A touchscreen that lights up is not necessarily a working HMI. It may have a dead strip along one edge, incorrect calibration, intermittent communications or a backlight that fails after ten minutes. To test HMI touchscreens properly, check the operator interface as a working part of the control system, not simply as a display.
For maintenance teams, the purpose is straightforward: establish whether the fault is in the panel, the touchscreen layer, the power supply, the communications path or the PLC application before ordering a replacement. A controlled test also reduces the risk of installing the wrong spare during a breakdown.
Start with the correct part and a safe test setup
Record the full manufacturer part number, including revision, display size, supply voltage, communication ports and any option codes. A similar-looking Siemens, Allen-Bradley, Mitsubishi, Schneider or Omron terminal may use a different project format, connector arrangement or firmware generation. The front bezel alone is not enough for procurement.
Before energising the unit, inspect the label and enclosure. Check for water ingress, impact marks, damaged seals, bent pins, missing terminal blocks and evidence of previous repair. On used equipment, pay particular attention to the membrane around the screen and keys. A unit can appear clean while a damaged seal has allowed contamination into the display assembly.
Use the specified DC supply, with adequate current capacity and correct polarity. Verify the supply at the HMI terminals under load rather than relying on an unloaded power reading. A weak 24 VDC circuit, poor earth connection or voltage drop caused by a long field cable can imitate an HMI fault.
Where testing is carried out on installed equipment, follow site isolation procedures and ensure that operating the screen cannot start machinery or alter a live process. If possible, test on a bench with a suitable power supply and known-good communications lead. This separates the panel condition from faults in the machine wiring.
Test HMI touchscreens for display and touch response
Power the terminal and allow it to remain on long enough to reach normal operating temperature. A short power-up check will not always reveal a failing backlight, unstable power circuit or display fault. Watch the boot sequence, brightness and screen stability rather than moving straight to the application pages.
Inspect the screen before running the application
Look for dim areas, flickering, vertical or horizontal lines, colour shift, image retention and uneven backlighting. LCD panels can remain usable with minor cosmetic marks, but a growing dark area or intermittent illumination should be treated as a reliability concern, particularly on a 24-hour line.
Check the screen at different brightness settings if the terminal allows it. A display that becomes unstable only at higher brightness may indicate an ageing backlight or internal power issue. Also inspect the viewing angle. Severe loss of contrast away from centre can make an HMI difficult to use at the machine, even if it passes a basic bench test.
Check every touch zone, not just the centre
Open the manufacturer’s diagnostic screen where available, or use a simple test page with buttons placed around the full display. Press the corners, edges, centre and each active object. Confirm that the selected object matches the point touched and that it responds consistently to light, normal pressure.
Pay close attention to the lower edge and corners. These areas receive the most repeated use on many operator panels and are common locations for resistive touchscreen wear. A touch panel that needs hard pressure, registers a press beside the intended button or misses one area may require recalibration. If calibration cannot correct it, the digitiser or touch overlay may be failing.
Test drag actions, scroll bars, numerical entry fields and acknowledgement buttons as well as simple page changes. Some faults only appear during a continuous press or when two touch points are used on a capacitive display. For a multi-touch HMI, confirm that the functions expected by the installed application work as intended. Not every panel or legacy project supports multi-touch behaviour.
If the unit has physical function keys, a keypad, selector control, USB port or memory-card slot, test those too. The HMI is often the operator’s only route to alarms, recipes and maintenance functions. A display with working touch but failed hard keys is not a complete pass.
Confirm the application and communications path
A healthy touchscreen can still be unusable if it cannot communicate with the controller. Connect the HMI to the intended PLC or a known-good test system, then confirm that values update, alarms appear and commands are received correctly. Watch for delayed updates, communication loss messages and values that freeze without generating an obvious alarm.
Check the interface used by the machine: Ethernet, serial, MPI, Profibus, EtherNet/IP, Modbus or another supported network. Confirm the correct port is being tested. Many terminals have separate programming and controller connections, and the wrong cable or port can lead to an unnecessary replacement order.
Review the terminal’s IP address, node settings, baud rate and protocol configuration against the existing machine documentation. When an HMI has been replaced or repaired, a saved project may be present but configured for a different network. That is a commissioning issue, not proof that the panel is defective.
Where access is available, make a backup of the existing application before changing settings or attempting a firmware update. Older HMIs may depend on a particular software version, PLC driver or panel image. Updating firmware to solve a touch issue can create a compatibility problem with the installed project, so it should be a deliberate decision rather than a first response.
Separate panel faults from machine faults
The fastest diagnosis comes from comparing known-good components. If the HMI boots but shows no live data, test its communication lead, switch port and PLC status before condemning the panel. If screen presses are visible in diagnostics but buttons do not act on the machine, investigate application permissions, PLC logic and safety interlocks.
Likewise, a completely blank screen does not automatically mean the display has failed. Check incoming voltage, fuses, terminals, earthing and cable condition. If the unit starts intermittently, monitor supply voltage during machine start-up, when contactors pull in and when nearby drives switch. Electrical noise and supply dips can present as random touchscreen freezes or reboots.
A practical fault record should include the part number, serial number, symptom, supply voltage, test duration, communication method and result of each touch area. This gives procurement a clear basis for sourcing and gives the repair team useful evidence if the unit is returned for assessment.
Check replacement compatibility before placing an order
When a touchscreen fault is confirmed, buy against the complete part number wherever possible. Screen size, mounting cut-out and logo are poor substitutes for an exact match. The replacement must suit the original supply, communication network, memory requirements, project software and physical environment.
A refurbished HMI can be a sensible option for legacy machinery or an urgent repair, provided the condition is stated clearly and the item is tested for the functions that matter. New and sealed stock may be preferred for critical spares policies, but availability, lead time and the age of the installed platform all affect the decision. For discontinued panels, a tested refurbished unit may be the practical route to restoring production while a longer-term migration is planned.
Also check whether the failure is limited to the touch overlay. On some models, the overlay, display and controller are separate service items; on others, they are supplied only as a complete assembly. Repairing a component can reduce cost, but it may not be the right choice where labour time, repeat failure risk and production loss outweigh the price of a replacement terminal.
Automation Planet UK can help source part-number-specific HMI stock across major automation brands, including new and sealed and refurbished options. Providing the full label details, photographs of connectors and a clear fault description helps avoid delays when a legacy panel needs replacing.
Build the test into your spare-parts process
Do not wait for a production stoppage to find that a shelf spare has a touch fault or incompatible firmware. When stock arrives, record its condition, power it up, test the display and touch zones, and confirm the part number against the machine asset record. Store the backup project, required cables and configuration notes with the spare or in the maintenance system.
A ten-minute functional check cannot certify every possible operating condition, but it can catch the faults that create the longest delays: wrong part numbers, dead touch areas, poor displays, missing terminal blocks and communication mismatches. Treat every HMI as a control-system component with a defined test result, and the next replacement decision will be quicker and better supported.