HID Viewer: A Complete Guide to Inspecting Human Interface DevicesHuman Interface Devices (HID) — keyboards, mice, gamepads, barcode scanners, many custom USB controllers — communicate with computers using standardized protocols that are both powerful and sometimes opaque. An HID viewer is a tool that reveals what those devices actually send and receive: descriptors, reports, usages, and raw report data. This guide explains how HID works at a practical level, how HID viewers help, popular tools, step-by-step workflows for troubleshooting and reverse-engineering, and tips for developers building or extending an HID viewer.
What is an HID Viewer?
An HID viewer is software that displays the structure and live traffic of Human Interface Devices. It typically shows:
- Device descriptors and configuration descriptors
- HID report descriptors (which define the meaning and layout of reports)
- Live input and output reports (raw bytes and parsed fields)
- Mappings between report fields and usage pages/usages (e.g., X axis, Button 1)
- Device properties such as vendor ID (VID), product ID (PID), and interface numbers
An HID viewer helps you see both the “what” (raw bytes) and the “why” (how those bytes map to actions).
Why use an HID Viewer?
- Troubleshoot non-responsive or misbehaving peripherals
- Verify device descriptors and report formats during firmware development
- Reverse-engineer proprietary devices or undocumented protocols
- Confirm compliance with HID usage tables and standards
- Capture input for testing automation or diagnostics
HID fundamentals — quick primer
HID devices describe their capabilities through a HID report descriptor, a compact binary format that defines one or more reports. Important concepts:
- Usage Pages: categories like Generic Desktop Controls, Button, Consumer
- Usages: specific controls like X, Y, Button, Volume Up
- Reports: structures sent from device to host (Input) or host to device (Output/Feature)
- Report IDs: identifiers when multiple report formats are present
- Logical/Physical ranges and units: how raw values map to real-world units
When parsing a report, an HID viewer typically decodes fields into named controls using those usage definitions.
Common features of HID viewers
Most HID viewers offer a subset or all of the following:
- Raw report logging (timestamped)
- Parsed report display (human-readable fields)
- Descriptor viewing (HID report descriptor and USB descriptors)
- Filter/search for usages or report IDs
- Live visualization (e.g., axis graphs, button toggles)
- Export/import logs (CSV, JSON, PCAP)
- Scripting or plugins to add custom parsers
- Cross-platform support (Windows, macOS, Linux)
Popular HID viewer tools
- USBlyzer / USBPcap + Wireshark (Windows): capture USB traffic; Wireshark can dissect HID reports.
- HidView / HidViz (various): lightweight tools focused on HID descriptors and live reports.
- hidapi + custom scripts (cross-platform): developers often build quick viewers with hidapi in Python, C, or Go.
- Linux tools: usbhid-dump, evtest, hid-recorder; kernel debugfs entries (/sys/class/hidraw/) offer raw access.
- macOS: IORegistryExplorer and custom IOKit-based tools for HID inspection.
Step-by-step: Inspecting a device with an HID viewer
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Identify the device
- On Windows: Device Manager / USB device listing or use tools that show VID/PID.
- On Linux: lsusb, dmesg, or checking /dev/hidraw* and /sys/class/hidraw/.
- On macOS: System Information or IORegistryExplorer.
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Open the device in the viewer
- Ensure privileges: capturing raw HID often requires administrator/root.
- Select correct device by VID/PID and interface number if present.
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View descriptors
- Read the HID report descriptor; map usages to fields.
- Check for multiple reports or report IDs.
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Capture live reports
- Start logging; exercise the device (move, click, press).
- Observe raw bytes and parsed fields; note timestamps.
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Analyze and correlate
- Match changes in parsed fields to user actions.
- If values seem inverted/scaled, compare logical/min/max and units in the descriptor.
-
Save and share
- Export logs or screenshots for firmware teams or issue trackers.
Troubleshooting examples
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Keyboard keycodes not matching expected characters:
- Check report descriptor for usage page (Keyboard vs Consumer).
- Confirm host-side mapping (layout, language) and whether device sends keycodes or consumer usage codes.
-
Joystick axes jitter or drift:
- Inspect logical/min/max and report resolution; apply deadzone or scaling if raw resolution is high.
- Check for multiple reports sending overlapping axis data.
-
Device stops sending reports intermittently:
- Look for USB power or reset events in the OS logs.
- Validate that the device isn’t switching interface or entering an alternate setting.
Reverse-engineering a proprietary HID device
- Capture a broad dataset: lots of actions and repeated sequences.
- Group reports by report ID and length.
- Identify candidate fields: bits that toggle with button presses; multi-byte fields that change with axis movement.
- Use controlled inputs (one control at a time) to map fields to usages.
- Build a parser that converts raw reports into structured events; iterate until all meaningful fields are accounted for.
Example: Mapping a 6-byte report where bytes 0–1 change with stick X/Y, byte 2 toggles bits for buttons, bytes 3–5 are constant. Hypothesize endianness and signed vs unsigned values, then test.
Building your own HID viewer — key tips
- Use an existing cross-platform library (hidapi, libusb, pyusb) to handle enumeration and transfers.
- Parse HID report descriptors with a library when possible; writing a correct parser is tricky.
- Provide raw and parsed views simultaneously; users need both.
- Offer scripting or plugins so unusual devices can be parsed without rebuilding the app.
- Respect device ownership and avoid exclusive claiming unless necessary; that prevents other software from using the device while debugging.
Security and safety considerations
- Avoid sending arbitrary output reports to unknown devices; they may change device state unexpectedly.
- Be cautious with devices that implement firmware update or mass-storage functionality on a special interface.
- When reverse-engineering, respect licensing and lawful-use restrictions.
Example: Minimal Python HID viewer using hidapi
# Requires: pip install hidapi import hid import struct import time VID = 0x1234 PID = 0xABCD dev = hid.Device(VID, PID) try: while True: data = dev.read(64, timeout_ms=500) if data: ts = time.time() print(f"{ts:.3f} raw: {data}") except KeyboardInterrupt: dev.close() This prints raw input reports; extend by parsing according to the device’s report descriptor.
When to use higher-level OS APIs
If you need key mapping, HID usage-to-key translations, or integration with system input stacks (e.g., synthesizing keyboard events), use platform-specific APIs (Windows Raw Input, IOKit on macOS, evdev on Linux) rather than raw hidraw access.
Conclusion
An HID viewer turns opaque HID traffic into actionable information: it reveals how devices describe themselves, what data they send, and how that data maps to user actions. Whether you’re debugging firmware, reverse-engineering a peripheral, or simply curious, a reliable HID viewer and a systematic workflow will save hours and clarify device behavior.
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