Update/evaluation of internal relays/contacts...PLEASE HELP

More Modicon

A 984 type Modicon programm can also be divided into segments which may be scheduled more than once per scan. The inputs are read just before the segment is executed and the outputs are written just after the segment is executed. This means that I/O is being updated continously and synchronously, even while executing ladder in the next or previous segment. This just-in-time I/O is superior to most other PLCs have to offer. I have been told that the Modicon IEC programming does not support this feature. Is that a step forward?

For anyone born in North America, the "normal" PLC scan just makes more sense then modicoms.

1> (Input Update): Find out what's happening with the equipment that's being controlled
2> (User Program Scan): Make decisions based on the above
3> (Output Update): Send new commands to the controlled equipment
4> Go back to step 1

When making decisions (2>User Program), stick with the way we do things in North America. When we write, we start at the top of the page and go from left to right. (some people think modicom has a better idea! don't include me!)

Most people would normally place conditions first, followed by actions:
(condition) IF start button is pressed (then action) Motor = On

This works pretty well with any programming language:
If Start=ON then Motor=ON else Motor=OFF
although the following works just as well, but it would break the rules:
Motor=Start (action) = (condition) = disallowed in Ladder logic

The PLC reads your program just as you would. Start at the top and read each line from left to right. Decide what actions have to be taken, make note of them, and when finished, go to step 3 and send your noted actions to the equipment. Now either go for lunch or start over with step #1.

Modicon

Peter,
The segment scheduler is a function of the Modicon S908 remote I/O system, not the programming software/language.

Bill,
Modicon does follow the normal PLC scan, as you described it. Every PLC has an asynchronous method for servicing remote I/O that differs from its "normal" I/O update.

Everyone,
FYI, the Modicon 984 ladder logic method of top to bottom, left to right ladder processing is really no major problem. It can only effect, at most, 7 consecutive rungs of logic (based on 1 line of logic per rung), after that you are into the next network. It is possible to develope progams with only one rung of logic per network and they would scan identical to any other left to right, top to bottom PLC.

Posted by Ron Beaufort:


LADDER1 :
| I:1 O:1
|---||-------()---------
|
| O:1 O:2
|---||-------()---------


Suppose that I:1 is OFF - for many scans. Both O:1 and O:2 are also OFF during these scans. Then suppose that I:1 comes ON for "this" scan. Both O:1 and O:2 will be ON at the end of "this" scan.

Click to expand...

Viswesh:
Just to show how important rung placement can be. See the following examples.

LADDER2 :

| O:1 O:2
|---||-------()---------
|
| I:1 O:1
|---||-------()---------
|

Suppose that I:1 is OFF - for many scans. Both O:1 and O:2 are also OFF during these scans. Then suppose that I:1 comes ON. O:1 will be turn ON at the end of "this" scan. On the next scan O:2 will also turn on.

LADDER3 :

| I:1 O:2 O:1
|---||-----]/[-------()---------
|
| I:1 O:2
|---||---------------()---------

Suppose that I:1 is OFF - for many scans. Both O:1 and O:2 are also OFF during these scans. Then suppose that I:1 comes ON. Both O:1 and O:2 will be ON at the end of "this" scan. On the next scan O:1 will turn off. This is the logic for a one shot.