I am trying to do a very simple round-robin to run devices in order.   I think the simple test program that I have made looks like it should work, but in the simulator it fails if more than one device has demand.  The idea is that there are three devices.   Each device has a demand input.  If the device has demand then as long as neither of the other two devices are running then that device starts up and runs for a pre-determined time and then stops, regardless of the state of its demand input.   Now the next device has a turn, and the next device, etc.

I would LOVE to add the program as an attachment but apparently .LLD is not a valid extension.

The problem I am having is that while the program works perfectly if any one device has demand, and even works perfectly if I turn on the demand for one device, then turn the demand off again so it continues running until the timer runs out, if I have more than one demand input turned on then after the first one finishes its turn it then rapidly switches between the two devices with demand.   The speed is high enough that it appears at first glance that both are turned on.

I have tried moving a few things around, thinking it was a code sequence issue, but no luck.

Any thoughts?

I'd settle for information regarding how to post a ladder logic program, or how to post under my name instead of 'AutoGen'.

Mike

Edit:  Program is now attached.   Simulate and turn on two or three of the inputs and you will see what happens after the 5 second timer runs out.

AutoGen_7634532

I would LOVE to add the program as an attachment but apparently .LLD is not a valid extension.

..

I'd settle for information regarding how to post a ladder logic program, or how to post under my name instead of 'AutoGen'.

Hi Mike.

Simply put the project file in a ZIP file. 

There are some issues with changing the forum name. First make sure the name is set in your account configuration. Then also mention here which nickname that is. We will ask the Moderator to have a look to help you out.

Personal advice -- Better to use FBD for programming the LOGO! This is the most natural way for the processor to do its task.

Hi Mike,

You will find the option to edit your username under "personal data" when clicking on your name on the top right corner of the page.

Sometimes the username does not update automatically. In that case I will help you to activate it.

Kind regards, Moderator_Lan

qwazee

Personal advice -- Better to use FBD for programming the LOGO! This is the most natural way for the processor to do its task.

 This may be true, but I'm used to ladder logic and FBD is really.... alien to me.   I'm not a fan.   I don't program often PLCs often, but all I have used is ladder logic for 30 years or so.

ixo65

If you want to attach a LSC (equal FBD or LAD) then you must ZIP it and attach the ZIP file.

 Thank you!    I have added it to the original message, and added information about simulating the problem.

Mike in Canada

Thank you!    I have added it to the original message, and added information about simulating the problem.

Thanks for the code segment.

I am thinking that the type problem you are seeing has to do with the way the code is scanned. With the FBD it is easier to control the scan sequence. Here you cannot.

But let us say you had the code doing what you were expecting. The moment you have all 3 requests coming up, the code you have will not allow the 3rd request to go through. You will need to code a sequence handling. Have that handler in turn switch the outputs.

Pseudo coding idea:

1. Check if request made.
   Yes: do next step
   No: Check next request
   
   
2. Start motor.
   
   
3. Motor timed out AND (Other request or Not this request)
   Yes: do next step
   No: (keep motor on)
   
   
4. Stop motor.
   Check next request.

Moving from one step to the next step can be done in the following style:

  M14    Q1      M14  
--] [----]/[--+--(R)
              |
              |  M21
              +--(S)

 It really surprised me that I was unable to find a single example anywhere.   Regular sequencing I have done a bunch of times on different machines using M flags as 'latches' to specify what has been done already,, but this round-robin concept has just never come up, and I don't yet understand why it's so difficult.

ixo65

 In theory it may be quite simple.

But implementing this "simple method" into an executable control program is anything but easy and can't be implemented with just a few LAD networks.

qwazee

 But let us say you had the code doing what you were expecting. The moment you have all 3 requests coming up, the code you have will not allow the 3rd request to go through. You will need to code a sequence handling. Have that handler in turn switch the outputs.

Pseudo coding idea:

1. Check if request made.
   Yes: do next step
   No: Check next request
   
   
2. Start motor.
   
   
3. Motor timed out AND (Other request or Not this request)
   Yes: do next step
   No: (keep motor on)
   
   
4. Stop motor.
   Check next request.

Moving from one step to the next step can be done in the following style:

  M14    Q1      M14  
--] [----]/[--+--(R)
              |
              |  M21
              +--(S)

 

First off, let me say that I appreciate the response.

 If  I was writing this in any procedural language then what you put above would make perfect sense.   It doesn't seem to make sense to me for ladder logic, but it might be reminiscent of one of my earlier trials where I had something like:

[I1]---[/Q2]----[/Q3]------------(M1)

[I2]---[/Q1]----[/Q3]------------(M2)

[I3]---[/Q1]----[/Q3]------------(M3)

M1--------------------------------(Q1)

M2--------------------------------(Q2)

M3--------------------------------(Q3)

The result was the same, but maybe adding /Qx and /Qy on the Qn lines would help... another layer of only-one-at-a-time...  I will try that.