On priority and Off priority self holding

Hi all,
I am beginner for PLC.
I know about self holding(Latching).
Someone has told about 2 types of self holding.
1.On priority self holding
2.Off priority self holding

My questions are
How to draw this two in ladder language)LAD)?
What is the different between this two method?

NskNSK said:

Hi all,
I am beginner for PLC.
I know about self holding(Latching).
Someone has told about 2 types of self holding.
1.On priority self holding
2.Off priority self holding

My questions are
How to draw this two in ladder language)LAD)?
What is the different between this two method?

Click to expand...

Never heard or On/Off Priority, other than the reset/set should have priority over the latch...

I did find this and it may help http://www.azatrax.com/latching-relay-circuits.html

I haven't heard of those terms before either.

It almost sounds like you are talking about set / reset(dominate) VS reset / set(dominate).

In Siemens there are blocks with both set and reset built into them. An 'RS' block and an 'SR' block.

Basically whichever letter is last R or S is what is dominate in the logic (most likely due to it being scanned last in the internal logic).

Basically you just need to select which one you want to have priority if for some reasons both R and S are active at the same time.

Is that what you were asking?

Off-priority (when B opens, C will be off regardless of A):

 

   A            B                          C

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

        |

   C    |

--| |---+

On-priority (when A closes, C will be on regardless of B):

   A                                           C

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

                    |

   C          B    |

--| |------| |---+

The first one is more common but the second one sometines has its uses.

Its a good explanation LadderLogic.
Its like SR and RS latches.
Thank you

Thanks all Friends.. :site:

I would be very wary about using latch-unlatch in a program without proper structure, program flow.

In my opinion, it's a poor practice in actual control of a machine, but sometimes use those as indicators for status that have no control of the machine.

I swear, the next rig I have to hook into that has copious amounts of latch-unlatch instructions in a cobbled up program that makes no sense...

I think it's time to get the Bushmaster out for some target practice.