3 Techniques to Solve Slow PLC Program Execution and Instantly Boost Device Performance

Five years ago, on a late night, I was urgently called to a packaging production line. The client was frantic—the newly commissioned equipment was running 30% slower than expected, packaging 20 fewer products per minute, leading to significant daily losses. Upon inspecting the PLC program, I shook my head internally: another typical case of “functionality working but performance poor.”

This debugging experience that night led me to summarize three killer techniques to specifically address the issue of slow PLC program execution.

Technique 1: Scan Cycle Optimization—Slimming Down the PLC

Many beginners write programs as if packing a suitcase, trying to stuff everything inside.The PLC scan cycle is like a human heartbeat, with each scan running the program from start to finish; the bulkier the program, the slower the heartbeat.

I once encountered an exaggerated program with a scan cycle of 150ms! Normally, it should be controlled between 10-50ms. That program was filled with numerous useless intermediate variables, repetitive calculation logic, and a bunch of “zombie code” that was never executed.

The optimization secrets are simple:

• • Remove useless code: Just like cleaning up junk files on a computer, regularly check and delete unused program segments.
• • Merge duplicate logic: Perform the same calculation only once, storing the result in an intermediate variable for multiple calls.
• • Use conditional execution: Not every function needs to be executed on every scan; make good use of conditional checks.

That night, I spent 2 hours slimming down that “fat program,” reducing the scan cycle from 150ms to 35ms, and the device speed immediately increased by 76%.

Technique 2: Data Processing Strategy—The Art of Intelligent Laziness

Good programmers are “smart lazy people”. In PLC programming, this “laziness” is reflected in data processing—calculating only what needs to be calculated and not calculating what doesn’t need to be calculated.

I once encountered a temperature control program where the programmer placed the PID control algorithm in the main program, calculating it every scan cycle. It’s important to note that temperature changes are slow processes, and such high-frequency calculations are unnecessary. I changed the PID calculation to timed execution, calculating once every 100ms, which instantly reduced the CPU load by 60%.

Practical tips include:

• • Hierarchical processing: High-frequency processing for urgent, real-time data, and low-frequency processing for statistical and display data.
• • Cache mechanism: Store complex calculation results first, and call them directly when needed.
• • Batch operations: For example, accumulate data records to a certain number before writing them all at once, rather than writing one at a time.

A seasoned engineer once told me: “The performance of a PLC is not calculated; it is saved.” I couldn’t agree more.

Technique 3: I/O Response Optimization—Letting Signals Fly

The I/O response speed is often the bottleneck of the entire system. It’s like a toll booth on a highway; no matter how wide the road, you still have to queue after the toll booth.

One of my most memorable optimizations was on a high-speed sorting system. The equipment needed to process 50 products per second, but the sensor signals were always “half a beat slow.” The problem turned out to be the input filtering time setting—set too long to prevent interference, the signal stabilized but the response slowed down.

Optimization strategies are as follows:

• • Set filtering time reasonably: Find a balance between anti-interference and response speed; usually, 3-10ms is sufficient.
• • Optimize scan order: Process critical I/O points at the beginning of the program to reduce delays.
• • Use interrupt functions: For particularly urgent signals, use interrupt programs for immediate response without waiting for the main program scan.

After that optimization, the system response time was reduced from 80ms to 25ms, and the sorting accuracy improved from 92% to 99.5%.

**The essence of these three techniques can be summed up in one word: precision.** Programs should be concise, processing should be accurate, and responses should be precise. Whenever I encounter performance issues, I ask myself three questions: Does this piece of code really need to be executed every time? Does this calculation really need to be so frequent? Does this I/O really need such a slow response?

Over the past fifteen years, I have seen too many engineers lose sleep over performance issues. In fact, performance optimization is not some profound technology; it is more of a habitual way of thinking. When you start thinking about “how to make the program run faster” instead of “how to make the program run,” you have already moved from beginner to expert.

Remember, a good PLC program must not only function correctly but also perform excellently. After all, in this efficiency-driven era, every millisecond of optimization can translate into real monetary gains.