The IBM Model M keyboard is notorious for its loud clacking keys, solid construction and tactile feedback which has made it a favorite of many serious keyboardists despite the fact that its design is now decades old. Unicomp still manufactures Model M keyboards, and offers some modern updates such as Windows keys and a USB interface. This is a great option to have, but like many others I still rely on older versions of the Model M that have a PS/2 interface, which is no longer included on most modern computers. This makes a PS/2 to USB adapter a necessity, and there’s many out there to choose from. But as most Model M enthusiasts know, not all adapters work with the Model M.
The problem is that Model M keyboards consume significantly more power than modern keyboards, and not all adapters (or motherboards, for that matter) can provide enough current on the PS/2 port to power a Model M. I’ve known this for a long time, but never knew exactly how much current a Model M draws vs. a modern keyboard. I decided to find out.
Model M Keyboard power requirements
Several websites indicate that the Model M draws 112mA, and that modern keyboards only draw one-hundredth (1.2mA) of this amount. That’s quite a difference! But is it correct? These numbers get thrown around a lot, but without any data backing them up. The source seems to be an old Geocities web page, which is well written, but offers no details on how these numbers came about. Curious, I decided to test my own Model M collection and see what I get. Here are the results:
|Make||P/N||Date Manufactured||Current in mA, LEDS off||Current in mA, LEDs on|
I got slightly lower numbers than the 112mA figure that’s often quoted (with all LEDs off), but not by that much. Based on that, 112mA seems like a reasonable number. Note that the Unicomp Model M draws much less current – it seems the circuit has been updated to make it more efficient than the old Model M keyboards. (Note: I don’t have a P/N and complete date for the Unicomp because it’s actually installed in an old Lexmark IBM case.)
Modern keyboard power requirements
I’ve determined that the classic Model M keyboards draw 100mA or more. But are modern keyboards really 100x more efficient? They shouldn’t need much power, but 1.2mA seems pretty low. I wanted to know, so I tested several non-Model M PS/2 and USB keyboards. I don’t know when they were manufactured as none of them have dates, but all have the Windows keys that were introduced in 1995, so I consider them all “modern”. Here are the results:
|Interface||Make||Model||P/N||Current in mA, LEDs off||Current in mA, LEDs on|
I admit, it’s a small sample, but these are all fairly typical keyboards from large companies. And none of them come close to 1.2mA. I did round the numbers, but even then the smallest current draw I saw was approximately 2.6mA. That’s still very good, and clearly a huge increase in efficiency. But based on my findings, I’d say modern keyboards are approximately 25-40x more efficient than a Model M, not 100x.
Final thoughts: PS/2 to USB adapters that work
Okay, so maybe the Model M doesn’t use 100x more power than a modern keyboards. Numbers are useless if your adapter still doesn’t support it. So which adapters do work? I have two I can strongly recommend.
The first is the “Blue Cube” adapter. It’s a very generic “Made in China” adapter that appears to have no model or part number. ClickyKeyboards.com sells them on their website, and you can also find them on Amazon.com and Ebay.com. I assume they’re all the same, but I can only vouch for the ones sold by ClickyKeyboards since that’s where I got both of mine.
The second adapter I recommend is the IOGEAR Model# GUC100KM. It’s no longer manufactured, but may be available used online. I have several that I got from work after we phased out our PS/2-based KVM switches, and they’ve worked on every Model M I own. Also, they have an LED that blinks as you type, which doesn’t seem to be all that useful but it’s kind of cool, and they can support a PS/2 mouse if you have one.
Side note: How I conducted my testing
For all tests, I used an Amprobe AM-570 multimeter and a Dell Optiplex 990 PC.
For the PS/2 keyboards, I took an old PS/2 extension cord and cut the VCC wire so I can place the meter inline to measure current. Power came from an IOGEAR GUC100KM USB to PS/2 adapter connected to a USB port on the PC.
For the USB keyboards, I used a Fried Circuits USB Tester to connect my multimeter inline for current testing. Power came from a USB port on the PC.
Every keyboard was used while typing up the results to verify they were fully functional.