This video https://www.youtube.com/watch?v=srFbEVCa1T8&index=2&t=0s&list=LLelezlPlon2Y92eyikVNskg explains a basic PLC program for a garage door switching operation.
In 25:18 of the video, in what sequence should I press push buttons ( UP, DOWN, Stop) to execute this :
move up till some point , stop movement and then , move down till door closed . ?

 

That is a very basic circuit. Either of the UP/ DOWN buttons can be initiated, the energized side locks out the opposing direction while active. The respective L.S. ends the sequence.
BTW, there is quite a bit more logic required for the average G.D. I wrote one once for a small Picmicro.
P.S. the L.S. are not labeled.
It is more an example of ladder logic than a garage door application.
Example ladder circuits.
Max.

 

To complete the labeling, the N.C. P.B. should is the mutual stop or Emer-Stop, the top L.S. is the UP L.S. , likewise the bottom one is the DOWN L.S.
Max.

 

That is a very basic circuit. Either of the UP/ DOWN buttons can be initiated, the energized side locks out the opposing direction while active. The respective L.S. ends the sequence.
BTW, there is quite a bit more logic required for the average G.D. I wrote one once for a small Picmicro.
P.S. the L.S. are not labeled.
It is more an example of ladder logic than a garage door application.
Example ladder circuits.
Max.

Thanks for replying. I'm a complete beginner in PLC. I don't understand the action of NC 2CR when PB 1 is pushed.

 

2CR as is 1CR are lock out contacts on relay 1CR/2CR that when one side is energized, they are used to disable the opposing side from operating, which would cause a conflict.
IOW the two contacts are registering the state of the outputs 1CR/2CR.
Max.

 

Incidentally if this were a PLC program, the relay outputs 1CR and 2CR could be used to pick up the external motor relays, making the top two rungs superfluous.
It looks more like a hard wiring diagram.
Max.