The Passive Aggression Of Connecting USB To PS/2

Before Bluetooth and USB, computers had PS/2 ports. Microsoft veteran Raymond Chen took another trip down memory lane this week to explain just how dumb the USB-to-PS/2 adapters that shipped with Microsoft Mouse devices really were.

The Microsoft Mouse first shipped in 1983 with a DB-25 serial connector. This was later updated to a DE-9 serial connector, followed by a version compatible with PS/2 ports, and finally, the USB and wireless incarnations used today.

There was some overlap between the ports available on PCs and the connector required for the Microsoft Mouse. When Microsoft was shipping USB plugs on the cable, the PC might only have PS/2 ports.

For those too young to remember, the PS/2 sockets replaced the chunky DIN connector used for keyboards and the DE-9 serial connector used for mice. The color-coded sockets – purple for keyboards and green for mice – featured on IBM Personal System/2 computers and swiftly spread throughout the PC world.

These connectors are largely obsolete today; they were replaced by USB or wireless technologies. But they do crop up on some security-minded devices where USB ports are either disabled or not present.

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Microsoft's solution for a USB mouse that might need to connect to a PS/2 port was to include an adapter in the box. The problem, according to Chen, is that "USB and PS/2 are completely different protocols that are not compatible in any way. The adapter was purely mechanical (passive). It connected one set of pins to another, but it contained no circuitry. All of the smarts was in the mouse.

"The mouse detected whether it received USB-like signals or PS/2-like signals on the pins and changed its behavior accordingly. The mouse did all the work."

Decades later, the adapters remain available, and it is possible to pick up units that have the smarts built in rather than relying on the device.

Chen's recollections hark back to a simpler time when mixing up the keyboard and mouse cables or bending the odd pin was all a user needed to worry about instead of wondering why a Bluetooth connection has mysteriously developed lag or dropped entirely, or why the operating system is steadfastly ignoring that particular USB device. ®