Few newbie questions

[Squarewave]

I've been using my laptop, but now I've downloaded RSLogix micro and installed it on my desktop, as the licence is a free issue I'm guessing you could put it on every computer you use if you wanted?

 

[Squarewave]

Hi all.

I've been making some way by using this video


I'm wondering if someone could look at something for me when you get a chance. If you were to go to 2h56m and look at the latching contacts using binary bits as memory, I'm not sure if I'm missing something as I've copied what she's done, I can do the following;

Button press - output on
Button press - output off
Button press, some of the bits change but I can't get the output on again.

Is there something I'm missing, as really this latch is supposed to be for on/off/on/off with the one button isn't it?

Thanks.

 

[plvlce]

I can't watch the video right now to see what is said, but I would expect this logic to function as you describe (on then off, staying off).

It looks to me like this is intended to demonstrate how latching bits in sequence works with OTEs:
1. When you first push the button M0 becomes true, and is then latched in permanently*.
2. When the button is released M1 becomes true and is latched until M0 becomes false (which can't happen via the logic)
3. When the button is pushed a second time M2 becomes true and is latched until M1 becomes false (which only happens if M0 becomes false, which can't happen)
4. The output is on when M0 is true and M2 is false (see above steps to determine that this becomes the case at 1 and stops being the case at 3)
5. Thereafter pushing the button will result in I:0/0 changing state but nothing else, as all the internal bits are stuck on.

* 'permanently' in this case meaning until you either toggle it off manually or the processor leaves and re-enters run and the prescan turns it off.

Compare what happens if you add an XIO for M2 to the beginning of rung 0 and ask yourself why the result is different.

 

[drbitboy]

Hi all.

I've been making some way by using this video


I'm wondering if someone could look at something for me when you get a chance. If you were to go to 2h56m and look at the latching contacts using binary bits as memory, I'm not sure if I'm missing something as I've copied what she's done, I can do the following;

Button press - output on
Button press - output off
Button press, some of the bits change but I can't get the output on again.

Is there something I'm missing, as really this latch is supposed to be for on/off/on/off with the one button isn't it?

Thanks.

Yes you are missing summat. No, it is designed to stay on forever after the second press, no matter how many releases follow.

Three hours to talk about memory? Did she talk about the scan cycle? Did she talk about time?

 

[drbitboy]

Three hours in and the topic of memory just came up? Three hours and you have not been taught to read* a ladder diagram yet? Oh dear.

* "read" means think like the PLC to "play" the code in your head, to predict and know what the PLC would do i.e. how the memory bits will behave over time and over each scan cycle.

Your time might have been better spent watching the first 3-5 videos in Ron's series repeatedly.

 

[Squarewave]

Three hours in and the topic of memory just came up? Three hours and you have not been taught to read* a ladder diagram yet? Oh dear.

* "read" means think like the PLC to "play" the code in your head, to predict and know what the PLC would do i.e. how the memory bits will behave over time and over each scan cycle.

Your time might have been better spent watching the first 3-5 videos in Ron's series repeatedly.

Thanks for the comments. Without getting into an argument, as I said, all Ron does is point out what others have done wrong, I don't learn like that.

The video I linked to I find to be very good.

It doesn't stay on, it stays off after the second press.

 

[Squarewave]

I can't watch the video right now to see what is said, but I would expect this logic to function as you describe (on then off, staying off).

It looks to me like this is intended to demonstrate how latching bits in sequence works with OTEs:
1. When you first push the button M0 becomes true, and is then latched in permanently*.
2. When the button is released M1 becomes true and is latched until M0 becomes false (which can't happen via the logic)
3. When the button is pushed a second time M2 becomes true and is latched until M1 becomes false (which only happens if M0 becomes false, which can't happen)
4. The output is on when M0 is true and M2 is false (see above steps to determine that this becomes the case at 1 and stops being the case at 3)
5. Thereafter pushing the button will result in I:0/0 changing state but nothing else, as all the internal bits are stuck on.

* 'permanently' in this case meaning until you either toggle it off manually or the processor leaves and re-enters run and the prescan turns it off.

Compare what happens if you add an XIO for M2 to the beginning of rung 0 and ask yourself why the result is different.

Thanks, I will try that later and see what happens.

 

[drbitboy]

Without getting into an argument, as I said, all Ron does is point out what others have done wrong, I don't learn like that.

Without getting into an argument, with all due respect, and with a sincere desire to help you, I have to say that that is not all Ron does. Maybe that part distracts or annoys you to the point that it seems that way, but that is most definitely not the case.

As I said before, part of the reason he did that is because so many people come to this forum, and to his classes, with a Dunning-Kruger-esque understanding of PLCs, and would never get past that level if someone did not take the time to help them. With that in mind ...

• Look at the image below.
• Can you explain those results with what you learned from the 7h video?
  • Nearly 3h of which were not enough to understand a trivial example
• I.e. why is Lamp F Off when Switch A is On?
• If, as I suspect, the answer is no, then
  • I suggest you watch the first 4m15s of this video, where it is fully explained, step-by-step. Watch it multiple times if need be.
    • Yes, there will be several "the common way PLCs are taught will not explain this" statements,
      • but if you can grit your teeth and pay close attention for those 4m15s, the scales may fall from your eyes.