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KVM Tutorial 04 — Troubleshooting

Common problems and solutions for Openterface KVM devices.

Device Not Detected

Symptoms

  • "No devices found" in device menu
  • Keyboard and mouse indicators show orange or gray
  • Serial port shows "N/A"

Diagnosis

Linux:

Bash
lsusb | grep -E "534d|1a86"
dmesg | tail -20
ls /dev/hidraw*   # HID video chip
ls /dev/ttyUSB*   # serial chip

Expected: 534d:2109 (HDMI capture) and 1a86:7523 or 1a86:fe0c (serial).

macOS: Apple Menu > About This Mac > System Report > Hardware > USB — look for Openterface.

Windows: Device Manager > "Universal Serial Bus devices" and "Ports (COM & LPT)" — CH340 should appear as "USB-SERIAL CH340 (COMx)".

Solutions

Problem Fix
Device not in lsusb/System Report Try different USB cable/port. Needs USB 2.0+
Device appears but no nodes Check udev rules (Linux) or reinstall drivers (Windows)
Permission denied Add user to dialout and video groups (Linux)
Detected then disappears brltty claiming the serial port (Linux) — see below

BrlTTY Conflict (Linux)

The most common cause of keyboard/mouse failure on Linux.

The brltty (Braille terminal) service claims USB serial devices, including the CH9329/CH32V208 chip.

Fix

Bash
# Option 1: Remove brltty (if you don't need Braille support)
sudo apt remove brltty          # Debian/Ubuntu
sudo dnf remove brltty          # Fedora

# Option 2: Blacklist the device (preferred)
echo 'ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", ENV{BRLTTY_BRAILLE_DRIVER}=""' | sudo tee /etc/udev/rules.d/99-brltty-openterface.rules
sudo udevadm control --reload-rules

No Video / Black Screen

Steps

  1. Verify HDMI cable securely connected at both ends
  2. Check target device is outputting HDMI (test with a regular monitor)
  3. Try a different HDMI cable
  4. Reconnect the device — the app handles hot-plug events
  5. Check video chipset detection: Supported: MS2109, MS2109S, MS2130S

Backend Selection (Qt)

If one backend shows a black screen, try another via Preferences > Video > Media Backend: - FFmpeg — Most reliable (recommended) - GStreamer — Linux only - Qt Multimedia — Windows fallback

GStreamer Issues (Linux)

Bash
GST_DEBUG=3 ./openterfaceQT 2>&1 | grep -i error

Try a different sink:

Bash
OPENTERFACE_GST_SINK=xvimagesink ./openterfaceQT

EDID Mismatch

If the target doesn't recognize the EDID, it may not output a compatible resolution. Try changing the target's output resolution or edit the EDID via the app's display settings.

Keyboard/Mouse Not Responding

Steps

  1. Check USB switch toggle — make sure it's set to Target, not Host
  2. Verify serial port status — should show a port name, not "N/A"
  3. Try switching baud rate — 9600 or 115200
  4. Check control chipset — Supported: CH9329, CH32V208
  5. Verify CTS monitoring — The app monitors Clear-To-Send lines for HID events

Mouse-Specific Issues

  • Relative mode on macOS: Requires Accessibility permission. Check System Settings > Privacy & Security > Accessibility
  • Absolute mode: Verify aspect ratio matches target display
  • Mouse lag: Try higher performance preset or increase baudrate
  • Serial port conflicts (Linux): Close other apps using the port: sudo lsof /dev/ttyUSB0

Audio Not Playing

Steps

  1. Enable audio via the audio icon > Enable Audio
  2. Check microphone permission — System Settings > Privacy & Security > Microphone (macOS)
  3. Select correct input device — "OpenterfaceA" or capture device name
  4. Select correct output device — your speakers or headphones
  5. Check target's HDMI audio output — is the target configured to send audio over HDMI?

USB Serial Driver Issues

macOS

Bash
kextstat | grep com.apple.driver.usb.cdc

If needed, install WCH CH34x driver from WCH CH34xDriver GitHub. Enable in System Settings > General > Login Items & Extensions > Driver Extensions.

Windows

If serial chip doesn't appear in Device Manager, install the CH340/CH341 driver. The installer typically bundles it; for portable builds, download separately.

