HMI screen two-way radio interference: fix bonding and shields

What each symptom points to

Note exactly what goes wrong and whether it clears when the transmission stops; the symptom picks the check.

• Pixels scramble or bands sweep the picture during a transmission, and the picture is normal again seconds after release. Radio energy reached the display side. Start with the earth bond and the wiring behind the panel.
• The picture freezes or the panel reboots during a transmission and recovers afterward. The radio reached the supply or the logic. Check the bond first, then the power routing.
• The picture stays readable but PLC data stops updating while the radio transmits. The energy is arriving on the communications cable. Check shield continuity and the single-end shield termination.
• The fault persists after the transmission stops. That is a supply or link fault, not the radio. Work through the supply checks in PLC COM timeout after power-up: check supply and boot order before blaming the antenna.

Two symptoms can mix. Note which part of the behavior clears with the transmission and which part stays.

Confirm the cause with a keyed transmission test

1. Write down what misbehaving means on this panel: scrambled pixels, a frozen picture, a reboot or lost PLC data, and whether it clears when the transmission stops.
2. With the machine in a safe state, have an operator key the radio briefly at the distance that produced the symptom, about 2 ft in a typical control room case, while you watch the screen and the data. Use the radio at its normal power setting.
3. Record the radio model, the antenna position and the distance. Take the frequency and the transmit power from the label on the radio or from its programming software; the field estimate below needs them.
4. Repeat the transmission once, and only if the first attempt produced nothing and the machine can run safely through a disturbance. Never repeat a test that disturbs control of a running machine.
5. If the disturbance tracks the transmissions and clears after each one, the installation is the fix. If the fault persists with no radio keyed, treat it as a supply or link problem.

Example: a 5 W handheld keyed 2 ft from the screen

Inputs:

• Transmit power P = 5 W, the high setting of a typical VHF handheld. Read the exact figure from the radio.
• Distance d = 2 ft = 0.61 m, the distance that produced the symptoms.
• The benchmark: the industrial generic EMC immunity standard expects equipment to keep working in a field of 10 V/m from 80 MHz to 1 GHz. The IEC 61000-6 series covers the low end of the frequency range up to 400 GHz; the residential and light-industrial edition and the industrial edition both carry the RF field test, and the industrial edition is the one quoted here.

Rule in words: the far-field estimate E = √(30 × P) / d, with E in V/m, P in W and d in m, applies once you are past roughly a sixth of a wavelength (λ/2π). At 150 MHz, a common VHF land-mobile frequency, λ = 2 m, so the boundary sits at 0.32 m and 2 ft is well outside it. Treat the result as a floor, because antenna gain and the near field push the true value a little higher.

Arithmetic:

E = √(30 × 5) / 0.61 = 12.2 / 0.61 ≈ 20 V/m

Result: about 20 V/m of RF field at the screen, roughly twice the 10 V/m the standard tests.

Decision: a field at twice the immunity test level upsets panels freely, so the symptom says nothing about this panel's health. Field strength falls in proportion to distance: keyed at 4 ft, the same radio delivers about 10 V/m at the screen, the level the standard tests. Keep routine keying about 4 ft from the screen while you correct the bond and shield defects, so the panel rides through whatever geometry remains.

Check the protective earth bond

A steel turret or enclosure carries radio energy to earth only where metal meets metal. Painted joints, loose fasteners, corrosion or a bond made through structural steel leave the panel floating at radio frequencies even while the DC bond reads sound.

Inspect the panel's protective earth terminal and every jumper across a painted joint. Tighten the fasteners, clean the contact faces to bare metal and keep the bonding conductor short and direct; at 150 MHz a long coiled lead behaves as an antenna, not a bond.

Measure continuity from the panel's earth point to the facility earth bar using the site's approved method, and confirm the path lands in the facility grounding system. Correct a defective joint to the panel documentation and site electrical practice, then repeat the keyed test.

Leave the separate ground rod out of it: a rod that is not bonded into the facility system invites potential differences between earth points, which is a shock hazard as well as an interference problem.

Check the cable shields

Radio energy rides into the panel on the wiring connected to it. A comms-only symptom points at the communications cable; a screen symptom with comms normal points back at the bond and the power wiring.

Walk the cable from panel to PLC. Identify the cable type from its marking or the drawing, then look for unshielded pigtails, splices, a damaged jacket and any broken braid or foil. A short unshielded gap defeats the shield on the rest of the run, so repair a gap with shielded cable and a proper termination, not a patch of ordinary wire. On a long run, keep that continuity over the full distance; the per-interface distance limits are in HMI to PLC serial cable maximum distance by interface.

Terminate the shield at one end, at the end the equipment documentation names, and keep the drain connection solid. Grounded at both ends, the shield carries loop current and can act as an antenna for the radio; single-end and both-end shield grounding compared sets out when each termination is the right one. The shield is not a return conductor for the signal, and why the shield is not a return covers that job separately.

Check power wiring and routing

Feed the panel the way its documentation shows. Look for loose supply terminals, damaged insulation and any long run of supply wiring laid parallel and close to the radio antenna or its feed line; those runs couple the transmission straight into the panel supply. Keep the supply wiring as disciplined as the DC power wiring guide sets out.

While you work, use distance as the interim control. Field strength halves each time distance doubles, so moving the keying point from 2 ft to 4 ft drops the field at the screen from about 20 V/m to near the 10 V/m test level. Distance manages the symptom while you work; the checks above are the cure.

Make the repair and verify it

Correct the defects in the order you checked them: bond first, shields second, routing third. After each repair, repeat the same keyed transmission test at the recorded position. The repair is done when a brief transmission leaves the picture readable, the data updating and no reboot behind it.

Keep the keying habit even after the repair. Routine keying from about 4 ft is free immunity on a control room floor and costs the operators nothing.

A screen that recovers after each transmission is telling you the installation leaks; if the panel stays abnormal once transmissions stop, chase the supply and the link before ordering any hardware. Record the radio model, frequency, power, test position and the repairs, so a later radio or antenna change can be reassessed against a known baseline.

The same checks keep any screen usable around radios on a floor, and the screen-side basics for a floor display are in shop-floor screens and andon-style boards.

Recommendation: If the screen only misbehaves while a radio is keyed within about 2 ft, do not replace the panel. Verify the protective earth bond into the facility system, restore shield continuity with single-end termination on the comms and power cables, and separate the supply wiring from the antenna feed line; keep routine keying about 4 ft from the screen throughout. Repeat the keyed test after each repair and stop when a transmission at the recorded position leaves the screen and the comms unchanged.

Questions