S_r Flip Flop

hi 2 all.

i wanted 2 know what r the typical application of SR F/F in plc programming

Any place you need a reset-dominant latch.

Latch: A bit that can be turned on logically by a pulse (or level signal), and remains on until reset by another pulse (or level signal), independent of the logic preceeding the 'Set' part

Reset-Dominant - A latching circuit which has the 'Reset' command after (and usually immediately following) the 'Set' command, so that the controlled bit can be forced off at all times, no matter whether the 'Set' input is true or not.

I don't ever recall using any, since I started with PLC's in 1988.

Perhaps it is just the applications that I use.

Is this a student question?

In ladder logic I think of an SR flip-flop as a normal start/stop circuit.
A momentary push button input OR'd with the Output coil as a latch, with the Stop push button input as an XIO ANDed with the OR. The output stays true although the pushbutton is no longer pressed. It will remain true until the Stop button is pressed.

Dale

Oboy said:

In ladder logic I think of an SR flip-flop as a normal start/stop circuit.
A momentary push button input OR'd with the Output coil as a latch, with the Stop push button input as an XIO ANDed with the OR. The output stays true although the pushbutton is no longer pressed. It will remain true until the Stop button is pressed.

Dale

Click to expand...

That is actually a bit inaccurate. A SR flip flop, (or any flip flop for that matter) will retain it's last state even if the trigger conditions go away. Ideally, a PLC SR flip flop will not change states when the powers off and back on, or from run to program to run.

With a typical holding contact as you describe, in the powerup-scan, most PLC's (very obvious with AB) scan all rungs but with a forced false result as the result of logical operations, that would unseal the hold in circuit, violating 'flip-flop-ality'.

A SR flip flop is in generic terms:

Some_True_Condition - SET_OUTPUT

Some_Other_True_Condition - RESET_OUTPUT

SR Flip Flop Representations

 SET CONDITION              SET

 ---||---------------------( C1 )

 RESET CONDITION           RESET

 ---||---------------------( C1 )

 

 Or Possibly

 

 SET COND   RESET COND

 ---||---+-----|/|---------( C1 )

    C1   |

 ---||---+

 

RS Flip Flop Representations

 

  RESET CONDITION           RESET

  ---||---------------------( C1 )

 

  SET CONDITION              SET

   ---||---------------------( C1 )

  

  Or Possibly

  

  SET COND

  ---||-----------------+---( C1 )

     C1      RESET COND |

  ---||---------|/|-----+

  

For the record!

The S-R flip-flop is a digital electronic device, and may indeed be emulated in PLC programming. I was going to upload a .gif to demonstrate, but can not seem to get the upload to happen, even though it says that the picture has been uploaded. Oh well, more for another day. Always something to learn.

Randy when you upload the file you have to copy and paste it into the reply.

Let's give it a go

This is a nice view of what an R-S does:


Like I said before, always something to learn, thanks Ron.

thanks randy

ur drgs explained it all at one go