Oh look, it's another 1 button question (But I'm not asking you to do my homework)

[robpaige]

Hey guys, from the reading I've done here you mostly seem to dislike the one button scenario, because it's brought up by students who should be doing their own homework. I get that, and don't want to insult anyone. As I've mentioned before, I have been thrown into the deep end, and am working to learn PLC programming with no prior programming experience, using the resources I can scrape together from the internet. What I'd like is for you guys to take a look at the ladder below, and give me your feedback. I have a guess as to what I think is happening (below the ladder) but I want to make sure I have the right idea before I send this off to my boss.

|   I:1/0   B3:0/0          B3:0/1 |

|---]  [----]OSR[-----------( )----|

|                                  |

| B3:0/1    O:2/0          O:2/0   |

|---]  [---] / [------------( )----|

| |                |               |

| |                |               |

| |O;2/0   B3:0/1  |               |

| |-]  [---] / [---|               |

|---------------------------(END)--|

I'll post what I think is going here as a reply to this post.

 

[Lancie1]

Another alternator.

To save time, here is a collection of 10 methods, posted in the Downloads in 2008. Robs post is Method #5 on this list.

 

[robpaige]

Scan A: Button hasn't been pressed
Rung 0000
I:1/0=0, XIC=FALSE B3:0/0=0. B3:0/0 gets a 0 written in its box
Rung 0001
B3:0/1=0, XIC=False, O:2/0=0, XIO=TRUE
Branch:
O:2/0=0, XIC=FALSE, B3:0/1=0, XIO=TRUE
Output O:2/0 has false logic going in, does not energize. This pattern will hold as long as the button is not pressed.

Scan B: Operator pushes the button:
Rung 0000
I:1/0=1, XIC=TRUE B3:0/1 gets a 1 written to its box
Rung 0001
B3:0/1=1, XIC=TRUE, O:2/0=0, XIO=True
Branch:
O:2/0=0, XIC=False, B3:0/1=1, XIC=FALSE
output O;2/0 has a path of true logic to it, light comes on

Scan C: Operator hasn't released the button yet:
Rung 0000
I:1/0=1, XIC=TRUE, B3:0/1=1, still.
Rung 0001
B3:0/1=1, XIC=True, O:2/0=1, XIO=False
Branch:
O:2/0=1, XIC=True, B3:0/1=1, XIO=False
Output O:2/0 has false logic coming in.
And this is where I am uncertain:
Since the logic to the output is false, does the output de-energize, or does it remain unchanged?

Moving on:
Scan D: Button is released
Rung 0000
I:1/0=0, XIC=False, B3:0/1 gets a 0 written to it
Rung 0001
B3:0/1=0, XIC=False O:2/0=1, XIO=False
Branch:
O:2/0=1, XIC=True, B3:0/1=0, XIO=True
O:2/0 has true logic going into it, output will be energized.

Scan E: Button is pushed again
Rung 0000
I:1/0=1, XIC=True, B3:0/1 gets a 1 written to it
Rung 0001
B3:0/1=1, XIC=True, O:2/0=1, XIO=False
Branch:
O:2/0=1, XIC=True, B3:0/1=1, XIO= False
O:2/0 has false logic coming to it, and if the output de-energized (see my uncertainty above) then the light should turn off, and will stay off after the button is released.

BUT...The OSR function for B3:0/0 should lock B3:0/1 into a one once the button is pressed, shouldn't it? And if that's the case, my interpretation of scan C is incorrect, and the output on rung 0001 would have true logic to it, thus staying on, wouldn't it?

 

[robpaige]

Thank you Lancie. The pics definitely cleared up some of my confusion.

The thing that seems to keep tripping me up is the button release stuff.

 

[Lancie1]

You might want to get a copy of the LogixPro PLC Simulator program. Then you could test this type of program on your computer, and see for yourself how it works.

 

[robpaige]

Indeed, it's on the to-do list, along with hammering out bugs in the RSLogix 500 emulator software. I'm starting to thing I need a bigger hammer.

 

[Lancie1]

You can download LogixPro from thelearningpit.com (free 15 day trial).

http://www.thelearningpit.com/

 

[leitmotif]

I think this is the perfectly written student question
Oh look, it's another 1 button question
HUMOR
(But I'm not asking you to do my homework)
up front about it.

BEST OF ALL
Posted example of work so far.

I predict if you keep this up
1. You will get very few if any nasty comments
2. You will get confused with all the help you get.

Dan Bentler

 

[robpaige]

Dan,

Thank you, that's kind of what I was going for. I have tutored a bit, and can tell when someone actually wants to understand the material, or is trying to dump it on the engineering major. I've also dug around a bit on this site, and have seen behavior from students that I consider appalling. The trick is, I find those posts, and try not to do that. As far as the humor thing goes, I figure if engineering doesn't work out, there's a brick backdrop out there with my name on it somewhere.

 

[Lancie1]

Critiquing your analysis, using LogixPro at Scan Speed = 1 to view changes in instructions:

Scan C: Operator hasn't released the button yet:
Rung 0000
I:1/0=1, XIC=TRUE, B3:0/1=1, still.

This is your first error. B3:0/1 stays on for ONLY 1 SCAN. Remember the OSR on Rung 000? It only keeps B3:0/1 energized for the FIRST SCAN after the pushbutton is ON. On the second and subsequent scans, B3:0/1 = 0.

Therefore, after 1st scan, O:2/0 will be ON and seal itself in through bottom branch of Rung 001. It will stay on until this seal-in is released by subsequent button press.

 

[bikerdude]

Hi,
The PLC rungs in post #1 will work fine in a SLC500 or a PLC-5 controller, I have used it many times. Only thing to note is that the status of the O:2/0 output will most likely be a zero upon power up or fist scan after a transition from program mode to run mode. Maybe your application has this behavior as being desirable? You will want to test this power up behavior if it is important.

I havent tried this code on a CLX processor yet.

BD

 

[robpaige]

This is your first error. B3:0/1 stays on for ONLY 1 SCAN. Remember the OSR on Rung 000? It only keeps B3:0/1 energized for the FIRST SCAN after the pushbutton is ON. On the second and subsequent scans, B3:0/1 = 0.

Therefore, after 1st scan, O:2/0 will be ON and seal itself in through bottom branch of Rung 001. It will stay on until this seal-in is released by subsequent button press.

I knew I was forgetting something. I'd forget my head if it weren't firmly glued on. Thanks Lance

 

[robpaige]

Hi,
The PLC rungs in post #1 will work fine in a SLC500 or a PLC-5 controller, I have used it many times. Only thing to note is that the status of the O:2/0 output will most likely be a zero upon power up or fist scan after a transition from program mode to run mode. Maybe your application has this behavior as being desirable? You will want to test this power up behavior if it is important.

I havent tried this code on a CLX processor yet.

BD

For the problem I was given, it wasn't specified, so I left it alone in order to not complicate things unduly. That said, I thought the best way to approach these things is to have things be off at start-up unless safety matters dictate otherwise?

 

[olsq]

here is what i came up with. I work in an AB heavy shop. After the other examples didn't work, I started playing

 

[Lancie1]

I would like to know which examples did not work. Every time I have tested, they all worked like a sewing machine. I can upload pictures to prove it, too.

Besides, your example is a rung rearrangement of Method #5, which does work, as it has for years.