Linux

The CH340 driver (ch341 module) is built into the kernel:

Bash
lsmod | grep ch341
dmesg | grep ch341

Firmware Update Fails

USB Stability

  • Do not unplug during flashing
  • Do not suspend the host computer
  • Use a direct USB port (avoid hubs)

Recovery

  1. Power cycle: unplug USB, wait 10 seconds, reconnect
  2. Re-enter ISP mode (some devices: hold button during power-on)
  3. Use Serial Reset Tool to re-flash bootloader
  4. Contact support if bricked

Performance Problems

High CPU Usage

  1. Enable hardware acceleration — Preferences > Video > Hardware Acceleration (VAAPI, V4L2-M2M)
  2. Lower resolution — 720p uses significantly less CPU than 1080p
  3. Lower frame rate — 15fps halves decode workload
  4. Switch backend — FFmpeg with HW acceleration typically uses less CPU than GStreamer

Frame Drops

Check the FPS counter in the status bar. If actual FPS is below target, the pipeline is bottlenecked. Enable frame dropping in the FFmpeg frame processor to prioritize smooth playback.

Logging and Diagnostics

Enable Logging

  • macOS: Settings > Logging Setting > Log to file (~/Documents/openterface.log)
  • Qt: Preferences > Log > set log level and file path

Serial Console (Qt)

Open via Device > Serial Port Debug — shows real-time serial protocol messages with filters for Keyboard, Mouse, HID, Chip Info.

Platform-Specific Issues

Linux: Qt Platform Plugin

This application failed to start because no Qt platform plugin could be initialized.

Bash
export QT_QPA_PLATFORM=xcb

Linux: Wayland Video Issues

Bash
QT_QPA_PLATFORM=xcb ./openterfaceQT

Windows: CH340 Driver

If the driver fails to install: disable Driver Signature Enforcement temporarily, then install manually via Device Manager.

Raspberry Pi: Video Stuttering

On Pi 3 or low-memory Pi 4: 1. Lower resolution to 720p 2. Use FFmpeg backend (not GStreamer) 3. Use 9600 baud for serial stability

Android-Specific Issues

Device Not Detected

Symptom: Video preview shows a placeholder, not the target's screen.

  1. Check the USB OTG connection — unplug and replug the cable
  2. Verify OTG support — try connecting a USB flash drive to confirm your phone supports OTG
  3. Check the KVM device — is it powered on? Are any indicator lights on?
  4. Try a different cable — some OTG adapters are faulty
  5. Restart the app — close it fully (swipe from recent apps) and reopen
  6. Check USB permission — if a system dialog asked for USB access, make sure you tapped Allow

No Video

Symptom: Device is detected but the screen is black or frozen.

  1. Check the HDMI cable — is the target computer's HDMI firmly connected to the KVM HDMI input?
  2. Check the target's output — is the target computer actually displaying something?
  3. Try a lower resolution — open settings → Video Format → pick a lower resolution
  4. Check camera permission — Android Settings → Apps → Openterface → Permissions → ensure Camera is allowed
  5. Restart the app

Mouse Not Responding

Symptom: Video works but tapping the screen does nothing on the target.

  1. Check USB connection for HID — open settings → Device and confirm the device is active
  2. Try a different mouse mode — switch from Absolute to Relative or vice versa
  3. Disconnect and reconnect — use the red Disconnect Device button, then reconnect
  4. Check the target computer — does it recognize a USB keyboard/mouse? Try unplugging and replugging the USB cable on the target side

Keyboard Not Sending Keys

Symptom: Mouse works but typing does nothing.

  1. Make sure the keyboard is open — tap the keyboard button
  2. Check the serial connection — open settings → Device and confirm it's active
  3. Check the baudrate — open settings → Baudrate and make sure it matches your device (115200 is the default)
  4. Check the keyboard layout — make sure the correct layout (US, JP, DE) is selected

App Crashes or Freezes

  1. Close and restart the app
  2. Lower the video resolution and frame rate — high settings can overwhelm devices with limited memory
  3. Check available storage — low storage can cause instability
  4. Update the app — check for a newer version on Google Play or GitHub Releases

Screenshots or Recordings Not Saving

  1. Check Storage permission — Android Settings → Apps → Openterface → Permissions → Storage
  2. Check available storage space on your device
  3. Recordings and screenshots are saved to your device's default media folder

Collecting Logs (Android)

If standard troubleshooting doesn't help, collect logs to share with maintainers:

Bash
adb logcat | grep -i openterface > openterface.log

Include this file when opening a GitHub issue.

Factory Reset

  1. Use the Serial Reset Tool from Settings (macOS) or Device menu (Qt)
  2. This resets the HID chip to factory defaults
  3. Reconnect the device after reset

Connection Recovery

The applications handle automatic recovery for: - Device disconnect/reconnect (hot-plug) - Communication timeouts - Chipset fallbacks (MS2109 → MS2109S → MS2130S) - Serial port recovery

Submitting Defect Reports

  1. Enable log file logging
  2. Reproduce the issue
  3. Submit via GitHub Issues or email to info@techxartisan.com