could anyone help me how to solve this problem via rslogix? thank you
- Thread starter ufomec
- Start date
Steve Bailey
Lifetime Supporting Member + Moderator
Depends on how the physical stop button is wired.
In the US it's typical to use NC contacts on a stop button. If that is the case when you're not pressing the stop button the XIC addressed to that device evaluates as true, so rung 0 should be an XIC. If the stop button is wired using a NO contact, then the XIO instruction is correct.
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drbitboy
Lifetime Supporting Member
Thanks for the prompt feedback; I was able to update the image before editing was no longer possible.
Agreed.
NC stop buttons is the approach we use too. Although it's common to see machines from overseas arrive with N/O contacts on start and stop buttons.
My comment was more so just to have rungs 0 and 1 on @drbitboys examples equivalent.
drbitboy
Lifetime Supporting Member
Wait. I still don't have it right? Oh dear ...
I used [XIO STOP(I:0.0/0)] initially so I could do everything on the MicroLogix 1100 without playing with wires.
I tested it. Or so I thought. hahahaha.
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drbitboy
Lifetime Supporting Member
Steve Bailey
Lifetime Supporting Member + Moderator
That gives the correct time delay before turning on the lamp.
drbitboy
Lifetime Supporting Member
Excellent! Well done: the light [O:0/0] turns on 20s after the input [I:0/0] becomes 1. This is a classic TON (Timer-driver ON-delay) circuit.
Also, the [OTL O:0/0] latches the light ON (assigns 1 to O:0/0), so when the timer is reset and [T4:0/DN] become 0, the light will stay on. That can be good too*, but at some point we will want the light to turn off as well, even though T4:0/DN may be 1. Also, if the input switch is turned off ([I:0/0] becomes 0) after 20s, then the timer will reset and [T4:0/DN] will become 0, but the light will say latched on. So there may be some more logic (instructions) added to Rung 0001 later to disable the latch [OTL] on every scan, and there will need to be a way for the light to turn off that looks at the input switch, but this is good for now.
The next step is to reset the timer 20s after it starts timing, so it can start the next 20s cycle.
- What is the trigger to do that reset?
- Hints: what bit becomes 1 at 20s after the timer starts? You did something similar on Rung 0004 of Post #7
* The light could also be turned on, and kept on, with an [OTE O:0/0] fed by rungs that contain additional logic, instead of the OTL triggered by [T4:0/DN]. Sometimes an OTL is not preferred: for example if the PLC lost power, then the light would be on ([O:0/0] would be 1) when the power came back, and that might not be what was desired. But leave it as a latch for now.
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drbitboy
Lifetime Supporting Member
Perfect!
Consider that the T4:0/DN Bit will behave like this:
Code:
Time ===>
T4:0/DN=1: - - -
| | | | | |
T4:0/DN=0: --------- --------- --------- ---- ...Can you find a way to count those rising edges, and then use that count to inhibit the the timer, and turn off the light, after the third rising edge?
Hint: you will have to edit one, or more, of the existing rungs in such a way that it changes their behavior at the third 20s event, but does not change their behavior at the first [T4:0/DN]=0 event.
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drbitboy
Lifetime Supporting Member
Also note that, with the current program, if the input switch is turned off between 20s and 60s, then the light will remain ON, because [O:0/0] was latched to 1.
So you will need some logic to turn off the light if [I:0/0] becomes 0. The timer will reset because the [I:0/0] controls the timer input rung, so you will not need to either do anything to the timer rung or add another reset.
So you will need some logic to turn off the light if [I:0/0] becomes 0. The timer will reset because the [I:0/0] controls the timer input rung, so you will not need to either do anything to the timer rung or add another reset.
drbitboy
Lifetime Supporting Member
And another way to say that is that T4:0/DN will have a false-to-true transition, although it would be better to say it will have a zero-to-one transition, every 20s.
I add this because the problem statement seems to come from a book's chapter 8 "Programming Counters" cf. https://sanamrao123.files.wordpress.com/2015/04/ch-8.pdf, and that phrase "false-to-true" is used fifteen times in that chapter.
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