The overarching reason we need optocoupled I/O is to provide electrical isolation between the internal control circuitry of the PLC and the external devices it interacts with. This isolation overcomes a number of issues out of the gate. The biggest problem in Industrial settings is a difference in ground potential for various reasons. Anyone who has experienced ground loop hums in audio equipment has experienced this problem; the root cause is the same. With 3.3V logic basically having a threshold of around 1.5V a signal from a source whose ‘0 volt ground’ is more than 1.5V below the ‘0 volt ground’ of the input device will never see the signal go above its threshold for a HIGH; it will always see a LOW, because it sees a -1.5V for a low and about +1.5V for a high. The difference in ground voltage gets added to the signal. The reverse is true for its outputs to the device; its output LOW (0 volts) will be seen as 1.5 volts and taken as a HIGH.
In industrial environments, electrical noise and interference are omnipresent challenges that can compromise the accuracy and stability of signals. Running cables near high voltage or high current cables, near motors that may produce magnetic fields are all situations susceptible to having damaging voltage spikes induced into them. These spikes become just like Electrostatic Discharge (ESD), like getting a shock when the air is dry and you touch a door knob; they can destroy electronics.
The term "optocoupled" refers to the integration of optoelectronic components, typically light-emitting diodes (LEDs) and photodetectors, within the input and output circuitry of a PLC. This technology offers several compelling advantages, making it an indispensable feature in modern industrial automation. Besides removing the ground loop problem by allowing the source and input device to happily have their own ground voltage references (the digital data being transferred by light not voltage), optocouplers do not transfer spikes. Also the LED in the optocoupler is much more robust than the transistors in electronics, the spikes having very little effect.
Furthermore, optocoupling ensures a higher level of safety in industrial applications. In scenarios where PLCs control machinery and processes with potentially hazardous voltages, the isolation provided by optocoupled I/O becomes a critical safety feature. It prevents the risk of electrical shocks to personnel and mitigates the possibility of damage to the PLC due to external electrical faults. This inherent safety feature is especially crucial in industries such as manufacturing, energy, and chemical processing, where worker safety and equipment integrity are paramount.
Another significant advantage of optocoupled I/O lies in its ability to enable communication between systems operating at different voltage levels. The input side of an optocoupler can be designed to interface with low-voltage control signals from the