USB to PS/2 dongle adapters - how do they work?

Hi,
I have a question about those little adapters that come with some mice (e.g. MS optical mice) that are USB or PS2 capable, to accomplish this, the mouse cable has a USB Type A plug on it and it comes with a dongle/convertor device that plugs into that, then this whole lot plugs into the regular PS2 (mouse) port to make the formerly USB mouse PS/2 compatible.

Now, my question is this:

Presumably the mouse actually is a USB device, so what 'smarts' does the PS2 convertor adapter device have in it that converts the USB signals to PS2 ones?
[or is this not the case?]

Presumably if its has some smarts in it then the electronics in this device are some form of USB controller - enough of a "controller" to talk to the USB mouse [the mouse will be a USB slave device, which would normally require a USB master to control it e.g. the PC's USB ports], but when the PS2 adapter is being used there is no USB master - unless the adapter is one.

I can't see that the USB mouse cable has both PS2 and USB signals down the same wires and the convertor merely selects the PS2 signal wires to the PS2 port.
The USB Type A plug only has 4 'connectors' and these are for the voltage and USB data +/- and thats all.

The reason I'm interested is that I have one of these in my drawer and I was wondering how the heck they actually work!

MY (MS) USB keyboard comes with both USB and PS2 connectors so the cable obviously has both sets of wires on it but the USB mouse does not.

Mind you if there are smarts there, then it must be a pretty small chip as the convertor is pretty small - barely enough for a USB type socket and a PS2 plug.


I must admit, I've never pulled out the multimeter and checked to see the pin outs - for all I know it may well be a simple plug adapter, then again it may not.

I can't see that the USB mouse cable has both PS2 and USB signals down the same wires and the convertor merely selects the PS2 signal wires to the PS2 port.

The convertor acts as a "dongle" to the USB mouse, and clues it in to start doing PS2 protocol down the wires instead of USB protocol (perhaps by having the voltage wires the other way round -- or perhaps simpler than that; isn't PS2 5V whereas USB is 12V?). The convertor will be specific to that USB mouse and is not a generic USB-to-PS2 convertor.

Peter

Both USB and PS/2 use 5v. Also, PS/2 mice only use 4 of the 6 pins: 5v, GND, Data, and Clock. I imagine these are just rerouted to the proper pins on the USB side, and that the signals don't change at all.

I think the main thing to note though, is that the adapter does not contain the circuitry needed to convert a PS/2 signal to a USB signal or vice versa. The USB mouse that you have is doing the actual protocol swapping, not the adapter.