A note on the USB-to-PS/2 mouse adapter that came with Microsoft mouse devices

Original Article ↗ Hacker News Discussion ↗

Recognition of the source / style

  • Many commenters immediately recognized the article as being from Microsoft’s long-running Windows internals blog based solely on the URL and title.
  • The blog is praised for quirky, highly detailed explorations of obscure Windows/PC behaviors, especially around Win32 and hardware edge cases.

How the USB–PS/2 mouse adapters actually work

  • The “green dongle” is not a protocol converter; it’s mostly just rewiring.
  • The mouse itself contains a dual‑mode controller that can speak either USB or PS/2, and detects which to use by looking at electrical conditions (e.g., USB D+ vs PS/2 clock), with algorithms documented in linked references.
  • Because of that, these adapters only work with devices explicitly designed for both protocols; plugging a pure USB mouse into one will not work.
  • Some vendors (including the same big one) shipped different pin mappings over time, so visually identical adapters are not always interchangeable.

Need for active adapters and retrocomputing use

  • To use a real PS/2 keyboard or mouse (e.g., Model M, older samplers, vintage PCs/terminals) on USB hosts, you generally need an active converter with a microcontroller.
  • Multiple hobbyist projects (USB4VC, HIDman, ps2x2pico, Arduino hacks) actively translate protocols and allow USB HID devices to work on PS/2 or older proprietary interfaces.
  • These projects also tune things like polling rates to get smooth behavior on old OSes.

USB vs PS/2: protocol and behavior

  • Commenters stress that USB is a tightly timed, packetized protocol with differential signaling and a host stack; you cannot “just wire it through” like simple serial.
  • PS/2 is a simpler 0/5V clock+data serial line, easy to bit-bang on microcontrollers.
  • PS/2 is interrupt-driven, which can reduce power and (in some setups) give lower input latency and better n‑key rollover, which is why some “gaming” boards still include PS/2.

DisplayPort–HDMI and other “passive adapter” analogies

  • The mouse dongle is compared to cheap DisplayPort‑to‑HDMI adapters that are mostly passive because the GPU can switch to HDMI/DVI signaling (DP++).
  • There’s debate over what counts as “passive” when level shifters or coupling caps are involved.
  • In contrast, truly passive USB–FireWire adapters seen online are called out as essentially bogus: just wires and glue, electrically incompatible, but still sold.

Laptop and internal keyboard interfaces

  • Several comments note that many laptops still expose internal keyboards/trackpads as PS/2/i8042 via an embedded controller or Super I/O over LPC/eSPI, for simplicity and early‑boot reliability.
  • This design also saves power versus constantly running USB polling and lets machines work without a full USB stack initialized.

Compatibility pitfalls, color coding, and legacy

  • The green (mouse) and purple (keyboard) coding comes from late‑90s PC design guides; the connectors are electrically the same.
  • Users recall that some passive adapters worked only with specific models of mice or keyboards, which now makes sense given dual‑mode vs USB‑only differences.
  • There’s discussion of why PS/2 ports persist: support for legacy KVMs, environments with USB disabled for security, and some BIOS/firmware behaviors that favor PS/2 at boot.