flip flop logic

Unregistered · Apr 29, 2005

how can i create a flip flop in ladder logic? It needs to be triggered by one input and has two out puts that flip flop with no time limit for each output.
Input turns one output on and when it goes off the next time the input comes on the the other output comes on.......ect

randylud · Apr 29, 2005

Search and ye shall find!

Use the search feature on this site and look for posts and threads on the subject. This is the flip-flop, traffic light and NPN-PNP site of all time. More space has been dedicated to these topics than nearly all else. I think!

Ken Moore · Apr 29, 2005

Use the search feature near the top of the page, this topic has been dicussed many times.

BTW,
Is this a homework assignment?

rsdoran · Apr 29, 2005

I have avoided the traffic light but have flip flop:
http://www.patchn.com/flipflop.htm

And NPN-PNP:
http://www.patchn.com/npnpnp.htm

Flasher too:
http://www.patchn.com/ab_plc_flasher.htm

DanielCamacho · Apr 29, 2005

I added the safety light as added security... But not a good idea starting and stopping somthing with just one input....

I think this will work in most cases...

if others think not just tell me why, please...

TConnolly · Apr 29, 2005

Check out the attahced PDF file.

I prefer and usually use the method shown in the first rung. It is a single rung implementation of a flip flop and it incorporates flip flop reset logic in the same.

The second shows another way you could do it with a counter. I just threw those on real quick to show how it could be done. The LSB of your counter accumulator changes state each time the coutner increments its value by one (binary counting). I've seen this method used quite a bit, the rungs are simple.

TConnolly · Apr 29, 2005

DanielCamacho said:
I added the safety light as added security... But not a good idea starting and stopping somthing with just one input....

I think this will work in most cases...

if others think not just tell me why, please...

Daniel, I've seen similar implementations, but I can spot a problem with this one. Assuming that you are starting with the pump output off, when the button is pushed the top line of rung 0 will solve true just fine and turn on your pump output. Now lets solve the branch. We have just set O:2/0, so the first instruction on the branch will evaluate as true and then the OTU instruction will immediately clear O:2/0. Remember that while the actual outputs are not updated until the end of a scan, the output data table is updated immediately.

To make this work a third bit is required. A OTE instruction must be placed in a parallel branch round the OTL instruction in the top branch. This bit will then be on for a single scan when O:2/0 is turned on. Then in the lower branch that same bit is referenced with an XIO instruction just before the OTU instruction.

Look at the first rung in the PDF file attached to my previous post in this thread to see a different way.

I agree that it is better to start and stop with two inputs whereever possible, and the stop input should come from a N/C contact, so that if the input power goes away, you still get a stop signal.

Terry Woods · Apr 29, 2005

"Cat Skinning Method #42 & #43."

There are MANY ways to skin this cat... here are just two...

METHOD 42: Only for those PLC's that can do that damned "RE-what-cha-ma-call-it". (We never have settled on a name for this particular concept! But, I'm feeling more strongly that the concept is called "Re-Entry".)



             WAS

   PB        PB

---| |---+---|/|------------+-----( SET ) WAS PB

         |                  |

         |                  +-----( SET ) FLIP

         |

         |  FLIP    OUT_1

         +---| |-----| |----+-----( RST ) OUT_1

         |                  |

         |                  +-----( SET ) OUT_2

         |                  |

         |                  +-----( RST ) FLIP

         |

         |  FLIP    OUT_1

         +---| |-----|/|----+-----( SET ) OUT_1

                            |

                            +-----( RST ) OUT_2

                            |

                            +-----( RST ) FLIP



           WAS

   PB      PB     +-------+

---|/|-----| |----+ TIMER +------( RST ) WAS PB   < --- This is a "debouncer" for the PB.

                  | 1-Sec |                             Not necessary if using a good snap-switch.

                  +-------+                             If so, simply remove the "Timer"

                                                        but keep the rung.

METHOD 43: For those PLC's that can not do that damned "RE-what-cha-ma-call-it".



           WAS

   PB      PB

---| |-----|/|----+-----( SET ) WAS PB

                  |

                  +-----( SET ) FLIP



  FLIP    OUT_1

---| |-----| |----+-----( RST ) OUT_1

                  |

                  +-----( SET ) OUT_2

                  |

                  +-----( RST ) FLIP



  FLIP    OUT_1

---| |-----|/|----+-----( SET ) OUT_1

                  |

                  +-----( RST ) OUT_2

                  |

                  +-----( RST ) FLIP



           WAS

   PB      PB     +-------+

---|/|-----| |----+ TIMER +------( RST ) WAS PB   < --- This is a "debouncer" for the PB.

                  | 1-Sec |                             Not necessary if using a good snap-switch.

                  +-------+                             If so, simply remove the "Timer"

                                                        but keep the rung.

Not to worry about using SET/RST in this context... these SETs and RSTs are self-correcting.

rdrast · Apr 29, 2005

Alaric, welcome... and regarding your post above, and Daniels, you, Alaric, are 100% correct.

Daniel (no offense intended) makes assumptions based on the sequence of operand evaluation. That **MAY** work, on some platforms, but may (probably) NOT work on others.

Not that I want to moralize, but you really shouldn't rely on any particular platform (PLC, Operating System) to exibit quirks that you enjoy. Rather, take the extra bit, and the extra logic, to try to ensure that any logic you develop works on ANY platform.

Scan time really doesn't get hurt by an extra couple of rungs of ladder logic (pure boolean logic), so there is little reason to eek out every possible microsecond in a routine which makes it PLC Operating System dependent.

IMHO, of course.

Mtn_Bkng_Dave · May 1, 2005

Method 225

Ok try this...........should work on all platforms.

Dave

CaseyK · May 1, 2005

Now that I think about it, traffic lights have a need for a flip flop, when they go to manual flash. Now, how to incorporate an elecvator into it???

Mtn_Bkng_Dave · May 1, 2005

LOL
How a light on each floor to tell ya when to not step into the open shaft so we can eliminate the doors?
Dave

CaseyK · May 1, 2005

Very Good Dave!

Someone give that guy a gold star, and TWO student questions, too!

regards.....casey

River Rat · Feb 6, 2011

geniusintraining · Feb 6, 2011

If this does not bring Terry out of the Woods' (pun intended) then nothing will

((744))

Like seeing Ron as a lifetime supporting member.... he posted enough and helped for a few lifetimes (thanks Phil, nice touch)

flip flop logic

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