Which contact should I use for a stop button? (Part I)

Contacts and stop button

In this post about contacts and a stop button, we are going to determine which contact is better to use for this type of button. In this part I, we will see the hardware logic and software logic.

Hardware Logic

First of all, we are going to talk about digital inputs and digital actuators.
The digital inputs are signals of logic level with two states, respectively:

  • LOW and HIGH
  • 0 and 1
  • FALSE and TRUE

The digital actuators are the components in any machine that allows movement and can also have the previous states. Moreover, they can have two functions:

  • Normally Open
  • Normally Closed

The main difference comes to connecting Normally Open actuators and Normally Closed actuators to a digital input because the situation is different when you have to activate the actuators.
Then, what is the difference between Normally Open and Normally Closed?

Input Actuators-Hardware Logic-Which contact should I use for a stop button? (Part I)

Normally Open Input Actuators

The default state of a Normally Open Actuator (NO) is open (deactivated), in this case, the input bit is 0.
In the case that the input bit changes to 1 (active), the Normally Open Actuator (NO), will close (activate).

Normally Closed Input Actuators

The default state of a Normally Closed Actuator (NC) is closed (deactivated), in this case, the input bit is 0.
In the case that the input bit changes to 1 (active), the Normally Closed Actuator (NC), will open (activate).

Remember that the actuator’s state has an impact on the input’s state and, as a result, on the input bit.

Software

Now, rather than having hardware logic with actuators, we will have boolean instructions or ladder logic contacts.
We know it is a bit confusing since this other kind of logic is quite similar to the hardware logic. Actually, the objective when ladder logic was created was to be alike to electrical circuits.
Moreover, the Normally Open and Closed contacts can be called Examine if Closed (XIC) and Examine if Open (XIO).

Input Actuators-Software Logic-Which contact should I use for a stop button? (Part I)

Now, the input bit values are the determinant of the boolean instructions result (XIC and XIO). Also, the input bit state is the determinant of the actuator’s state.

State of actuator > Input bit > Result of boolean instruction

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