Let's check out one of the most important

Let's check out one of the most important programming concepts in automation and controls. The seal-in circuit. Picture a setup with two push buttons meant to control something like a pump or a motor. One button is meant to start and one button's meant to stop. Both buttons are momentary, which means they're only shown as inputs true to the PLC while you're actively standing there holding the button in. But you want the basic application to be, press the start button and walk away, that pump or motor is running. Press the stop button, and then the motor will stop. For the ladder diagram for this circuit, you want to have examine if closed or normally open contact for the start button, followed by examine if closed or normally closed contact for the stop push button, and these will energize the output. But to have this work with momentary push buttons, you need to add a branch around the start push button that looks at the actual status of your output. This way, when the start button gets pressed, and the output turns on, the whole rung will stay true until someone walks back and hits the stop button. The sort of logic appears most frequently for pumps and motors, but any sort of station where you want simple latching control from momentary inputs, this is a super useful concept to know about.