PLC Scanning

gimli · Oct 29, 2018

Hello all,

Was wondering if someone could clarify...I understand that
plc logic is scanned top to bottom, left to right. How about at the end of a rung that has multiple outputs? Do the outputs get executed at the same time or top to bottom also?
thanks for any input

roxusa · Oct 29, 2018

Top to Bottom

gimli · Oct 29, 2018

thanks for clarifying !

Aabeck · Oct 29, 2018

In RSLogix double click on the line number, where your screenshot shows the highlight and it will show the text version of the line. This will let you know exactly the order the items in a line is scanned

Your line would be:

BST ADD PRESSURE P_OFFSET PRESSURE NXB ABS PRESSURE PRESSURE_ABS BND

EDIT: If you are doing online edits, the first double click will start the line edit, the next double click will show the text.

Mickey · Oct 29, 2018

Just to be picky...

Execute Program:
PLCs generally will solve the logic from left to right, top to bottom. The output of the rung before is available for the next rung.
This is like some of the popular bands of PLCs like Mitsubishi, Allen Bradley, Siemens, Omron, Automation Direct, etc. There are some exceptions like older Modicon(SCADAPACK) models which solve the logic top to bottom, left to right. Always refer to the manufactures manual to ensure the program execution method.

https://www.google.com/search?q=plc.....69i57j33.20461j0j8&sourceid=chrome&ie=UTF-8

gclshortt · Oct 29, 2018

https://accautomation.ca/understanding-the-plc-program-scan/

Note:
The above works with a synchronous PLC scan. Controllers like Allen Bradley CompactLogix using the RSLogix 5000 software use asynchronous I/O scan. This means the that the updating of I/O will happen whenever it can. Usually you would assign the IO to internal memory bits to ensure the updates happen when you expect it.

Regards,

PLC Scanning

gimli

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roxusa

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gimli

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Aabeck

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Mickey

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gclshortt

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