How do I tell a failed backlight from a dead HMI?

Power the unit and aim a flashlight at the screen from an angle. A faint ghost of the normal display means the computer and LCD are working and only the backlight is gone — a routine repair. No image at all under the flashlight points at the display assembly, its board, or a unit that isn't booting.

Is a black-screen HMI worth repairing?

Backlight, power-section and display repairs typically run $300–$1,000 and 1–2 weeks — usually worth it on a current model. On a discontinued model, check its lifecycle status first: repairing into a family with no spares and unobtainable software can be money toward a dead end.

The screen is black but the machine keeps running — how?

The HMI is only the operator's window; the PLC runs the machine autonomously and doesn't care whether the panel lives. That's also your opening: bridge the operator view first, then repair or replace the panel without production pressure.

Can I get the operator a screen back today?

If the controller speaks EtherNet/IP, Siemens S7 or Modbus TCP, yes — the free LineKeeper app for iPhone and iPad connects directly; on Allen-Bradley Logix it reads the whole tag list automatically. A dashboard takes about 15 minutes; the operator panel's start/stop buttons come once the right tags are identified. No changes to the PLC logic.

HMI black screen: dead unit or $40 fix? How to tell in five minutes

The five-minute separation

What you see	What it means	Severity
No LEDs, nothing at all	Supply problem — measure 24 VDC at the panel's own terminals; check the fuse and the supply under load, not just its idle LED	Often a $40 fix
Power LED on, screen black — flashlight shows a faint image	Backlight failure. Aim a flashlight at an angle: a ghost of the screen means the display and computer are fine	Repairable; routine on CCFL-era panels
Power LED on, truly no image under flashlight	Display assembly or its board	Repairable, costs more
Shows boot logo, then black	Firmware or application failure — the display just proved it works. Look for maintenance/safe-mode boot options for your model	Software-side; often recoverable
Screen black, but touch beeps or the PLC reacts to presses	Runtime alive, display-side fault only — blind operation is a burn risk; stop guessing at invisible buttons	Repairable

Full step-by-step triage, including what to check in the cabinet before blaming the panel: the 10-minute line-down guide.

Brand notes

Allen-Bradley PanelView: symptoms decoded model by model in the PanelView Plus 7 guide; touch-specific faults in the touchscreen guide. Older PanelView families may not be worth the repair — check the obsolescence timeline first.
Siemens KTP / Comfort Panel: the same flashlight logic applies; symptom walkthrough in the Siemens guide, and if the unit is from a retired TP/OP generation, the replacement options guide covers the paths.
Mitsubishi GOT: touch the screen before anything — the GOT's screensaver blanks the backlight on a timer and fools everyone: the GOT guide.
Weintek, Delta, Pro-face, Red Lion, Magelis: the electrical triage is identical — supply, backlight, display, boot. Brand changes the recovery doors and where the project files live: Weintek · Delta · Pro-face · Red Lion · Magelis.

Repair, replace or bridge

Repair: backlights, supplies and display assemblies are routinely rebuilt for $300–$1,000, typically 1–2 weeks with shipping. The right call for a healthy panel with one dead subsystem.
Replace: the honest math depends on whether your model is current or retired — a discontinued unit pushes you into scarcity-priced spares or a migration decision. That's a strategy question, not a shopping question.
Bridge first: either way, the operator needs a screen today. That's the 15-minute job below — and it takes the panic out of the repair-or-replace decision.

Keep producing while you wait

Have the panel project export? Drop a FactoryTalk View, TIA Portal, Weintek or CSV export into our free HMI project converter — it runs in your browser, the file never leaves your PC, and your displays' buttons, indicators and values become app pages with the real tag names already in place. Graphics and alarm logic aren't carried over, and it takes the project export — a bare .mer runtime can't be converted.

Today, from a phone: the free LineKeeper app for iPhone and iPad speaks EtherNet/IP, Siemens S7 and Modbus TCP. On Allen-Bradley Logix it reads the controller's own tag list automatically; on S7 and Modbus you add the addresses you care about. Build a dashboard in about 15 minutes — the operator panel's start/stop buttons come once the right tags are identified, ideally mapped in advance. Mount a spare iPad in full-screen panel mode, and the operator has a screen before the first repair quote arrives. Reading is free; writes are a one-time unlock with confirmation and limit checks on every value.

Tag browser in the LineKeeper app: every tag of a ControlLogix or CompactLogix controller with live values and search, read straight from the PLC — The Logix controller hands over its whole tag list — live, searchable

Temporary operator panel in the LineKeeper app standing in for an HMI with a black screen: start and stop push buttons, hold-to-run jog, speed setpoint and status lamps — The temporary panel: start/stop, jog, setpoints, status lamps

Guarded setpoint write in the LineKeeper app: allowed range shown and confirmation required before the value reaches the ControlLogix PLC — Writes are range-checked and confirmed before they reach the PLC

Free app · iPhone & iPad

A temporary screen in minutes

Zero-config tag browser for Logix, drag-and-drop dashboard, operator panel, alarms — no changes to the PLC logic.

Download on theApp Store

On a desktop? Scan with your iPhone or iPad to get the free app.

See the app tour →

For the machine itself: the LineKeeper device — in development — is a self-contained 7″ or 10″ touchscreen built to live on the machine permanently — the same drag-and-drop screens, plus what a phone shouldn't carry:

Live view of every mapped tag, refreshed twice a second — plus per-tag unlocked writes, confirmed and audit-logged
Operator panel: start/stop, indicators, setpoints — hold-to-run releases when your finger lifts (not a safety function)
Stays useful after the repair: data logging, dashboards, machine diagnostics

Small hardware that helps while the panel is out

GL.iNet Opal travel router — Wi-Fi into the machine network from a spare switch port, so the phone app can stand in as the screen today.
Anker USB-C to gigabit Ethernet adapter — wired straight into a spare switch port, the phone sees the controller in seconds; USB-C iPhones and iPads need no drivers.
VIVO pole-mount VESA arm — puts the temporary screen at eye level on a machine post instead of a chair.
TICONN IP67 enclosure with clear cover — houses the gateway or Wi-Fi bridge on the machine without bare boards on the rail.

Amazon affiliate links — they never change the price you pay.

The plug-in LineKeeper device — a 7″ or 10″ touchscreen version of the same idea, with 24/7 logging — is in development ($1,290 planned, one-time). No preorders, no deposits; details on the pricing page.

HMI black screen: dead unit or $40 fix?