dsperber. While that info is very useful, I think the OP wants to go from a USB keyboard to a PS2 connection.
Ahh... I didn't pick that up. I see he's now edited his original post to clarify just that.
So I guess I'm now puzzled. Why wouldn't you plug a USB keyboard into a USB port? Why would you want to use a PS/2 serial port (which are of course harder and harder to find on new machines)? Is there some functional advantage gained? Will all of the new additional key codes on newer keyboards (which were not on the original IBM PS/2 keyboard layout) be supported?
I can certainly understand why one (such as myself) would want to continue using an IBM-style PS/2 keyboard (which, by the way, are still made today... although not by IBM... as exact duplicates of the old original ones first made 25 years or so ago, to satisfy people such as myself who just like the touch and sound of these dinosaurs) on a machine which didn't have a true PS/2 serial port to connect to. That's why the Adesso adapter is exactly what you want, allowing the old PS/2 keyboard and mouse to be used through a USB port on the newer PCs which lack the PS/2 serial port.
I guess I don't appreciate the reverse situation's viability. i can't imagine why it would be beneficial or desirable to just want to utilize the PS/2 serial port on your PC if you had one, for a USB device.
If you lack sufficient available USB ports on the PC to connect the USB keyboard, just get a USB port-multiplier (i.e. "hub")
like this USB 2.0 one from Sabrent, and plug the USB keyboard into one of the new ports on the hub. They are also available in USB 3.0 style, if that's what you need.
Seems like a much simpler and more useful and practical and inexpensive gizmo, if you're looking for a gizmo that allows you to use what is really today's standard USB keyboard on a PC that doesn't have enough USB ports.