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

Original Article ↗ Hacker News Discussion ↗

USB-to-PS/2 “adapters” bundled with older Microsoft mice and keyboards turn out to be mostly passive: the real work was done by dual‑mode controllers in the peripherals that could speak either USB or PS/2, depending on how they were wired. Commenters explore how this design explains why many cheap adapters never worked with other devices, and contrast passive pin‑rewiring with modern active converters that translate protocols for retro hardware. The thread branches into related topics such as PS/2’s latency and power advantages, legacy i8042 keyboard controllers in laptops, and why PS/2 ports still appear on some contemporary motherboards for security and compatibility reasons.

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.