PLC Newbie - Simple Question

[RunRabbit]

Hi All,

I've just recently completed a basic training course on PLCs.

I've been playing around with Step7 from Siemens and I'm trying to perform a simple function but for the life of me can't seem to get it done.

Basically I have 8 outputs, let's say QB5.

I want to basically do the following:

Every 1 second cycle through turning on outputs 1-8. Only one output is on at any one time and I just cycle through all of them every second.

I am doing it in Ladder as I don't quite know STL yet.

Any ideas?

 

[L D[AR2P#0.0]]

How are you generating your 1 second timer - do you have edge detection so that the QB is only changed on, for example, the rising edge of the 1 second timer?

What approach are you taking - a simple state machine?

 

[RunRabbit]

Ok seems I'm out of my depth. I hadn't even considered what you're asking.

I know I needed a constant one second interrupt to initiate the change but that's about it.

I don't really need a solution moreso just some different methods of solving the problem.

 

[L D[AR2P#0.0]]

Are you using TIA portal or Simatic manager V5.x
Are you using a real plc (which) or plcsim

 

[RunRabbit]

I'm using Simatic Manager 5.5 and PLCSim.

 

[L D[AR2P#0.0]]

Here's a project with 3 examples for cycling bytes:

 

[RunRabbit]

Many thanks for that code mate. Loaded it up and it works fine. I'll try go through it and make sense of it.

I noticed in FB1 your one shot timer. I tried to follow it logically but I can't seem to work out how the Input is triggering the counter continuously. I read the help file on SFB4 and it requires input to go high to start. I can see how the first cycle will work but how does Input get taken high again on the second cycle?

 

[L D[AR2P#0.0]]

As the input to the timer is the timer output inverted, when the timer completes the timer input will be false causing the timer output to be false for one scan, the next scan the timer input will be true (as it is the timer output inverted) and hence the cycle repeats.