Scheduler - Modicon using Proworx 32

PeterW

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Jun 2006
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Not used proworx for awhile and just trying to reacquaint myself with its little oddities.

I'm working with an existing system which has multiple segments scheduled, all set to continuous, but only the first few actually contain code.

I was a little confused why this was done and then realised that the number of segments scheduled matched the number of drops for I/O points.

So every Drop has a segment scheduled where it is the Drop in and drop out. Even where the drop point only has input cards, its included on drop in and drop out.



I'm sure this is not needed as I thought (from past history) that the I/O is updated at the start and end of the program, just like most other PLC's. I thought the drop in and drop out fields in the schedular were if you needed to specifically update a drop during the program, and then it could be different on the in and out depending on where the inputs and outputs are located.

In most cases I thought the drop in and drop out would be left blank....

Am I correct or am I going crazy? .... Both are possible.
 
I just looked at the Modicon Ladder Logic Block Library User Guide, 840USE10100, Version 2.0. This states that the number of segments should be greater than or equal to the number of I/O drops. Jobs I've done typically had and extra segment that contained all subroutines; this segment is not scheduled as continuous in the segment scheduler.

It's a bit quirky, but I've found the instruction set to be pretty rich.
 
It may also have been done for communications servicing. Comms are serviced in each 'inter segment delay'. The more segments the more comms per scan
 
PeterW said:
I'm sure this is not needed as I thought (from past history) that the I/O is updated at the start and end of the program, just like most other PLC's. I thought the drop in and drop out fields in the schedular were if you needed to specifically update a drop during the program, and then it could be different on the in and out depending on where the inputs and outputs are located.

Modicon 984LL does not update all I/O at the beginning of each scan, instead it updates the I/O associated with each segment at the begging of each segment. Depending on how the segment scheduler is used you can update some of the I/O multiple times within one scan of a program. There does not need to be any logical relationship between segment I/O scheduling and I/O located at any particular drop. This was very helpful in the old days when processors were slow, with newer processors this feature is rarely used. Most programmers now associate all of their I/O with the first segment.
 
Jim Dungar said:
Most programmers now associate all of their I/O with the first segment.

That's interesting, how do they do that. The drop down box only allows you to select one, which is why I presume we have 10 segments and only 3 used (we have 10 I/O drops).
 
It actually depends on the remote I/O structure. If you are using the traditional "S908" style RIO, you must have one segment per drop even if there are no networks in those segments. Each I/O must be assigned to at least one segment or else it will not be scanned.

My point, was that today's programmers rarely take advantage of I/O scheduling and program segmenting in order to effectively decrease scan time and increase throughput.
 
Peter,

Here's an example if it's any help.

This is done on Modsoft but the principle is the same for any 984LL program.

There are 2 I/O drops.
The program logic was split into 10 segments.

The I/O physically present on Drop 2 was used in Segments 2, 3 & 4 so Drop 2 inputs were read at the start of Segment 2 and Drop 2 outputs written at the end of Segment 4.

The I/O physically present on Drop 1 was used in Segments 5, 6 & 7 so Drop 1 inputs were read at the start of Segment 5 and Drop 1 outputs written at the end of Segment 7.

984LL_scheduler.jpg
 

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