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I plug a USB to PS2 converter into my computer and then plug a PS2 to USB converter (different brand) into that and then plug a thumb drive into that... and the drive works.

What the heck?! Is USB somehow electronically backwards compatible with PS2 and they are just pass through devices?

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migrated from serverfault.com Sep 16 '10 at 17:44

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@Warner: USB is also powered. I'm not sure why you're worried... –  hotei Sep 16 '10 at 17:48
    
If you're going from USB to USB why are you even attempting the PS2 in the middle? –  KronoS Sep 16 '10 at 17:56
    
You can (or could at one time) get mini-DIN to DB9 passive adapters for serial/PS2 mice and do the same kind of thing. –  Dennis Williamson Sep 16 '10 at 18:19
    
@KronoS I was just fiddling around while on Hulu –  BCS Sep 16 '10 at 18:37

3 Answers 3

up vote 34 down vote accepted

The adapters you have are pin compatible with each other and there are no electronics within them. They simply convert four pins one way, then back again. In essence, no converting is going on.

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Worth a thousand words (and one upvote) –  Pops Sep 16 '10 at 18:03
    
This is because the PS/2 mouse is really a PS2_or_USB mouse it's sending USB data as well. –  Martin Beckett Sep 16 '10 at 19:09

Both USB, and PS/2 are used for serial communication, and are very similar.. The largest difference is the PS/2 bandwidth is much slower.

Also see this section on the USB wikipedia article

Mice and keyboards are frequently fitted with USB connectors, but because most PC motherboards still retain PS/2 connectors for the keyboard and mouse as of 2007, they are often supplied with a small USB-to-PS/2 adaptor, allowing use with either USB or PS/2 interface. There is no logic inside these adaptors: they make use of the fact that such HIDs are equipped with controllers that are capable of serving both the USB and the PS/2 protocol, and automatically detect which type of port they are plugged into.plugged into.

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So if I got a true/pure PS2 mouse and a true/pure USB port, that thing would be useless. Bazaar. –  BCS Sep 16 '10 at 18:40
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"Are very similar" ?!?! USB is a differential, asynchronous, massively complex bus. PS/2 is a single-ended, synchronous, dumb serial pipe. There are absolutely huge differences at all implementation levels (hardware, OS HAL, OS device driver, and almost always application). This is like saying a canoe is very similar to the space shuttle because they both are used as human transportation devices. The reason USB->PS/2 connectors exist is that the mice detect which totally different system they are connected to and implement entirely different ways to communicate to the host system. –  darron Apr 23 '11 at 17:01

PS/2 and USB are both 5v powered, but the protocols for each port are different. The adapters linked to are designed for devices which can detect what type of port they are connected to and adjust their protocol accordingly.

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