Smart student question.

Bill from NH

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B
Hi all expert.

I have this project in school but am having some problems to actually do the program.

I will not mention the exact nature of it for having read the past posts, I know it will not be welcomed.

Here is what I have done.

I have made a list of all the Inputs and Outputs.
I have written what I beleive the sequence should be.
I have grouped my sequences into 4 sections.

My main concern here is WHERE THE HECK DO I START?

The sequence is an ongoing thing. On and on and on.

How will I decide the first rung?
 
Starting point

Initialize the system and check ALL system SAFE options, Estop, tensions, overtorque, things along these lines. Make SURE the system is SAFE to operate before starting ANYTHING. SAFE is for HUMANS, warnings and cautions are for EQUIPMENT.
 
Actually, you can probably just pick a step of the sequence at random and start there. If I'm understanding your application, it's a continuous sequence, so eventualy you'll come to a step that leads back to your initial step.

If it's a real system, then select the step where the machine is ready to be started from following power-up.

Remember, the PLC will continuously execute its program. When it reaches the last rung, it will send fresh data to the outputs, read the latest status of the inputs, and go back to the first rung to start over. For someone accustomed to programming in Visual Basic or C, this can be a tricky concept to get straight in your head.
 
This guy seems to have done his homework - about US anyway...

For me, I usually start with the system that I understand best. Typically this is some motor somewhere.

When the entire program is eventually written, in essence "all that the program does" is turn on/off outputs. So I start with those.

I start with the output drawn out in space, on a peice of paper or whiteboard. Some programming software packages let you do this also:

|
| MOTOR
|---- ---( )
|
|



Then I look over all the sequences, and ask the two questions:
When does it start?
When does it stop?

The sequence may say that it starts when the START PB is pressed, but not if a second motor was started within the last 30 seconds, and stays running, stopping only when the STOP button is pressed.

So I program that:

| MOTOR2 STARTED
| START WITHIN 30 SEC STOP MOTOR
|------| |---------|/|------+-----| |-------( )
| |
| MOTOR |
|------| |------------------+



But now I've got a loose end - there's nothing driving the "MOTOR2 30 sec" bit. So I program that, which will then have a loose end of needing to drive the MOTOR2 bit, and so on.

Eventually all the loose ends get tied up.

In programming sequences, it's the same thing.
When does Valve X open? - When the sequence is on Step 2.
When is the sequence on Step 2? - When it is on Step 1, and condition XYZ happens.

You just take it step by step, and don't be affraid to use lots of internal bits.

Just make sure that you give them a clear expanation of what they do. One good rule of thumb is that every description should include a verb.

It shouldn't be START or even START PB, but START PB IS PRESSED above. That way, when you read STOP PB IS NOT PRESSED on the rung above, you know that the STOP PB is wired Normally Closed, and pressing it interrupts current flow, resulting in a 'zero' in the I/O image table.

Another technique that's useful is to make the rung comment about what you intend the rung to accomplish BEFORE you program the rung. Nothing clarifies thinking better than trying to write Complete Sentences.
 
Last edited:
Allen Nelson- Thanks....I owe you.

In my past dealings, I adopted the thought that 90% of the task is done in 10% of the time. It is the last 10% of the job that takes 90% of the time. That is until PLCs came around. Every time I've started a new project, I was met with about an hour of brain-lock in the start up of the ladder design. I've recently learned another tid-bit...."you learn best from those who do." I wander what that sounds like in Latin?
 

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