Looking to increase PLC speed?
Modern process industries depend on quick, reliable data whether it is the level in a storage tank, the flow rate through a line, or the pressure inside a reactor. Behind all this real-time action stands the Programmable Logic Controller (PLC) the brain of automation.
But what if your PLC seems a bit slow?
Delayed updates on the HMI, sluggish alarms, or slow control loops are all signs that your system needs attention. The good news is, improving PLC performance is possible and that’s what this guide from InstrumentationBlogs is all about.
In this article we will learn how you can increase PLC speed. Also we will learn 7 proven techniques that can dramatically improve scan times, enhance reliability and ensure your instrumentation data is always up to date.
Understanding PLC Speed
First let us understand what makes a PLC “slow.”
A PLC works in a scan cycle, which typically includes:
Reading input data from sensors (flow, level, pressure, temperature, etc.)
Executing the control logic.
Updating outputs and communicating results.
This cycle repeats continuously and the time taken to complete one loop is called the Scan Time.
If your PLC’s scan time increases, even by a few milliseconds, your process control can lag especially in high-speed applications such as flow control, batching, or safety interlocks.
7 Proven Techniques for PLC Speed Optimisation
Each of these methods helps to reduce scan time and make your PLC run smoother — with real-world instrumentation examples.
1. Simplify Your Code Structure
The problem:
Complex or unorganised code with multiple nested IF statements and unnecessary loops can slow down execution.
The solution:
Keep your logic clean and modular. Use subroutines or function blocks for repetitive tasks.
Instrumentation example:
Instead of calculating the same pressure alarm condition in every routine, make a subroutine for alarm logic and call it wherever needed. This reduces scan load and makes the code easier to maintain. This will increase plc speed.
2. Use Interrupts Wisely
The problem:
Not all signals are equally important. Treating all inputs the same can waste scan time.
The solution:
Use interrupt routines for high-priority signals like emergency shutdowns or high-speed counters.
Instrumentation example:
In a flow measurement system, use interrupts for turbine pulse inputs (which come fast), while routine temperature or pressure readings can stay in the normal scan.
3. Optimize Data Types
The problem:
Using heavy data types (like REAL or STRING) when not required increases CPU load.
The solution:
Choose lighter data types such as INT or BOOL wherever possible.
Instrumentation example:
If your flow transmitter outputs a discrete pulse count, store it as an integer, not a float. For binary alarms (like high/low pressure), use Boolean tags. It saves memory and speeds up processing.
4. Manage Timers and Counters Smartly
The problem:
Too many timers running in parallel can overload the PLC.
The solution:
Combine timers where possible and reset unused ones.
Instrumentation example:
For example, in a tank level monitoring system, don’t use separate timers for every valve open/close delay. One timer structure can handle multiple delays depending on the valve ID which will increase plc speed.
5. Optimize Communication Networks
The problem:
Heavy data traffic from multiple HMIs, SCADA, or Modbus devices can cause communication bottlenecks.
The solution:
- Use faster industrial protocols like EtherNet/IP, Profinet, or Modbus TCP.
- Minimise polling intervals.
- Transfer only essential data instead of every tag.
Instrumentation example:
If 20 transmitters are connected through Modbus RTU, set proper polling intervals. Or group signals by priority fast-changing variables (like flow) can update frequently, while slow-changing ones (like tank level) can update slower.
6. Improve Documentation and Comments
The problem:
Poor documentation makes debugging and optimisation difficult, especially when multiple engineers work on the same PLC project.
The solution:
Use clear tag names and structured comments. This won’t directly reduce scan time but will drastically reduce troubleshooting time improving overall performance of your automation workflow.
Instrumentation example:
Instead of “AI_01”, name your tag “Level_Transmitter_TK101”. This clarity helps quickly locate performance issues during plant shutdowns or maintenance.
7. Evaluate Your Hardware
The problem:
Sometimes, no matter how well you optimise your code, the PLC hardware itself becomes the bottleneck.
The solution:
Upgrade to a faster CPU or distributed I/O system.
Instrumentation example:
If you are collecting hundreds of analog signals from multiple transmitters, using remote I/O panels connected via Ethernet can drastically reduce communication delays and improve PLC scan time.
Advanced Tips for PLC Speed Optimisation in Instrumentation Systems
Once you’ve implemented the basics, these advanced techniques can push your PLC’s performance even further.
A. Use Structured Text or Function Blocks
Structured text (ST) executes faster for complex math and conditional logic. Use ST for calculations like flow compensation, pressure correction, or temperature conversion.
B. Enable Parallel Processing (Multitasking)
Modern PLCs can run multiple tasks in parallel. Assign critical control logic (like PID loops) to high-priority tasks, while non-critical functions (like data logging) run in the background.
C. Filter Sensor Data Intelligently
Instead of averaging every scan, use digital filters (like moving average with adjustable window size). This gives smoother readings without overloading the CPU.
D. Optimise HMI and SCADA Updates
- Reduce the number of tags scanned per screen.
- Avoid large image files or excessive animations.
- Use historical trending efficiently (log only necessary parameters).
This ensures that the PLC isn’t overwhelmed by unnecessary data requests from HMI panels.
E. Integrate Predictive Maintenance Logic
Instead of continuously checking all devices, monitor only critical parameters.
For example, create a predictive logic that tracks differential pressure across filters alerting maintenance only when required. This keeps the PLC efficient while improving reliability.
F. Use Event-Driven Communication
Whenever possible, switch from cyclic to event-driven data exchange i.e., transmit data only when a variable changes beyond a defined threshold.
G. Monitor and Benchmark Performance Regularly
Use built-in PLC diagnostics or performance counters to measure scan time, communication delay, and memory usage.
Set benchmarks and compare them periodically this helps maintain long-term efficiency.
Frequently Asked Questions (FAQ)
Q1. What is PLC Speed Optimisation?
It is the process of improving the PLC’s performance by reducing scan time and enhancing processing efficiency through coding, communication, and hardware adjustments.
Q2. Does PLC Speed Optimisation affect accuracy?
No. When done correctly, it improves both speed and reliability without affecting accuracy of instrumentation readings.
Q3. How often should I check PLC scan time?
At least once every 3–6 months or after major program changes.
Q4. What’s an ideal scan time for process PLCs?
Depends on the process. For instrumentation control, 10–50 ms is common. Fast control loops may need <10 ms.
Q5. Can communication protocols affect PLC speed?
Yes. Ethernet-based protocols (Profinet, EtherNet/IP) offer much faster response compared to serial-based ones (Modbus RTU).
What we learn today?
PLC performance directly impacts how well your instrumentation system responds to process changes. By applying these PLC Speed Optimisation techniques, you can increase PLC speed and ensure faster scan times, smoother communication, and more reliable control.
Remember — faster doesn’t just mean “better.” It means safer, more efficient, and more productive.
For more practical guides like this, stay tuned to InstrumentationBlogs.in, your trusted source for learning instrumentation and control engineering the easy way.
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