accomplished by having an I/O update task that runs in parallel with the logic solving engine. At the beginning of a scan the logic solving engine makes it's own copy of the I/O image table, and uses this for the remainder of the scan. Thus you get the dual benefit of a consistent view of the inputs for the application program, and allowing input scanning, and the setting of outputs to run in parallel, thus not delaying the logic solving engine.
I think we should adopt this technique.
--
Stan Brown [email protected] 843-745-3154
Westvaco
Charleston SC.
--
_______________________________________________
LinuxPLC mailing list
[email protected]
http://linuxplc.org/mailman/listinfo/linuxplc