Programming question for LogixPro?

plc101

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May 2007
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Hi to all. I'm new to the site but what brings me here is this: I'm just learning how to program in RSLogix and I'm practicing with this program called LogixPro Simulator. I'm doing the door exercise number 4 for those of you familiar with the program. I can't seem to figure out how to program the door to stop opening when I hit the close pb and stop closing when I hit the open pb. What takes place now is when the door is opening and I hit the close pb, the door starts closing. There is no halt to the door. Same goes for the door closing when I hit the open pb. I've tried timers and one shots but still not right. Is there someone out there familiar with the door exercise #4 in LogixPro?
 
I'm not familiar with the LogixPro simulator or the exercise you speak of, but if you post your code along with a description of what you're trying to do, we can try to help you out.
 
There is no halt to the door.
How could you create a halt? That is what PLC programming does: create functions that do not exist already. If everything had built-in "halts" and other functions, PLC's would not be nearly as useful.

I assume you have struggled with this for awhile, so here is one method (out of many possible ways). Use four intermediate (interposing) relays to create 2 halt functions.
Door_Opener_Ex_4.jpg


PS: Notice how the rung comments describe what the rung is going to do? It is as if a good rung description pre-ordains success. If you can describe it, then programming it becomes easier. I find it amusing how beginners put off this step (writng good comments) "until I get it working". By the time you get it working, the comments are not needed, except for the next poor slob who has to figure it out.
 
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Lancie1 said:
PS: Notice how the rung comments describe what the rung is going to do? It is as if a good rung description pre-ordains success. If you can describe it, then programming it becomes easier. I find it amusing how beginners put off this step (writng good comments) "until I get it working". By the time you get it working, the comments are not needed, except for the next poor slob who has to figure it out.

I couldn't agree more!
I've been hammering on about commenting programs for years, in my opinion it's more important than good well written code.
Even badly written code can be understood if it's commented well.
 
Lancie1 said:
I assume you have struggled with this for awhile, so here is one method (out of many possible ways). Use four intermediate (interposing) relays to create 2 halt functions.

Shouldn't both of the STOP PUSHBUTTON instances be -|/|- ?
 
Lancie1,

Thank you for your help. I have been working on solving this/learning this for a while and I can't wait to plug in your logic. I agree about the rung comments simply for the guys that have to troubleshoot the equipment after installation...
You seriously just got added to my buddy list.

Thanks again,
Paul
 
plc101 said:
I agree about the rung comments simply for the guys that have to troubleshoot the equipment after installation...

Rung comments aren't just for troubleshooting after installation.

I just finished a system with 6500 rungs... I needed good documentation by the time I got to startup because it had been several months since I had written the first part of the code.

It's just good practice to document as you go because you'll never go back and document when you're done. You're always too busy getting along to the next project to go back and document the last one.
 
Shouldn't both of the STOP PUSHBUTTON instances be -|/|- ?
No, the Stop Pushbutton in the LogixPro Door Simulator Exercise is Normally Closed, so what must the PLC instruction be to look at an "On" Input and return a "1" bit? Is not the proper instruction XIC (Examine if Closed), correct?

Don't worry, this is a common mistake. If an input is NC, you use an "NO" (XIC) to look at it on the PLC ladder. Remember it is two different places we are talking about, one a physical device, the other a PLC software program function.

I have tested the LogixPro progam that I posted, and it works with the switches as defined in the official exercise. Now, if the Stop pushbutton had been a Normally Open switch, then you would be correct.
 
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Lancie1 said:
No, the Stop Pushbutton in the LogixPro Door Simulator Exercise is Normally Closed, so what must the PLC instruction be to look at an "On" Input and return a "1" bit? Is not the proper instruction XIC (Examine if Closed), correct?

Don't worry, this is a common mistake. If an input is NC, you use an "NO" (XIC) to look at it on the PLC ladder. Remember it is two different places we are talking about, one a physical device, the other a PLC software program function.

I have tested the LogixPro progam that I posted, and it works with the switches as defined in the official exercise. Now, if the Stop pushbutton had been a Normally Open switch, then you would be correct.

"Stop" buttons are (should be) wired normally closed for a reason. If there is an electrical failure such as a power supply failure, a broken wire, a bad input etc.... you want your system to fail SAFE ie: stop. It is much better to have a control system that won't start as opposed to one that won't stop.

This is why in a hard wired system an E-Stop will alway be wired normally closed.

edit: Lancie

I know you know this, I just wanted to point it out.
 
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