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

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In this second part, we will finally know which is the adequate contact for a stop button and why. If you have not read the part one, click here.

Merging Software and Hardware

Every boolean instruction in a PLC program has a specific address, which is of a single bit and it will be given by you. After that, the state of each bit will be the state of its instruction.

This case will consist of a coil latch with an instruction to detach the latching, but… which contact should we use for a stop button? Let’s find out!

Which is the adequate contact for a stop button, a Normally Open contact or a Normally Closed contact?

Normally Open Contact

First of all, we are going to see how the Normally Open Contact will work as an input actuator for a stop signal.

Normally Open as input actuator for stop signal
Normally Open Contact as input actuator for stop signal

If the input actuator is turned on, the input bit becomes ON or 1. As seen in the other illustration, if the input is 1, the result of a Normally Open will be 1.

On the one hand, this is advantageous because the instruction must interrupt the connection and therefore prevent the output from latching.

On the other hand, we have to keep in mind that the input’s state must be 1 to interrupt the connection (result=0), so the input actuator must switch the input to ON or 1 when it is activated.

In the first image, we can see that when the N.O. contact is activated, the input’s state will switch to 1.

For the moment, the Normally Open contact is not the perfect option for a stop button. As we will see now, it does not make sense because to make it work, we would have to press the two buttons at the same time. Moreover, it can be dangerous.

The principal issue is that we could have is related with the wires. What would happen if one of these breaks? Here you have an illustration that shows the danger perfectly:

Wire-break with Normally Open

After this wire-break, the N.O. contact, which actuates as a stop button, will no longer work because the current won’t reach the contact. The wire-break is a failure, but it causes another one: the stop button is unusable.
In conclusion, the Normally Open contact does not work well as a stop button, as it fails, it generates risky situations and makes this button inoperative.

What would happen with a Normally Closed contact?

Normally Closed Contact for a stop button

Wire-break with Normally Closed

The Normally Closed contact won’t generate risky situations as the Normally Open contact. Even if the wire breaks, the input will perform as if the Normally Closed contact is energized and the stop button will act as if it is being pressed. The latch will fail.

In the following illustration, you can perceive a Normally Closed input actuator as a stop button:

When we change the input actuator, the state of the input automatically changes. Before, when we had an N.O., if the actuator was deactivated, the input state was always 0 or OFF. Now, if the actuator is deactivated, the input state is 1 or ON. What does this mean? It means that since the input’s state is always ON or 1, it will work because when it is deactivated, the current will always pass through the contacts. In conclusion, the best connector for a stop button is a Normally Closed contact.

Normally Closed as input actuator works good
Normally Closed Contact as input actuator for stop signal

Thanks to the stop button, the hardware and the software work together. In the following representation, you can see that when the stop button is activated, its state changes to 0.

Stop button activated
Stop button activated

Why don’t we do the same with the START button?

First of all, keep in mind that these types of failures do not have to produce dangerous situations.
Even so, the start button is not a critical function. So, if the wire breaks, the START button won’t work, just as the STOP button.

Moreover, if we change the contact of the START button to a Normally Closed, at the moment we press the button, the state will change to 0 and the current won’t pass.