Carl-Daniel Hailfinger wrote:
All of them have PS/2. However, some only have one PS/2 port which is either a pure keyboard or a combined keyboard/mouse connector. The latter may pose challenges regarding pinouts and/or electrical interfaces (multiplexing?).
PS/2 uses four signals; power, ground, clock and data. Dual-purpose mini-DINs (e.g. page 10 in http://www.pcengines.ch/schema/alix1c.pdf) use the two last pins in a 6-pin connector for the second clock and data pair.
Y-cables are simple splitters.
The 6-pin connector is backwards compatible with 4-pin plugs, so plugging a keyboard directly into it will work without a splitter.
Carl-Daniel Hailfinger wrote:
Short version: Some keyboards are dual USB+PS/2 keyboards and have an USB connector by default. They often come with a pure passive adapter which will convert the USB pinout to a PS/2 pinout. http://us.st12.yimg.com/us.st.yimg.com/I/directron_2019_60686738 is an example image of such a female USB -> male PS/2 adapter.
The image shows a mouse adapter.
Did you actually take one apart to verify that it is nothing more than a passive adapter?
As you know, USB signals are much faster than PS/2, and I would be very surprised to see electronics in keyboards clever enough to support both over the same wires. That's not a simple trick.
I would however not be at all surprised to learn that those green and purple adapters actually have electronics in them to translate between USB and PS/2.
Some time ago, a few pranksters (myself included) suggested to use such an adapter to plug USB flash drives into legacy PCs without USB ports. Of course that wouldn't work due to the passive nature of the adapter (electrical and protocol mismatch).
I would instead guess that it will not work because the electronics in the adapter was designed and implemented to support one specific type of USB HID device. (Keyboard, or mouse, but not both. Oh, and if it was a passive adapter there would be no difference between keyboard and mouse adapters.)
The PS/2 port is generally seen as a pure input "device" and using it for complex output (i.e. not just switching keyboard lights) will certainly baffle lots of people and confuse those "of course that won't work" experts.
Those experts might be well served by reading up on the PS/2 protocol. The signalling is very much bidirectional between the keyboard controller on the mainboard, and the microcontroller in the actual keyboard itself.
//Peter