I have a DEC VT525 terminal base (recently repaired) that requires a PS/2 keyboard. The Digital OEM keyboard was a LK 411, with a PS/2 plug. There was also a PC-style keyboard available.

I could get lucky and find a good keyboard for $100 or so, factoring in shipping to Australia. But I have a lot of USB keyboards and rather than get another bulky keyboard, is there an adapter that takes a modern USB keyboard and converts this to a PS/2 male plug with correct PS/2 serial protocol? The one PS/2 keyboard that I have is the kind that used to cost about $5, works just well enough to show that the terminal is OK. Can't stand it for long.

There is a small plug-style converter around - I researched and found they should not work and I even wasted a couple of bucks getting one to be sure - and physical-only adapters, sure enough, do not work.

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(Before you think about whether this is a duplicate, there are many questions asking how to connect a PS/2 keyboard to USB. This is about going the other way. I've looked, so if it's really a duplicate, I missed it and will be happy to find I was wrong.)

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    keyboard USB -> PS/2 adapters where in magenta color. The green ones where for mouses. I never saw a gray or white one before for USB/PS2 but they where common for PS2/DIN5 adapters... – Spektre Sep 24 at 9:11
  • @Spektre the one I have that didn't work is purple. – Adam Eberbach Sep 24 at 9:37
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    $100 to get a useful PS/2 keyboard in Australia? Seriously? – Doc Brown Sep 24 at 11:02
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    I always thought those green and purple adapters that came with Logitech mice/keyboards were just plug adapters---no electronics. I always thought it was the processor in the keboard or the mouse that figured out whether to speak USB or speak PS/2. Am I wrong? – user10478 Sep 25 at 15:13
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    @besmirched mostly they are - I discovered the keyboard controller needs to support both USB and PS/2 if these are to work. But the startech device linked looks to be a genuine protocol translator. – Adam Eberbach Sep 25 at 22:56
up vote 10 down vote accepted

The short version of this answer boils down to something pretty simple and easy:

Cheap, easy: try more different keyboards with your passive PS/2<->USB pin adapter
OR
Not cheap, still easy: buy a really expensive, nice keyboard that also works over PS/2 and take it as an excuse to get yourself a nice keyboard for day-to-day computing!



The passive PS/2<->USB pin adapters not working may be merely a factor of your choice of USB keyboard to be converted being too new.

If you have that many more USB keyboards stashed away, it might be worth trying the adapter on several different keyboards based on a few key factors.

I'll try and communicate what I've noticed about this stuff as best I can.
These aren't hard and fast rules for finding a PS/2 adaptable keyboard, as much as a short gist of guidelines that can hopefully point you at a compatible keyboard in your spare parts pile.


In the below image, take notice of the styling of both the meta/"Windows" key - a flat modern icon of Microsoft's recent motifs, and the OEM Dell branding - indicating this keyboard likely came bundled as part of desktop system.
Both of these points would suggest this keyboard is probably too new to utilize a multi-mode controller IC.
Lacking a multi-mode controller IC, this keyboard would be incapable of detecting when it was plugged in to a PS/2 port with a passive pin adapter and would lack the internal logic circuitry to enable to function as anything but a USB keyboard.


It also features an thematically darkened aesthetic common to a newer era of consumer computer hardware - probably the loudest indicator of a keyboard's age outside of chiclet keycaps.


Supposing you have enough old USB keyboards around, finding one that's closer to the era of when PS/2 devices were still popular may allow the use of one of these passive adapters.

In the image below, it can be seen that this keyboard features an older styling overall.
The icon on the "Windows key" hails from the increasingly distant era of Microsoft's Windows Vista design motifs - an OS at it's prime in a time when PS/2 hardware was still in style.
The OEM Logitech indicates this keyboard is more generic than the Dell (or HP, Acer, etc...), and was probably purchased discretely for use with any given desktop system of the time.
This keyboard also features a thematically lighter aesthetic that comes from a remarkably obsolete era of computing.
These points don't add up to guarantee a cross compatible keyboard, but are positive indicators in that direction.
This is a keyboard more likely to have a multi-mode controller IC which would function with the typical passive PS/2<->USB pin adapter.


The second linked image could easily be a better example, but still illustrates the point.



