PLC controller Input/output Interface

PLC controller inputs

An automated system depends to a large extent on the ability of a PLC controller to read signals from different types of sensors and input devices.

To detect a work piece, see a mechanism in motion, check the pressure or the level of liquid, you need specific automatic devices such as proximity sensors, marginal switches, photoelectric sensors, level sensors, etc. Therefore, the input signals can be logical (on / off) or analog.

One of the most frequent analog signals is a current signal of 4 to 20 mA and a millivolt voltage signal generated by several sensors. The sensors are generally used as inputs for PLCs. You can get sensors for different purposes. They can feel the presence of some parts, measure temperature, pressure, or some other physical dimension, etc. (ex. inductive sensors can register metal objects).

Input adjustment interface

The input adjustment module changes the actual logic level to a level that suits the CPU unit (for example, the input of a sensor that operates with 24 VDC must be converted to a 5 VDC signal so that the CPU can process it). This is usually done through optical isolation, and this function can be seen in the following image.

Optical isolation means that there is no electrical connection between the external world and the CPU unit. They are separated “optically” or, in other words, the signal is transmitted through light.

The way this works is simple. The external device brings a signal that turns on the LED, whose light in turn incites the photo transistor which in turn starts to drive, and the CPU sees it as a logical zero (the supply between the collector and the transmitter drops below 1 V).

When the input signal stops, the LED goes off, the transistor stops conducting, the collector voltage increases and the CPU receives logic 1 as information.

PLC controller output

the most commonly used devices are motors, solenoids, relays, indicators, sound signaling and the like. When starting a motor or a relay, the PLC can manage or control a simple system such as the system to classify products into complex systems such as the service system to position the head of the CNC machine

The output can be analog or digital type. The digital output signal works like a switch; The line is connected and disconnected. The analog output is used to generate the analog signal (for example, a motor whose speed is controlled by a voltage corresponding to a desired speed).

Output adjustment interface

The output interface is similar to the input interface. The CPU brings a signal to the LED and turns it on.

The light incites a photo transistor that begins to conduct electricity, and therefore the voltage between the collector and emitter emits at 0.7 V, and a device connected to this output sees it as a logical zero.

Conversely it means that there is a signal in the output and it is interpreted as logic. The photo transistor is not directly connected to an output of the PLC controller. Between a photo transistor and an output, there is usually a stronger relay or transistor capable of interrupting stronger signals.


Instrumentation Engineer

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