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It's a historical thing. When USB was still fairly new manufacturers (mostly Chinese) wanted to make keyboards and mice for the new USB-only machines but they were still selling replacement equipment for older systems. They cleverly made keyboards and mice with firmware that could test to see whether it was connected to USB or to PS2 via a cheap passive adapter. That way they could sell the same components into both markets at very little additional cost.
Many manufacturers continued this practice for years, simply not providing the adapter as the number of PS2 machines dropped and that market dwindled. As new USB-only keyboards were being designed, with additional functionality, built in USB hubs, improved rollover etc., support for PS2 was dropped. This version of the firmware was then used across almost the full range of devices.
Even now you'll find that some USB keyboards and mice still have dual functionality, but they are usually the cheap end, pretty basic devices where the manufacturer is using old firmware to keep the manufacturing cost to the bare minimum.
Going the other way, using a PS2 keyboard or mouse into a USB port can't use any sort of passive adapter, the process is too complex and requires digital processing. These intelligent adapters are available (or you can build one using a Pico!). I have a friend who is currently using a lovely old IBM Model M keyboard into a USB port of a Mac mini using such a device. :) |
