PLCs in automation systems are either centralised in control rooms or dispersed on field equipment. Though mostly surrounded by the harsh electromagnetic environment of power circuits and equipment, the PLC is designed specifically for industrial settings — multi-layer anti-interference measures and carefully selected components give it strong tolerance of harsh environments, stable operation and high reliability, so it usually needs no special measures for industrial use. But because it connects directly to field I/O, external interference readily enters via power or I/O lines and causes malfunctions. Interference divides into external (random, unrelated to system structure, sources cannot be eliminated, only limited case by case) and internal (structure-related, entering mainly via AC mains and analogue inputs, reducible by sound circuit design). Improving PLC reliability means strengthening immunity from several directions — analysed below with hardware measures.
PLCs need no special measures for industrial use, but excessively harsh environments or bad installation degrade reliability. Ambient temperature 0–55℃; avoid direct sunlight and heat sources; ensure ample heat dissipation and ventilation. Humidity below 85% for good insulation. In corrosive gas, heavy fog or dust, install enclosed. Strong vibration also reduces reliability — apply damping.
The PLC itself resists interference well. Simply wiring its supply separately from power equipment usually suffices against mains-borne noise. In unusually severe cases add a screened isolation transformer to reduce earth-coupled interference. If expansion units exist they must share one switch with the basic unit — powered up and down together. Good earthing is vital: give the PLC a dedicated earth, separated from other equipment (Fig. 1a); if impractical, common earthing (Fig. 1b) is acceptable, but series (daisy-chained) earthing (Fig. 1c) is forbidden — it creates potential differences between devices that inject interference. Earth leads thick, resistance small, point as close to the PLC as possible.
Earthing serves safety and interference suppression; a complete earthing system is a key anti-interference measure. Here "earth" means the system-potential reference, not the signal return. Metal frames floating in the PLC room are aerials picking up airborne interference and need a potential-reference earth. The AC earth is required for supply via the transformer neutral — one of the two supply loops — and carries currents and harmonics that are a serious interference source, so AC earth, DC earth, analogue earth and digital earth must be separate. The common point of digital and analogue earth is best left floating. Minimise potential differences along earth runs, thicken earth leads, use ring earths where possible. The system-earth terminal (LG) is an anti-interference neutral terminal, usually ungrounded; when electromagnetic interference is severe, connect it to the earth-electrode terminal (GR).
The input circuit is the PLC's port for digital and analogue signals; component quality and wiring soundness directly affect reliability. For digital inputs, keep pushbutton and limit-switch contacts in good order and wiring tight. Mechanical limit switches are failure-prone — prefer reliable proximity switches. Pushbutton contact choice matters too; choose reliable components. Analogue inputs are commonly 4–20 mA / 0–20 mA current or 0–5 V / 0–10 V voltage with 24 V DC supply.
Digital outputs come as relay, thyristor or transistor types — choose per the load, or reliability falls and the system may stop working altogether: thyristor outputs for AC loads only, transistor for DC only. Output terminals have limited load capability — beyond the maximum, external relays or contactors are required. The quality of those external contactors, relays and solenoids strongly affects reliability: typical faults are coil shorts and mechanically stuck or dirty contacts. Use quality components, and in high-reliability applications diagnose key output points from current anomalies, switching to programmed fault handling on detection. Where inductive loads sit on outputs, fit protection to spare the PLC contacts.
To limit external wiring interference, route AC and DC I/O in separate cables; for integrated-circuit or transistor equipment I/O, screened cable is mandatory — screen floating at the I/O side, earthed at the control side (Fig. 2).
Hardware measures aim to keep interference out, but given its randomness — especially in industry — hardware alone cannot shut everything out. Software flexibility combined with hardware raises immunity.
1. Watchdog monitoring. PLCs are rich in soft elements — timers, counters, auxiliary relays — usable to mask bad input signals and prevent erroneous outputs. Define a timer as a watchdog for a moving part, set to the maximum plausible travel time; start it with the motion command. If the part reaches position in time it signals completion and clears the timer — normal; otherwise alarm or stop.
2. Debouncing. In vibrating environments limit switches and buttons emit spurious short glitches; delay with an internal timer to obtain a reliable, debounced signal.
3. Software digital filtering to raise signal-to-noise. For large random spikes, amplitude-limit: sample five times and discard any sample far above the others. For frequently fluctuating parameters — flow, pressure, level, position — use arithmetic averaging of n samples (n=12 for flow, n=4 for pressure are considered best). For slow signals such as temperature, take three samples and keep the median. For integrating A/D converters, sample over whole power-frequency periods (20 ms multiples) — proven better against mains interference than a plain integrator.
