PLC Scan

Tweeker · Oct 3, 2011

Greetings all. My question is rather general in nature. Do brands of PLC's vary in their method scan execution? Do all major brands conform to the read inputs, solve logic, update outputs format?

Somewhere in the back of my mind I remember someone telling me that ControlLogix uses a slightly different method. If so, how is it different?

sthompson · Oct 3, 2011

Tweeker said:
Greetings all. My question is rather general in nature. Do brands of PLC's vary in their method scan execution? Do all major brands conform to the read inputs, solve logic, update outputs format?

Somewhere in the back of my mind I remember someone telling me that ControlLogix uses a slightly different method. If so, how is it different?

As a general rule yes. Most PLCs Scan right to left, top to bottom. The one exception I know of is the Modicon 484/984 PLCs that Scanned top to bottom, left to right.

Most PLCs Read the Inputs, Scan the Logic, Update the Outputs, take care of some Housekeeping, then repeat.

The ControlLogix line updates the I/O Asynchronously to the PLC Program Scan. They have a dedicated I/O Processor for the ControlLogix, and a hidden Routine for the CompactLogix, so the value of I/O can change during the scan, possibly more than once.

Stu....

Tweeker · Oct 3, 2011

sthompson said:
As a general rule yes. Most PLCs Scan right to left, top to bottom. The one exception I know of is the Modicon 484/984 PLCs that Scanned top to bottom, left to right.

Most PLCs Read the Inputs, Scan the Logic, Update the Outputs, take care of some Housekeeping, then repeat.

The ControlLogix line updates the I/O Asynchronously to the PLC Program Scan. They have a dedicated I/O Processor for the ControlLogix, and a hidden Routine for the CompactLogix, so the value of I/O can change during the scan, possibly more than once.

Stu....

Thank you. Where in the AB documentation can I find this information? I have tried to search the help files but come up empty.

sthompson · Oct 3, 2011

Start with this Manual.

http://literature.rockwellautomation.com/idc/groups/literature/documents/pm/1756-pm004_-en-p.pdf

Stu....

Clay B. · Oct 3, 2011

One of the things I use alot is the Online Books option in your Rs Logic 5000 software.

If you go to the Tools pull down menu you will see a function call Online Books. Click this and it will give you a list of links to Manauls for specific functions for Comapct/ Control Logix PLC's.

Working thru the quick start guide is a very good to get an idea how the Compact/Control Logix PLC handles things.

Mickey · Oct 3, 2011

Most PLCs Scan right to left, top to bottom. The one exception I know of is the Modicon 484/984 PLCs that Scanned top to bottom, left to right.

Add ControlMicrosystem's "SCADAPack" to this one.

Tweeker · Oct 3, 2011

Thanks all. Helpful information and tips.

GeoffC · Oct 4, 2011

Modicon 484/984 PLCs that Scanned top to bottom, left to right.

Not entirely true
This is the case within each 'network' (a network is an 11x7 matrix)
If you choose to have only 1 rung in each network then the scan is top to bottom.
That is each network is solved one after the other from top to bottom

PLC Scan

Tweeker

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sthompson

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Tweeker

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Tweeker

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