Din5 plug adapter to USB

There is a simple reason why it does not work. The keyboard does not talk the USB protocol.

With most mice and keyboards you have the following options:

• PS/2 mouse, using the PS/2 protocol. Works only in PS/2 plug.
  Does not work with a PS/2 to USB plug.
• USB mouse, using the USB protocol. Works only via and USB plug.
• A mouse understanding both PS2 and USB, auto-detecting which wiring is in use. This one will work both when plugged into a PS/2 port and when (with a convertor) plugged into an USB port.

The same is true for keyboards, though there are many more mice that support both PS/2 and USB than keyboards.

Now your keyboard has a old din plug. That means it is probably very old. Thus it is unlikely that it groks both the pre-PS2 din way and the modern USB method. This is why it fails.

If you want to get it to work you will need to add an active device which speaks both the old protocol and the new one. A mere plug which connects a few cables will no do.

Some nice picture to help me gain rep let the younger readers understand just what AT Din5, PS/2 etc is:

And some convertors with active electronics inside them. These will work, even for old PS/2 keyboards. I never tested them with an additional AT keyboard plug.

DIN-5 used and PS/2 both use the same protocol, colloquially known as "AT". (in the case o PS/2, it's "ATX".) That's why the PS/2 is simply smaller, using a mini-DIN-6 connector instead o a DIN-5

Please note that really old keyboard (early 1980's keyboards), though using the 5-DIN connector, used the older XT protocol, so it probably wouldn't be compatible.

As far as the computer is concerned, there shouldn't be any difference between a PS/2 keyboard and a DIN-5 keyboard with a PS/2 adapter. So either the adapter is malfunctioning, or something isn't working on the USB side o things.