AB PLC scan time

Arun · Sep 22, 2011

We are using AB L35 processor in our new machine this is 4th machine we are selling and ready to ship. This is my new job. I observed some operation issues and discussed with our team and they have similar problem in older projects. My opinion is PLC scan time is more and we need to work on that either make good PLC program or may change CPU, so can anyone suggest the best way to monitor the PLC scan time.
Also I seen many AFI in the project and I am trying to clean the project. Any tips to reduce the PLC scan time are greatly appreciated.

dmroeder · Sep 22, 2011

You can monitor a task or program right in RSLogix5K by right clicking on either one and selecting properties. You will see a monitor tab.

The best way though is to install the optional tool called "Logix5000 Task Monitor"

MarkNightingale · Sep 22, 2011

Arun,

I'm not sure if you can get the scan time of the entire PLC code, but I do know that you can get the scan time of a specific program by using the Get System Value instruction.

I have attached a picture as an example.

Hope this helps

Mark

elmatador · Sep 22, 2011

In regards to scan time. Depends on how much of FBD, ST etc you are using. Ladder would have the fastest scan time of them all. Please don't tell me you are using Rockwell Alarm Blocks??

jkerekes · Sep 22, 2011

elmatador said:
In regards to scan time. Depends on how much of FBD, ST etc you are using. Ladder would have the fastest scan time of them all. Please don't tell me you are using Rockwell Alarm Blocks??

Why? I haven't really used many of them. Except for their memory footprint, what are the issues you've seen?

elmatador · Sep 22, 2011

jkerekes said:
Why? I haven't really used many of them. Except for their memory footprint, what are the issues you've seen?

It depends on how much of the functionality of the Rockwell Alarm Block you are using. We tested Rockwell Alarm blocks as compared to our own ladder and we were able to cut the scan time to 1/4. Yes we didn't have the PLC time stamping the data and we had to map the tags to the HMI but it gave us a much faster scan time.

If its a tiny little system with a small I/O count then it won't really matter. I am talking about 14 17-slot controllogix racks.

In Arun's case it might not even be a problem because he is using a L32E, I can see it having little I/O.

allscott · Sep 22, 2011

elmatador said:
It depends on how much of the functionality of the Rockwell Alarm Block you are using. We tested Rockwell Alarm blocks as compared to our own ladder and we were able to cut the scan time to 1/4. Yes we didn't have the PLC time stamping the data and we had to map the tags to the HMI but it gave us a much faster scan time.

If its a tiny little system with a small I/O count then it won't really matter. I am talking about 14 17-slot controllogix racks.

In Arun's case it might not even be a problem because he is using a L32E, I can see it having little I/O.

Wow can I ask what you were controlling?

TConnolly · Sep 22, 2011

elmatador said:
In regards to scan time. Depends on how much of FBD, ST etc you are using. Ladder would have the fastest scan time of them all.

Not so. ST is faster, and for math intensive operations it blows ladder out of the water.

bce123 · Sep 22, 2011

My opinion is PLC scan time is more and we need to work on that either make good PLC program

what is your current scan time? do you have high speed I/O application or servo moves ect...? can you post your logic?

OZEE · Sep 23, 2011

Also - what scan time do you need?

elmatador · Sep 23, 2011

Alaric said:
Not so. ST is faster, and for math intensive operations it blows ladder out of the water.

Not sure which PLC platform you are talking about. If its RSLogix5000 based, then sir I challenge you to a duel until death!

Go to RSLogix5000, click on Help, then Online Books. On the left side select Instructions, then select the document on the far right called "Logix5000 Controllers Instruction Execution Times and Memory Use"....the answer for you will be in Black and White.

elmatador · Sep 23, 2011

allscott said:
Wow can I ask what you were controlling?

Racks included a massive gas processing facility and eight 3.3MW generators I/O.

Ron Beaufort · Sep 25, 2011

We are using AB L35 processor ... I observed some operation issues and discussed with our team and they have similar problem in older projects. My opinion is PLC scan time is more ... Any tips to reduce the PLC scan time are greatly appreciated.

DISCLAIMER: all of the following material relies on ASSUMPTIONS - which are based on the small amount of information that you have posted so far ...

naturally we can't tell for sure from what you've told us so far, but your problem MIGHT NOT be related to "scan time" at all ...

since the COMPACT-logix and the CONTROL-logix platforms are both programmed using the RSLogix5000 software, it's easy to assume that the two platforms operate identically "under the hood" ... they do not ...

the COMPACT-logix has an unusual/weird scan sequence when compared to all other Allen-Bradley platforms – particularly when it comes to updating the real-world inputs and the real-world outputs ...

moreover, much of the official documentation for the COMPACT-logix platform contains serious mistakes regarding the I/O update sequence ...

secret handshake: unlike the CONTROL-logix platform, the COMPACT-logix uses a "dedicated task" (read: "invisible task") which handles the I/O update operations ... this dedicated task has a priority of SIX ...

so ...

(1) suppose that some of your other tasks have been given priority assignments HIGHER THAN SIX (specifically, given LOWER numbers) – then it's possible that those tasks MIGHT BE interfering with the I/O task's ability to move input and output signals between your processor and the real-world field devices ...

or ...

(2) suppose that some of your other tasks have been given priority assignments LOWER THAN SIX (specifically, given HIGHER numbers) – then it's possible that those tasks MIGHT NOT BE able to execute properly since the I/O task MIGHT BE interfering with their timing ... (in most systems the first scenario is more likely than this one) ...

in either case, the symptoms might indeed APPEAR to be related to "scan time" issues – and in other Allen-Bradley systems that would probably be the actual cause ... but ... due to the way the COMPACT-logix system uses that special "dedicated task" to handle its I/O update, it is possible that the same symptoms MIGHT BE related to "task priority assignment" issues instead ...

suggestion: post your complete ACD program file here and let us take a look at it (you'll have to ZIP it first) ... it certainly might turn out that these ideas have NOTHING to do with your problem at all ...

but ... (and here's the tricky part) ...

IF (that's a big IF) your problems are indeed related to your "task priority" settings – then NOTHING that you do to improve your "scan time" is going to solve your problems ...

suggestion: read the material in the following links – and pay particular attention to the "tip" in the figure attached below ...

http://www.plctalk.net/qanda/showthread.php?p=438401&postcount=13

http://forums.mrplc.com/index.php?showtopic=21633&view=findpost&p=104275

good luck with your project ...

.

daba · Sep 25, 2011

Good info Ron, as always

I learnt a lot from that, good to see you back.

DamianInRochester · Sep 25, 2011

moreover, much of the official documentation for the COMPACT-logix platform contains serious mistakes regarding the I/O update sequence ...

Have these errors been assembled in a list or an addendum by AB? I use the Compact Logix extensively so this is of great concern to me.

AB PLC scan time

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