What is Cycle Time?
Cycle Time is the fixed time period in which an industrial Controller, such as a PLC, performs a complete processing cycle. This includes reading inputs, executing logic and driving outputs.
In real-time systems, a consistent cycle time is essential for predictable and deterministic behaviour.
⚙️ What happens in one cycle?
Each cycle typically consists of three steps:
-
Input scan The system reads all digital and analogue inputs.
-
Logic execution The programmable logic is executed (e.g. in ladder diagram or structured text).
-
Output scan Outputs are driven based on the processed result.
These three phases repeat continuously — every
Xmilliseconds — depending on the configured cycle time.
🧠 Characteristics of cycle time
| Property | Explanation |
|---|---|
| Fixed time interval | Usually expressed in milliseconds (e.g. 5 ms or 20 ms) |
| Determines resolution | The shorter the cycle time, the faster the system can respond |
| Affected by logic | Complex logic or communication can lengthen the cycle time |
| Relation to determinism | Essential for deterministic behaviour |
🏭 Cycle time in OT systems
| Application | Typical cycle time |
|---|---|
| Motion Control | 1–4 ms (high speed, low latency) |
| Standard PLC control | 5–50 ms |
| SCADA visualisation | 250–1000 ms (not real-time) |
| Safety systems (SIS) | 10–20 ms (depending on SIL level) |
Cycle time must be matched to the physical process: too slow → inaccurate, too fast → overloaded.
⚠️ Issues with unstable cycle time
- Jitter (variations in cycle time)
- Delayed response to sensor values
- Incorrect timing during synchronisation
- CPU or network overload
See also: Jitter, RTOS, Network Congestion
✅ Optimising cycle time
| Measure | Explanation |
|---|---|
| Use RTOS | A real-time OS provides consistent timing |
| Split logic into phases | Avoid overloading a single cycle |
| Move communication out | Time-consuming network requests in separate processes or tasks |
| Monitor CPU load | Cycle time grows under heavy load |
| Use an event-driven approach | Not everything needs to run in every cycle |
📌 In summary
Cycle time defines the rhythm of your control system. For industrial automation this means predictability, stability and control over your process — provided it is properly tuned and monitored.