If you absolutely need a different but specifically PS/2 keyboard at the end of the day, good 'ol ebay tends to be an acceptable place to check.
In between all the random and poorly priced stuff one can sometimes find some good deals.
Please note, the above link is not in any way an endorsement of the listing or seller (and indeed I expect it to be well expired by the time this question is in google results for anything), but a mere example of the point.



It may also bear worth reframing of the problem as an opportunity to buy a nice buckling spring Model M keyboard - they came in PS/2 natively back in the day, and are within the ballpark of $100-200AUD depending on various factors.
If my understanding serves me, unicomp and cherry may also have some PS/2 compatible keyboards available which are generally well regarded.
I would also not be surprised to learn of any modern "gaming" oriented keyboards on the market that have snuck a PS/2-compatible mode into their controller IC for use with the passive adapters, but that's far more web research than fits the scope of this answer.

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    Regarding gaming keyboards and such—my CODE Keyboard from WASD came with a passive PS2 adapter, so I would assume it has the controller necessary as well. – Wildcard Sep 24 at 18:19
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    @Wildcard Not overly surprising. Gaming keyboards were an area where until the last few years PS/2 was a feature for some users. The issue was that the Generic USB Keyboard driver could only report 5 simultaneous key presses (aka 5 Key Roll Over vs NKRO), PS/2 could do more (never saw a limit stated, might be indefinite if the keyboard was capable of supporting it). In recent years that limitation's gone away, AFAIK because a new generic keyboard driver that does support NKRO. Inertia probably means that there're still a decent number of gaming keyboards with PS/2 support today. – Dan Neely Sep 24 at 20:56
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    No excuses for getting better keyboards required, the one that doesn’t work is a Topre 87. @Wildcard thanks for the reminder, I have a CODE (with Dolch SA caps) so I pulled it out, and it works through the adapter! Also I bet my 1999 Keytronic has a good chance too. – Adam Eberbach Sep 25 at 11:39

Not at all. Well, at least not with any simple plug / converter. USB works completely differently from PS/2; any converter will need an active component. And no, those pesky PS/2-USB plugs that were given away with many keyboards during the 2000s are not active, but just rearrange wires - the keyboard controllers themselves were able to handle either interface.

Making an active component would mean to program an active system, like an RasPi, to do the conversion. Way more work than worth it.

But then again, PS/2 keyboards can still be bought new for anything between 10 and 30 Euro. Even in Shops. A quick browse through Aussi dealer websites and Ebay did show tons of offers for new PS/2 keyboards starting at AU$ 5. Give it a chance.

  • I don't think the issue is finding "a" PS/2 keyboard - OP states he has a keyboard that works, but he can't stand using it for long. What he wants is to use a "good" keyboard with this machine - and preferrably without paying the 100-ish dollars that he says it would cost to buy a new one, so a (cheaper than $100) adapter to use a good (USB) keyboard he already has would be preferrable – Syndic Sep 24 at 8:45
  • hmm I am still using USB keyboards on DIN5 devices (oscilloscopes Win9x) and it still works with new keyboards (USB->PS2->DIN5) (I am using the magenta USB/PS2 convertors the green ones where for mouse) so either I am lucky and all my devices/keyboards works with rewired interface or my adapter is active one (small USB MCU could fit into the connector but I doubt its the case). The same goes for older PS2 PCs I got here. But I can see your point for the pre USB devices. – Spektre Sep 24 at 9:05
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    @Spektre I'm not aware of any active USB->PS/2 converter in the market. The ones that work, work because the keyboard itself provides both protocols. This dual support is, however, no longer present in recent keyboards. – tofro Sep 24 at 10:29
  • @tofro good to know (so I guess I am lucky to have old firmware in keyboards) – Spektre Sep 24 at 11:21

This is not the elegant solution you are requesting, but it will solve the problem:

  • Buy a Tripp-Lite B042 series KVM switch.

Specifications: https://assets.tripplite.com/product-pdfs/en/b042008.pdf

This has two USB inputs for a keyboard and mouse, and outputs to USB or PS/2. There is an adapter that connects to the USB output connector to make it work as a PS/2 keyboard.

It also works the other way, accepting PS/2 input to output USB keyboard/mouse.

The downside to this is that the KVM is a huge rackmount black box and you now have a pile of additional wire to manage.

  • Nice box, but it might cost more than having a mint period-correct PS/2 keyboard shipped around the world. – Adam Eberbach Oct 29 at 22:23

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