An adapter is used to connect a device with a large connector to a host with a more compact connector or vice versa. Both devices need to use the same signal as the adapter is a completely passive component. Please note that adapters need to be supported by the keyboard! Compare this with a converter which acts as a "translator" between two devices and requires power to do so.
PC AT to PS/2
The original IBM PC had a 5-pin DIN connector. The IBM PC AT kept this connector for its keyboard, but the signal was changed to a newer bi-directional protocol to support the keyboard LEDs. Later on, IBM released its PS/2 line of computers, but this time they kept the protocol and changed the connector to a 6-pin Mini-DIN form. The end result is that PC AT keyboards can be plugged into any PS/2 port using the appropriate adapter.
USB to PS/2
Note: Check the manual for your keyboard or mouse to see if it supports this adapter, as PS/2 and USB are incompatible!
This special adapter allows certain USB keyboards and mice to be used as a PS/2 devices. When the controller chip on the keyboard or mouse detects that a PS/2 adapter is used, it will switch to the PS/2 protocol instead of its default USB protocol. These adapters are purely passive wiring adapters, and the mauve keyboard and green mouse variants are internally identical.
Normally USB would be preferred for its hot-plugging support, but there are a few reasons why PS/2 keyboards might be wanted:
- Most USB drivers will only allow for 6-key rollover (6KRO) whereas most PS/2 drivers can handle around 16KRO. If the keyboard supports 16KRO, it will now do so, but if for example you have a Cherry keyboard with no diodes, it will simply remain 2KRO.
- Certain environments might block new USB devices yet leave the PS/2 port free for keyboard use (PS/2 only supports keyboard and mouse input, so it can hardly be exploited).
- On certain motherboards, the event handling rate might be higher than USB. This is dubious at best and depends a lot on the drivers of the operating system.