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I own a couple of PS/2 to USB adaptor/converter connectors that look like the following:

Is there anyway to test if these adapters are passive without disassembling them (or risking damage by disassembly)? In all likelihood, they are passive as the active converters usually have a chip of some kind embedded in a cable to actively convert the protocol.

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  • \$\begingroup\$ All the active converters I've seen go the other way: Plug a PS/2 keyboard into a USB port. \$\endgroup\$
    – markrages
    Commented Apr 22, 2011 at 23:04

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We pulled apart one of those and it's all wire or passive. The mouse that they come with must detect what it's plugged into USB or PS2 and do the right thing.

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  • \$\begingroup\$ right on. there's no way a PS2 mouse or keyboard port is going to operate a USB device. the ones of these that i've seen came with M$ mice, that could tell which kind of port they were connected to. \$\endgroup\$
    – JustJeff
    Commented Apr 23, 2011 at 0:05
  • \$\begingroup\$ Both types exists. Passive is/were the most common though. \$\endgroup\$
    – winny
    Commented Apr 11, 2020 at 18:30
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I am named on the patents for the MSFT PS2/USB detection. I invented the first algorithm to accomplish this.

Yes, the MSFT adapters are just passive. This was one of my criticisms of this scheme. I knew that people might assume the adapters are active and try to use them with other devices.

All USB mice will not necessarily work on PS2 as well. Most USB interfaces do not have the capability to switch to PS2 (logic) signal levels. The USB versus PS2 protocols and data packets are different with both mice and keyboards.

My apologies to computer users for this invention. After USB was in place, the PS2 interface should have died a more rapid death.

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All USB mice are capable of sending to a PS/2 port. This was by design because there are still many people who need to use PS/2 (for KVM's and such), but it is not reasonable for companies to continue to manufacture 2 separate items. Because of this, adapters like those are purely passive as the mouse itself is the one that figures out what it is connected to.

However, it doesn't work the other way around since PS/2 devices never had any idea of what USB was.

I did come across something interesting though, it looks as though you can plug a flash drive into a USB->PS/2 converter and then plug that into a PS/2->USB converter. Not really sure how that actually works though.

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  • \$\begingroup\$ That must be passive on both ends. There's almost no logical reason for a PS/2 -> USB converter to detect USB on the input and pass it through. There's not a chance in hell that you can plug one half of a differential asynchronous massively complex serial bus into a synchronous bus' clock IO pin and the other half into it's data IO pin and expect anything remotely correct to happen. \$\endgroup\$
    – darron
    Commented Apr 23, 2011 at 17:15
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    \$\begingroup\$ All USB mice are capable of sending to a PS/2 port <sup>Citation needed</sup>. Why is this still true, since PS/2 has practically disappeared from modern motherboards? \$\endgroup\$
    – Kevin Vermeer
    Commented Jun 16, 2011 at 14:12
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    \$\begingroup\$ @Kevin Vermeer I actually doubt that it is true... I know I did lots of googling before posting this answer and found a place that said it was part of the USB mouse and keyboard standard that it had to be capable of using ps/2... i cant find that now though. Wikipedia says that "many" do, but has no citations. \$\endgroup\$
    – Kellenjb
    Commented Jun 16, 2011 at 14:19
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    \$\begingroup\$ Heh, how are they practically gone from modern motherboards. The last new computer I got (~year ago) came with a PS/2 keyboard. My computer I built 6 months ago has PS/2 support. I'm typing this on a PS/2 keyboard and using a PS/2 mouse. I think PS/2 is on it's way out, but it's definitely not going to die like floppy disks for probably another decade \$\endgroup\$
    – Earlz
    Commented Sep 26, 2012 at 3:34
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    \$\begingroup\$ Data point: I just tested two USB mice. One cheap Logitech one from ~2009 and one expensive Razor one from ~2013. The old one does work correctly through a PS/2 passive adapter. The new one does not. \$\endgroup\$
    – mgiuca
    Commented Aug 22, 2015 at 8:15
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You can check this with a regular multimeter on continuity mode. On an active adapter, there will be continuity on the power pins (VBUS and GND) and shielding only. On a passive adapter, the data pins will also have continuity.

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