You might be too young to remember this, but there was a transition period when USB mice and keyboards came with PS/2 adapters (or vice versa), which guaranteed compatibility. However, over time those adapters stopped working in most cases, and the only option to convert from USB to PS/2 is with an active converter. Now, why did those green adapters work well before? Raymond Chen of Microsoft explains it to us.
As an old enthusiast of mechanical keyboards, one of my many fears is ending up with a motherboard that doesn't have a PS/2 port on its I/O. Some might think that's an exaggeration, but the key to any legacy option is its simple availability: You never know when you'll need it.
Today, the USB interface dominates the world, but there was a point in time and space where USB and PS/2 had to work together, so to speak. Microsoft resolved their differences with the help of adapters, a practice that also extended to mice with serial connections. However, if someone tries to connect a USB mouse to a PS/2 port with a simple adapter, they're most likely wasting their time. The same problem arises when we try to go the other way, so… what was the trick?
USB, PS/2, and mice smarter than we remember
Raymond Chen of Microsoft shared a brief post on his blog recently, and it begins with something essential: The PS/2 and USB protocols are incompatible. Although there is some similarity in their links (there's a data pin, a ground connection, and 5 volts), there's no chance of a passive transformation like the one we saw between the AT keyboard connector and PS/2. In reality, Microsoft's adapters worked for a very simple reason: Their mice could detect the type of signal and change their behavior automatically.
Chen compares it to the modern use of power adapters. The adapter itself has nothing special inside; it just redirects the connections. The real 'intelligence' is in the device, or rather, in its power supply, which can negotiate a wide range of voltages (usually 100-240V). Perhaps the old green adapter doesn't make sense today, but USB <-> PS/2 conversion is not impossible. There are many active converters, and I still remember the classic 'Blue Cube' (now off the map), designed for keyboards.
Source: Microsoft Dev Blogs
