My 1989 IBM Model M Keyboard

Power Consumption of IBM Model M Keyboards

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 so many others I still rely on older versions of the Model M that have a PS/2 interface, which is no longer included on mainstream computers. So an adapter is necessary, 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 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
IBM (Lexmark) 82G2383 09/13/1995 97 126
IBM (Lexmark) 82G2383 09/29/1994 101 129
Unicomp 2001 7 29
IBM 1395240 12/07/1989 103 120

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 – clearly the electronics have been updated in it to be more efficient. (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 Model M draws somewhere around 100+mA. 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’s the results:

Interface Make Model P/N Current in mA, LEDs off Current in mA, LEDs on
PS/2 Dell RT7D00 025PGG 3 39
PS/2 Logitech Y-SAF76 867633-0403 3 35
PS/2 HP 5107 5187-3549 3 39
USB Logitech Y-U0009 820-004520 4 33
USB Dell SK-8115 3 51
USB Dell KB4021 0DJ454 4 16
USB Microsoft 1366 X818767-001 3 48

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 efficiently. 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.

Blue Cube PS/2 to USB adapterThe 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.

4 thoughts on “Power Consumption of IBM Model M Keyboards

  1. I just wanted to thank you for the ps2/usb connector suggestion. I just received it and it works flawlessly.

    Thanks again great article !

  2. Is that your industrial Model M in the picture? It looks awesome. Can I ask where you got the colored and clear plastic keycaps at? I’d love to trick out mine like that.

    1. Thanks Greg, it is indeed! That’s my ’89 IBM. As for the source of the key caps, they came from various places. Some came from other old IBM keyboards I purchased on Ebay. If you’re looking for a readily available source though, I suggest going to Unicomp which is the company that still builds Model M’s. They offer keys in several colors with whatever standard print you want on them. That’s how I got the custom colors on the top of my numeric keypad. Here’s a link to their buttons page!

      http://www.pckeyboard.com/page/category/Buttons

      They don’t appear to have clear keys right now, I think mine came from another old keyboard.

Leave a Reply

Your email address will not be published. Required fields are marked *