When it comes to the work of electricians, the older generation relied on abacuses and notebooks. To calculate loads, they would simply draw a line with a pen; to change wiring, they would manually sketch a wiring diagram. Later, relays were strung together like beads on an abacus, where any movement would trigger a response, and logic was entirely dependent on the opening and closing of contacts.

Back then, control cabinets resembled a wall of mechanisms, with wires crisscrossing everywhere, making it overwhelming. But times have changed, and now electricians have a new tool in their hands—PLC, or Programmable Logic Controller, commonly referred to as the ‘new abacus’ for electricians.

Why is it called an ‘abacus’?

What can an abacus do? It can perform calculations and keep accounts. The work done by a PLC is quite similar, but instead of calculating money, it calculates logic. Previously, you would need a row of intermediate relays to achieve interlocking; now, you can accomplish it with just a few lines of ladder logic.

In the past, a control cabinet could connect hundreds of wires, making it easy to miswire; now, a single PLC unit has just a few input and output terminals, and by arranging the logic in software, the machine can respond correctly.

For example: if you want to control a motor to rotate forward and backward with interlocking protection. The old method involved two contactors with interlocking contacts, plus a thermal relay.

The wiring diagram would look like a spider web. With the PLC method: write a logic statement—if the forward button is pressed, output Y1; if the reverse button is pressed, output Y2; then add an interlocking condition.

Three lines of logic, and it’s done. Doesn’t it resemble the movement of an abacus bead? The abacus moves beads, while the PLC manipulates logical contacts.

What does it look like?

Don’t think of a PLC as a large machine; it’s actually a small rectangular box. It has a row of input terminals on top (for buttons and sensors) and a row of output terminals on the bottom (for contactor coils and solenoids).

In the middle, there is a CPU, which acts like the brain, responsible for processing input signals according to logical operations and producing output results.

Imagine this: when you press a button, the PLC receives a voltage signal, equivalent to moving an abacus bead.

Once the CPU reads it, it calculates based on the logic program, resulting in the appropriate lights turning on or off, and the necessary actions being taken. This is how it operates.

How is it different from relay logic?

Previously, control logic relied entirely on metal relays. To introduce delays, you would add a timer relay; to count, you would add a counter; to maintain a state, you would add a latching contact. To achieve even a small function, several components had to work together, and there was always the risk of contacts burning out.

PLC is different; all relays, timers, and counters are ‘software components’. You can draw ladder diagrams on the screen, placing contacts and coils as needed, and if you’re not satisfied, you can delete and redraw without having to rewire. For electricians, this is the ‘upgraded abacus’, allowing them to manipulate thousands of logic lines with just a finger.

Where is it used?

The applications of PLCs are astonishingly broad. Factory assembly lines, automatic doors, elevators, packaging machines, stamping machines…

Wherever electrical control logic is needed, PLCs can be applied. In the past, a workshop’s control cabinet resembled a monster; now, a small cabinet with a PLC can replace an entire system.

For instance, in an automatic washing machine, the water level detection and speed control essentially follow the logic of a small PLC; similarly, traffic lights at intersections operate on a logic of timing and interlocking, which PLCs can easily implement.

How to learn it?

When electricians first encounter PLCs, they often feel a bit lost: why do they need to write programs? Don’t worry, PLC programming is not the computer code you might imagine, but ratherladder diagrams, which are drawn with horizontal and vertical lines, similar to wiring diagrams.

The vertical lines represent power, and the horizontal lines represent logical paths. By placing contacts and coils on the diagram, it can run. For those learning to be electricians, this is much more intuitive than writing a bunch of English commands. You can even translate previous relay wiring into ladder diagrams, which is almost like a beginner’s guide.

For example:

• Normally open contact is like a button.

• Normally closed contact is like a stop button.

• Output coil is like a contactor.

• Timer and counter are virtual and do not require physical connections.

In other words, the world inside a PLC is like a virtual relay factory.

Why can’t electricians live without it?

First, flexibility. When factory processes change, previously you would have to rewire; now you just need to modify the program. Second, reliability. Contacts won’t burn out, and logic won’t drop out. Third, convenience. One PLC can replace dozens of relays.

Moreover, PLCs can connect to computers, communicate with sensors, and work with human-machine interfaces. In the past, electricians would watch indicator lights; now they can see all statuses on a screen and even monitor remotely. No abacus can perform such tricks!

Key to Getting Started

To truly learn PLCs, there are three key points:

1. Logical Thinking: Electricians who can draw wiring diagrams already possess logic; they just need a different canvas.

2. Familiarity with Components: Understanding how to connect input and output terminals, and how to use normally open and normally closed contacts, requires hands-on practice.

3. Practice and Experimentation: Don’t be afraid to make mistakes; if you make an error, delete it. PLCs are cheaper and more forgiving than relays.

Remember this: PLCs are not meant to turn you into a programmer, but to help you transfer the logic of an electrician into software.