Hardware engineers solve a usability problem with the PS/2 connector, but inadvertently create a new one

Raymond Chen

The PS/2 connector was used for both keyboards and mice. The connectors were physically identical, but the protocols were different, so every computer had a label next to each PS/2 port indicating whether it is a keyboard port or a mouse port. By convention, the keyboard plug and port were purple, and the mouse plug and port were green. These connectors are not seen much nowadays, having been largely displaced by USB.

During a group reminiscence of earlier days of computing, one of my colleagues recalled a story from his time at a company that manufactured PCs: One of their hardware engineers had come up with a clever way to detect whether it was a mouse or keyboard that was plugged into the PS/2 port. That way, the user could plug the mouse and keyboard into either port, and the system would be able to figure out which was which.

The company shipped a system with this feature, leaving two unlabeled PS/2 ports and allowing users to plug their mouse and keyboard into whichever port they liked. “Our computers are so easy to use: Just plug it in wherever it fits, and it just works!”

Unfortunately, the company also was inundated with support calls from customers asking how to set up their PC, since there were no labels to tell you where the mouse and keyboard go. This was no doubt documented in the Quick Start guide, but as we all know, nobody reads those things.¹

Their ease-of-use feature turned out to be a support call generator.

They quickly modified their case design so that the two ports were labeled with mouse and keyboard icons, but the hardware secretly didn’t care.

Related: Microsoft invented a design that allows cylindrical batteries to be inserted in either orientation.

¹ I read those things.


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  • Matthew van Eerde 0

    So label both ports as “keyboard/mouse”

  • Luan Vitor Simião Oliveira 0

    i’ve seen a computer that had 2 ps/2 ports that where half purple and half green.
    guess this is why.

  • Mystery Man 0

    I remember a similar accessibility misfire.

    You probably know that downloading a copy of Windows Admin Center involves several steps, many clicks, more than a couple of keystrokes. A blogger posted a short note, writing, “to download Windows Admin Center directly, click on this link.” Having given us a useful shortcut, he concluded his blog post with “you are welcome.” This last line, which was a benign act of humility, however, was easily misinterpreted: One of my colleagues clicked on the link and received a file called “WindowsAdminCenter2009.msi”. Assuming this was a 12-years-old version of Windows Admin Center, he canceled the download. The expression on his face told me that he was assuming conceit and ignorance on part of the poster.

    I didn’t dare correct him.

    • Paul Jackson 0

      This numbering scheme is just horrible. I make this expression every time I see it. At least the Windows team realized it and released version 20H2 after having the utterly confusing version 2004.

      • M. W. 0

        This could have been fixed by pulling a reverse-Kurtzgesagt and adding 10000 to the version number; then people wouldn’t mistake it for a date.

        • Mystery Man 0

          The version number IS a date. (It is 09/2020.) People mistake it for a year instead of a date.

          Not sure what’s “reverse-Kurtzgesagt”.

          • Nathan Samson 0

            Kurtgezsagt is a youtube channel, they have made the human-era calendar for the past few years. And instead of having the year 2021 for example (which is pretty arbitrarly chosen on the birth of jesus – and wrong one at that at best) they estimated that it would actually be the year 12021 (taking into account when “civilization” started)


  • Erik Fjeldstrom 0

    I had an IBM laptop (yes, before Lenovo) that had only one PS/2 connector. For the longest time I wondered why it only had one, as the manual wasn’t super-clear (it simply said “either keyboard or mouse”): it was only after looking at the optional equipment list that I saw that you could buy a splitter that allowed you to plug in both at the same time. It was a clever feature to be sure, but was not described very well!

  • Kasper Brandt 0

    Just using the protocol as it is designed is hardly “clever”. PS/2 mice sends a device id on power-on, and PS/2 devices can also be identified by sending the identify (0xF2) command to the device.

    • Falcon 0

      The clever part could be the ability to swap the ports/addresses in hardware, so that code that accesses the hardware directly continues to work.

      (I don’t know if the system actually did that, just guessing.)

      • Azarien 0

        I guess the operating systems are hardcoded to expect a keyboard on the first port and a mouse on the second port.

  • John Elliott 0

    I wonder if Apple started getting similar queries about their ADB ports after PS/2 ports became a thing?

    To my way of thinking, IBM should have made the ports interchangeable in the first place, since it’s obviously possible, even with 1980s tech (I’ve got a 1980s PC clone where the BIOS detects which device is on which port, as Raymond describes here). Perhaps they didn’t because they wanted to save money by reusing functionality from the AT keyboard microcontroller, which didn’t know about mouse ports.

  • M. W. 0

    nobody reads those things.

    But the manuals to hardware, games, etc. are the best part; they’re what you read on the bus/train/etc. or while you have downtime at school or work.

  • Jonathan BarnerMicrosoft employee 0

    “By convention, the keyboard plug and port were purple, and the mouse plug and port were green.”

    It was specified in the PC97 System Design Guide.

    • Azarien 0

      I never learned which color is for the mouse and which is for the keyboard.

  • Alexis Ryan 0

    Sounds like asking for trouble some smart ass user is going to plug in two of the same type of device to see what happens.

  • Azarien 0

    What would happen if someone plugged in two mice? Or two keyboards?

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