Need help for Ladder Logic

riouxhugo

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I am a beginner with PLC and desire to learn more. Could someone tell me why this logic doesn't work (Auto/Man button).
N51:50/1 is a maintained push button coming from a MMI.
B59:0/0 is AUTO mode.
B59:0/1 is MAN mode.
At first pass, AUTO is selected.

Thanks.

automanpb.gif

automanpb.gif
 
The second rung will never be true....The first rung is always unlatching (setting to 0) N51:50/1
 
Well, not always. Not when B59:0/0 is true. Perhaps this subroutine is not being scanned?

D'oh....That's what I get for answering before my 3rd cup of coffee

Make sure the OSR bits are unique and not used elsewhere.

And when you say 'maintained' from the HMI, are you doing a SET of the bit and when? Make sure that if it is a SET, that it is only a SET on push of the button and not while the button is being held down. If it is the latter, the HMI could be setting the bit back true even after your unlatches.
 
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D'oh....That's what I get for answering before my 3rd cup of coffee

Make sure the OSR bits are unique and not used elsewhere.

And when you say 'maintained' from the HMI, are you doing a SET of the bit and when? Make sure that if it is a SET, that it is only a SET on push of the button and not while the button is being held down. If it is the latter, the HMI could be setting the bit back true even after your unlatches.
B59:0/0 and B59:0/1 are both unique. Could you suggest me a way to test the logic by step.
Thanks.
 
Robert, it is called a maintained Push Button (FT View ME 5). State settings, Next state based on: Current State. 2 States value are 0 and 1 and Connections Value is N51:50/1.

Thanks.
 
Robertmee meant are the OSR bits, B60:9/0 and B60:9/1, unique.

If both B59:0/0 and B59:0/1 were true at the same time this would never initialize and never switch. Place the S:1/15 initialization on its own rung with it as the only condition to make sure.

If S:1/15 then latch B59:0/0, unlatch B59:0/1, unlatch N51:50/1
 
Robertmee meant are the OSR bits, B60:9/0 and B60:9/1, unique.

If both B59:0/0 and B59:0/1 were true at the same time this would never initialize and never switch. Place the S:1/15 initialization on its own rung with it as the only condition to make sure.

If S:1/15 then latch B59:0/0, unlatch B59:0/1, unlatch N51:50/1
Ok,

I have tried several logic now including Robert's suggestion with your precisions. Thanks.
First pass work perfectly.
When I first hit PB work perfectly.
But, when I 2nd, 3rd, 4th and etc. hit the PB, B59:0/1 stay ON.

I am lost...

Still need help to get out of this (suppose) simple flip/flop circuit.

Hugo
 
Make sure that the control's ON and OFF states are distinctive. After hitting it the first time it should go to the ON state (possibly very quickly, you may not see it) then back to the OFF state if the control is basing its indicator on the PLC's current state.

Can you view the logic at the same time that the HMI is hooked up? It would be good to watch in a table the status of all the bits. Specifically B60:9/0 and /1 should normally be 0.

I guess my main question is, why don't you do an XIC of N51:50/1 directly to an OTE of B59:0/0. Then another rung with an XIO of N51:50/1 to an OTE of B59:0/1? Precede the whole thing with an rung unlatching N51:50/1 on the first pass? It would be much simpler.
 
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Why does S:1 have a 15 under it? is it really just suppose to be a 1 ?


S:1 is a word ( 16 bits) for processor mode and control status. Each bit means something different. See pdf's below for SLC and PLC5 status
 
Why does S:1 have a 15 under it? is it really just suppose to be a 1 ?

S:1/15 is a bit that's set to 1 by the processor on first scan of the logic (Program to Run, or power on in run). He's using it to force the default state to AUTO on power up, which I don't really agree with. If the system is in MANUAL when a power blip occurs, it's going to suddenly go to AUTO on power up.
 
Make sure that the control's ON and OFF states are distinctive. After hitting it the first time it should go to the ON state (possibly very quickly, you may not see it) then back to the OFF state if the control is basing its indicator on the PLC's current state.

Can you view the logic at the same time that the HMI is hooked up? It would be good to watch in a table the status of all the bits. Specifically B60:9/0 and /1 should normally be 0.

I guess my main question is, why don't you do an XIC of N51:50/1 directly to an OTE of B59:0/0. Then another rung with an XIO of N51:50/1 to an OTE of B59:0/1? Precede the whole thing with an rung unlatching N51:50/1 on the first pass? It would be much simpler.

I'm with Bernie here....Just use a Toggle Button from your HMI instead of a SET. Let the HMI handle the flip-flop condition. Latch or Unlatch N51:50/1 on first pass depending on whether you want MAN or AUTO as the default powerup condition.
 

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