Editing Ladder logic (section)

===Industrial STOP/START===
In common industrial latching start/stop logic, we have a "Start" button to turn on a motor contactor, and a "Stop" button to turn off the contactor.

When the "Start" button is pushed the input goes true, via the "Stop" button NC contact. When the "Run" input becomes true the seal-in "Run" NO contact in parallel with the "Start" NO contact will close maintaining the input logic true (latched or sealed-in). After the circuit is latched the "Stop" button may be pushed causing its NC contact to open and consequently the input to go false. The "Run" NO contact then opens and the circuit logic returns to its inactive state.
<!-- This diagram currently matches the description above.  Please do NOT alter it unless you can provide a reliable reference that the diagram (and the description above) is wrong. If you change the NC Stop to NO, the arrangement doesn't work. See talk page.  --->
{| 
|<pre>
 --+----[ ]--+----[\]----( )
   |   Start |   Stop    Run
   |         |
   +----[ ]--+
        Run
</pre>
<pre>
 -------[ ]--------------( )
        Run             Motor
</pre>
|}
The above realizes the function: Run = (Start [[logical OR|OR]] Run) [[logical AND|AND]] ([[logical NOT|NOT]] Stop)

This [[Latch (electronic)|latch]] configuration is a common [[idiom]] in ladder logic. It may also be referred to as ''seal-in logic''. The key to understanding the latch is in recognizing that the "Start" switch is a momentary switch (once the user releases the button, the switch is open again).  As soon as the "Run" solenoid engages, it closes the "Run" NO contact, which latches the solenoid on. The "Start" switch opening up then has no effect.
: Note: In this example, "Run" represents the status of a [[bit]] in the [[programmable logic controller|PLC]], while "Motor" represents the actual output to the real-world [[relay]] that closes the motor's real-world [[electrical circuit|circuit]].

For safety reasons, an emergency stop ("ES") may be hardwired in series with the "Start" switch, and the relay logic should reflect this.
{| 
|<pre>
 --[\]----[\]----+--[ ]--+---------( )
   ES    Stop    | Start |        Run
                 |       |
                 +--[ ]--+
                    Run
</pre>
<pre>
 -------[ ]--------------( )
        Run             Motor
</pre>
The above realizes the function: Run = ([[logical NOT|NOT]] ES) [[logical AND|AND]] ([[logical NOT|NOT]] Stop) [[logical AND|AND]] (Start [[logical OR|OR]] Run)
|}