What is the Difference Between PS/2 and USB Keyboards?

All Hail the PS/2! Long Live the PS/2!

Some people wonder why all these modern gaming and enthusiast keyboards still bother including a PS/2 connection, because isn’t PS/2 supposed to be a legacy connection? Well, yes and no.

The PS/2 connection type was considered a legacy port by the Intel/Microsoft PC 2001 specification in 2000, preferring to use the USB port for keyboards and mice. Hold your horses because PS/2 is far from dead.

Newer does not always equal better, for certain applications.

There are some limitations to the USB port, when you use a USB keyboard your computer has to spend CPU time to poll the bus to check to see if there is traffic. Now, the more you poll, the better response rate, but the more CPU time it takes away. After a certain point, increasing the polling rate does nothing but waste CPU cycles because most modern keyboards have a built in limiter.

This is where the PS/2 connection shines, unlike USB devices, the computer doesn’t need to poll the PS/2 port. Instead, when a key-press is detected, the keyboard will send a signal to the computer causing a hardware interrupt.

This forces the CPU to register every key-press, thus all the key-presses should be registered. This is why most manufacturers will advertise a full n-key rollover over PS/2, while a more limited 6 or 10-key rollover over USB.

So if you’re in a setting in which performance needs to be maxed out, with no latency, and pushing the limits of computer protocols, the PS/2 connection is a must. This is therefore, why most performance keyboards come with a USB-to-PS/2 adapter, because the keyboard can switch between these two protocols on startup.

So even though the PS/2 protocol was introduced in 1987 by IBM, it still has a place today.