Where to start learning the many PLC programming instructions? Welcome to continue reading this section: PLC programming instructions for self-locking.  

When you start to get involved in PLC, you will find that there are so many PLC programming instructions, and you will be a little dizzy at first glance, and you don't know where to start. To solve this learning problem for novices, here we will continue to explain the common instructions of PLC programming, which is the self-locking of PLC programming instructions.  

Normally, we can also realize the self-locking wiring function in the traditional relay hardware control, but there is no PLC programming self-locking simplicity, and the self- locking formed by the connection of the relay hardware contacts is relatively complicated, and it is more troublesome to find when there is a fault for a long time. But PLC programming self-locking instructions solve this problem. We can first define the terminal names of each corresponding physical object according to the figure above. And input the command keyboard into the programming software. 10.0 is a button for starting (normally open contact), iO.1 is the normally closed contact for emergency stop and stop button here, and the output is QO.O, which corresponds to the output signal to the relay coil (or air valve that controls the movement of the cylinder) that controls the start of the motor. The normally open contact of this output coil and the start button are connected in parallel.  

 The exported picture does not have color, friends who need color can scan the code at the end of the article to follow the message 

Let's look at the picture above, green represents energy flow, that is, signal, continuous green represents the closed-loop output of energy flow, that is, there is a signal output, if there is an interruption in the middle, PLC cannot output. If the QO.O normally open contacts are not connected in parallel in the above figure, then when we release the start button, the motor will also stop and cannot be continuous. When the normally open contact is connected in parallel, when the start button (QO.O can be regarded as the output coil), the normally open and normally closed state can maintain the continuous output of the energy flow. At this time, when the operator releases the button, QO.O will remain closed, which is the self-locking in PLC programming, as shown in the figure below: 

At this time, if the operator presses the emergency stop button, the energy flow will be interrupted, QO.O will stop the output, and the motor or cylinder will stop acting. This is the whole process of self-locking function in PLC programming.