Bullet Proof Sequence

I have used a few different ways to program sequencial events in PLC's. Is there a rock solid way to make sure your sequence gets followed? I'm doing a dozen or so steps most of the time in either AB or AD plcs... I don't program everyday as I wear many hats so I tend to forget if one works better than the other must me sometimers LOL...

I use:

Do task 1
When task 1 is done do task 2
When task 2 is done do task 3
...

or:

If x=1 do task 1
When task 1 is done change x to 2
If x=2 do task 2
When task 2 is done change x to 3
...

There always seems to be something that breaks the first way after you start adding to the code...

Just looking for ideas and preferences.

Thanks
John

I use the second method and go by tens, then if a new step needs to be added there is room to insert it and keep things in order.

I like the second example because it makes it easy to backup the logic in the event of a fault or jam. The best method is the one you understand best and of course KeepItSimple. Redundant rungs and contacts do nothing but get in the way.

I like the idea of 10's. Do you use an ADD or just do a MOVE to step?

Thanks
John

Here's what I do:
http://www.plctalk.net/qanda/showpost.php?p=179819&postcount=7

It's worked well for me for many years and now is how my brain thinkgs of programming sequences.

Hi Norm,

Thanks for the example. That is a good way too. I usually forget to do the and not step#x and that is where I fall short I think.

Thanks
John

Hello John,
If your AB PLCs support it (pretty sure AD doesn't), why not use Sequential Function Chart.
That way, the process has to be followed.

Failing that, I would go method 2.

Hey John,

That link that Norm posted was very good, when you get a chance read the whole thread http://www.plctalk.net/qanda/showthread.php?t=25856

I was asking the same question that you are after, there are other good examples in that thread

With the AD's have you looked at the drum sequence instruction? I have not but I think it may be what we are after???

I do a lot of material moving/conveying, I can dig up a few examples if you want, I have several other programmers that do work for me and they all do it different

One uses a pass down bit and he will pass that bit to the next rung for the next sequence

Thanks for the link to the entire thread. I have used the way Steve Baily describes in the other thread in the past and forgot about it...

Here is a framework for a generic sequence logic. Each step of the sequence requires at least two rungs like these. In addition, you may need to add rungs to define the "Step Trigger Condition" to pas from one step to another. A typical Step Trigger Condition might be some time after a limit switch closure. One of the steps will also need to be defined as the "Home" step and the logic for that step will need to have a "Reset" contact in parallel with the sealing contact.


Previous This Step Next Step General

Step Trigger Trigger Permissive This Step

--] [---------] [----+------]/[---------] [----------( )-

|

|

|

This Step |

--] [----------------+



Next Step

This Step Trigger Next Step

This Step Trigger Condition Trigger

--] [---------]/[----------] [----------( )-



The framework can easily be expanded to include sequences that might need to run in reverse or to accomodate single stepping.

Click to expand...

I had the program up and running in no time and with no issues.

Thanks
John