The Development of PLC: From Relays to Intelligent Controllers
——A lecture by an electrician teacher who transitioned from a “traditional electrical worker to an automation expert”
When it comes to PLCs, they are now the “intelligent controllers” that everyone in factories is clamoring for. However, if you really ask how they came to be and how they became so powerful, you might need to look into the “evolution of electrical engineering” to understand it clearly.
I entered the workshop in the late 1980s, when there were no PLCs, just a sea of relays, time relays, and control transformers, with control cabinet schematics stacked like accounting ledgers. There was no such thing as “downloading programs”; everything relied on wiring to connect logic, and even a self-holding circuit required a convoluted wiring setup. How did PLCs come about? It was because these electricians were driven to madness by wiring and forced the era to improve.
1. Starting from the Era of Clicking Relays
The earliest control systems relied on a series of relays connected in series and parallel. What is “electrical control”? It means that when you press a button, it energizes a relay, which then controls a contactor or another relay, stacking logic layer by layer.
In those days, creating a simple sequential control required drawing dozens of schematics, with wires piled up like a bird’s nest. Changing the logic meant completely rewiring everything. Maintenance relied on using a voltage tester to probe bit by bit; when something went wrong, it was a literal “firefight in the electrical cabinet”—a physical struggle to troubleshoot.
Back then, we didn’t even talk about “intelligence”; as long as the wiring was correct and the machines didn’t explode, we were already grateful.
2. Factories Became More Complex, and Relays Couldn’t Keep Up
As production lines began to grow more complex, it was no longer just a matter of “press to start, release to stop”; it started to involve conditional interlocks, sequential control, alarms, and interlocks. Sometimes, actions needed to be delayed based on time, judged by quantity, and required memory—relays began to show their limitations.
When we were working on automatic batching, we encountered a situation where a batching tank had four solenoid valves that needed to fill proportionally, delay closing after filling, enter a mixing phase, and interlock with weighing devices and mixing motors, all while providing alarms, counting, and recording interactions. In such cases, relays were simply insufficient, and even if they were, the wiring could drive anyone crazy.
People began to ponder whether there was something that could “remember states, judge conditions, and run automatically”—this is when PLCs made their debut.
3. The First Generation of PLC: The Programmable Controller Replacing Relays
In the late 1970s and early 1980s, the first generation of PLCs appeared on the industrial scene. The initial idea wasn’t very advanced; simply put, it was a small computer. Its “input ports” connected to buttons, switches, and sensors, while the output ports connected to relay coils, contactors, and solenoid valves. You didn’t need to wire complex logic; you just needed to write a simple program, and the equipment could operate in logical sequence.
Initially, PLCs didn’t even have screens; they relied on rows of lights and paper program sheets, with debugging done using dip switches, and programs input line by line using a “programmer”. Being able to write a self-holding + timing function was already a great achievement.
But this “programmable controller” reduced a bunch of relay cabinets into a small metal box.
4. The Second Generation of PLC: From “Logic Replacement” to “Digital Control”
By the 1990s, PLCs were no longer just tools to save relays; they had become the central brain of entire production lines.
Especially with the advent of PLCs like Siemens S7-300, which not only had powerful programming capabilities but also supported networking, analog control, PID regulation, and remote downloading… it was no longer just a “patch in the electrical cabinet” but a system-level hub.
I remember the first time I encountered the S7-300; I was helping a food factory upgrade their filling line. Originally, they relied on mechanical cams and a bunch of limit switches to determine bottle positions. We replaced all the mechanical limits with the S7-300, using photoelectric sensors to feed signals into the PLC, and created a program for “anti-overfill”, “delayed filling”, and “synchronized coding” control, which directly improved efficiency by 30% with almost no faults.
Since then, many factory electricians began to switch careers to learn PLCs; who still relies on wiring diagrams to make a living?
5. Today’s PLC: More Than Just a Controller, It’s the “Industrial Brain”
Today, PLCs are no longer just the “automation nerve” in workshops; they can even connect with upper-level systems, ERP management systems, and MES production traceability systems. With a tap on a mobile app, the PLC can immediately start up, switch batches, or issue alarms on the production line.
For instance, Siemens’ S7-300, combined with the WinCC monitoring platform, can achieve “real-time data collection + alarm analysis + historical review + recipe management”, even adjusting warehouse operations.
Once, we did remote PLC debugging for a chemical plant, and they said: “In the past, when our equipment malfunctioned, electricians would have to break in at midnight to check the wiring. Now that you have installed the PLC, when something goes wrong, we can check which segment of logic is faulty using a computer, and a remote program change fixes it.” Ultimately, it’s about intelligence replacing manual blind repairs.