Introduction
Connecting external devices to a PLC controller, whether input or output, is a special topic for the industry. If it is autonomous, the PLC controller itself is nothing. To function, it needs sensors to obtain information from the environment, and it also needs execution devices to be able to transform the programmed change into reality. A similar concept is found in the functioning of the human being. Having a brain is just not enough. Humans only reach their full activity by processing information from a sensor (eyes, ears, touch, smell) and by using their hands, legs or certain tools. Unlike humans who receive their sensors automatically, when it comes to controllers, the sensors must then be connected to a PLC. How to connect the input and output parts is the subject of this chapter.
Shipwreck-supply concept
The controller has input and output lines through which it is connected to a system that it controls. The inputs can be keys, switches, sensors while the outputs are directed to different devices, from simple signal lamps to complex communication modules.
This is a very important part of the history of PLC controllers as it directly influences what can be connected and how it can be connected to the inputs or outputs of the controller. The two terms most frequently mentioned when examining the connections to the entrances or exits are "shipwreck" and "supply". These two concepts are very important for correctly connecting an API to an external environment. The briefest definition of these two concepts would be:
SINKING = Common GND line (-)
SOURCING = Common VCC line (+)
The first thing that gets attention is the "+" and "-" power, the DC power. The inputs and outputs that are running or being supplied can only conduct electricity in one direction, so they are only supplied with direct current. According to what we have said so far, each input or output has its own return line, so 5 inputs would need 10 screw terminals on the housing of the PLC controller. Instead, we use a system of connecting multiple inputs to a return line as in the following image. These common lines are generally marked "COMM" on the housing of the PLC controller.
PLC
Input lines
Until now, the explanation of the input and output lines of the PLC controller has only been given theoretically. In order to apply this knowledge, we need to make it a little more specific. An example may be the connection of an external device such as a proximity sensor. The outputs of the sensors can be different depending on a sensor itself and also for a particular application. The following images show some examples of sensor outputs and their connection to a PLC controller. The output of the sensor actually marks the size of a signal given by a sensor at its output when this sensor is active. In one case, it is + V (supply voltage, generally 12 or 24V) and in the other case, GND (0V). Another thing worth mentioning is that the sinking-supply and supply-sinking pairing is still in use, not the supply-supply or sinking-sinking pairing.
If we were to make the connection type more specific, we would get combinations as in the following images (for more specific connection diagrams, we need to know the exact sensor model and a PLC controller model).
Output lines
The output lines of the PLC controller can generally be:
- transistors in PNP connection
- NPN connection transistors
-relay
The following two images realistically show how an API handles external devices. It should be noted that a main difference between these two images is a position of "output load device". By "output charging device" we mean a relay, a signal lamp or the like.
How something is connected to a PLC output depends on the connected element. In short, it depends on whether this element of the output charging device is activated by a positive power pole or a negative power pole.
author: Nebojsa Matic