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I remember when USB mouses came out those green PS/2 to USB adapters were around. (I cannot remember to have seen a purple keyboard adapter doing the same. But that doesn't mean anything.)

I always was sure they were passive with only wires inside.

But PS/2 is a completely different specification on the electronical level. So from that point of view there has to be an IC inside.

So was there an IC inside or not? Or maybe PS/2 is somehow compatible with USB?

Please note I am only talking about an adapter to connect an USB mouse to a PS/2 slot. Not any other adapters.

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  • 7
    I believe that those adapters only work with certain mice.
    – Eric Brown
    Jan 15 at 20:43
  • @EricBrown I have never seen a USB mouse that doesn't speak PS/2 as well.
    – AndreKR
    Jan 19 at 13:51
  • @AndreKR I've got multiple mice that only speak USB and I find plenty when I'm taking such an adapter, plus an active PS2→USB adapter, plus a laptop to the thrift stores to find ones that do. You're just used to experiencing the economies of scale of having so many mice using the same ASIC for so long.
    – ssokolow
    Jan 19 at 20:32
  • @ssokolow It might also be selection bias where I prefer certain brands and those brands implement PS/2.
    – AndreKR
    Jan 19 at 20:50

3 Answers 3

68

You are correct, they are only passive adapters with wires inside.

There is no IC inside.

And no, PS/2 protocol is not in any way compatible with USB protocol.

The trick is, the chip inside the mouse and/or keyboard can detect to which port they are connected and then either use PS/2 or USB protocol.

The adapters do not work with devices that do not understand both protocols.

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    @DavidFulton That's guaranteed to be an active adapter with a chip inside doing the conversion. But passive adapters did exist for keyboards as well.
    – vidarlo
    Jan 16 at 10:01
  • 8
    @DavidFulton No it doesn't. Either the devices are new enough to support both protocols, or, then you need an active adapter. Adapters were passive back then, these days having an active adapter to make those old devices work may be common.
    – Justme
    Jan 16 at 10:35
  • 4
    @DavidFulton you are describing a male-USB/female-PS2 adaptor. The opposite of what the question is about.
    – OrangeDog
    Jan 16 at 12:07
  • 4
    @CSM This wasn't about DIN and PS/2, but adapters between USB and PS/2. And not entirely true, because, if you go further back, PC and AT keyboards were not protocol compatible despite the identical DIN connector. I also don't recall tri-protocol mice, I think there were transition era between RS232 and PS/2 mice and later between PS/2 and USB. No need to support all three.
    – Justme
    Jan 16 at 14:19
  • 2
    Any idea how the detection was done? Did it just use the difference in power supply voltage to change the protocol and pin usage? Jan 16 at 16:25
24

The adapter is passive. The mouse is designed to talk either USB or PS/2 and can figure out which is currently connected

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  • I don't know whether there's any combination which makes use of this, but for completeness there are apparently 'phones etc. which have a physical USB port which can be conditioned for different purposes (e.g. debug mode, reflash) depending on the resistor on the ID line. xdaforums.com/t/the-samsung-anyway-jig.1629359 Hence /potentially/, something acting like a mouse could declare that it wanted simple serial protocol on the USB data pins: however I don't believe this applied to the simple passive (green) adapters which predate both this level of sophistication and the ID pin itself. Jan 17 at 13:16
  • @MarkMorganLloyd No it definitely does not work how you potentially pictured it to work, the mouse is unable to declare such things and PC side cannot switch the USB (old, USB1, USB Type-A) data pins into other protocols. It is weird how so many people stray from the asked question, speculating how it could work, but it's the mouse that detects to which kind of port it is connected. Perhaps if someone could post a new question like "How did a old mouse detect if it was connected to PS/2 or USB port" I could try to answer it.
    – Justme
    Jan 17 at 17:10
  • New question posted as suggested by @Justme - retrocomputing.stackexchange.com/questions/28298/…
    – dave
    Jan 17 at 17:16
  • @Justme Granted, I was wandering :-) Jan 17 at 19:55
11

As others are saying, they're passive adapters and, as these two pinout sites detail, they work by connecting the USB Data- line to the PS/2 Data line and the USB Data+ line to the PS/2 Clock line and relying on the mouse to recognize whether it's seeing PS/2 flow control signalling or the beginnings of a USB initialization handshake.

USB Pin Name USB Pin # PS/2 Pin Name PS/2 Pin #
VDC +5V 1 VDC +5V 4
Data- 2 Data 1
Data+ 3 Clock 5
Ground 4 Ground 3

(However, as the first of the two sites says, they might also contain 10k pull-up resistors on the data and clock lines since that can help with compatibility.)

I can personally confirm that they only work with mice whose controller ICs know how to speak both protocols. (I keep an eye out for such dual-protocol optical mice when thrifting because they're cheaper and/or less work than buying or building an active USB-to-PS/2 adapter in this day and age.)

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  • The PC won't send clock out to any device on a PS/2 interface. It is always the device that sends out clock when it wants to send data, and also when PC has requested to send data to device.
    – Justme
    Jan 17 at 21:44
  • @Justme Fair point. I phrased that badly.
    – ssokolow
    Jan 17 at 22:36
  • Can it damage an USB mouse (which doesn't support PS/2 signals) if you connect it to a PS/2 slot using an adapter?
    – zomega
    Jan 18 at 11:25
  • @zomega, I haven't observed such occasion. Wold be very unlikely. Jan 18 at 20:30
  • @ssokolow Can you confirm that a green adapter is the same as a purple adapter because pins are the same?
    – zomega
    Feb 1 at 17:28

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