Bently Nevada 120M8155-01 (HMI-UI) — Touch Panel Blackout After Power Cycling - Excellent PLC

Model: Bently Nevada 120M8155-01
Device Type Code: HMI-UI (Human-Machine Interface for User Interaction)
Industrial Role: Local display unit for vibration protection systems and machine health dashboards

Observed Field Behavior

After a scheduled maintenance shutdown, operators reported that the 120M8155-01 touchscreen remained completely black after cabinet power-up. The backlight did not activate, and no boot logo appeared. Communication between monitoring racks and the PLC remained normal, confirming that only the display subsystem was affected.

Context of Failure

Power cycling events occur frequently due to:

planned maintenance
plant black starts
UPS battery replacements
emergency turbine trips

A display blackout immediately after these events typically points to issues linked to power input quality or internal startup logic rather than catastrophic hardware failure.

Technical Root Causes Identified

Based on similar incidents across compressor, turbine, and refinery installations, three technical factors are most common:

1. Backlight Inverter Failure

The LCD inverter is sensitive to repeated inrush current after power cycling. When it fails, the panel may still boot but remains visually black due to lack of illumination.

2. Power Module Undervoltage

If control cabinet voltage drops below minimal threshold (often during black starts), the HMI may initiate boot but stall before LCD initialization.

3. Damaged Cold-Start Capacitors

Aged electrolytic capacitors lose capacitance and cannot deliver surge current during cold starts, causing blank display conditions.

Diagnostic Steps Used by Field Technicians

A structured diagnostic approach was taken to avoid unnecessary replacement:

Checked Input Voltage at HMI Terminal
Voltage fluctuated between 21.4–23.1 VDC (below typical 24V nominal).
Confirmed Panel Boot via Ethernet Pings
Device responded to network ping, confirming system firmware was active.
Inspected Internal Inverter Board
Mild discoloration and odor indicated thermal degradation.
Thermal Imaging
Infrared scan showed abnormal heating near inverter IC.

Result: confirmed inverter subsystem failure combined with unstable supply.

Corrective Measures Implemented

A two-stage repair was performed:

Electrical Stabilization
- 24VDC supply regulator added to prevent undervoltage events
- verified cabinet UPS output ripple within acceptable limits
Hardware Service Action
- replaced inverter board
- replaced aging capacitors around power conditioning circuit

Following service, the display recovered normally on re-energization, and no further blackout events were recorded during the following 90-day operational period.