This is a feature that certain keyboards have that let the keyboards register each key regardless of other keys being pressed. It is required for Plover. Read the Wikipedia.org(external link) article about Rollover.
How do I know if my keyboard has N-Key Rollover (NKRO)?
A simple test is to open up a text editor and--while holding down both shift keys--type each letter on your keyboard. If all keys are typed into the text editor, your keyboard probably has nkro.
This site provides a web application that lets you test your keyboard's capabilities for registering multiple key presses. To test, click where it says "click to use", then each key you press lights up green in the picture of a keyboard. Press the middle row keys asdfjkl; and with a normal keyboard only 6 of the 8 keys will light up green, maybe fewer, but if your keyboard has n-key rollover, all 8 will light up green. Then press other multiple-key combinations such as yuhj and see if they all light up green.
What if my keyboard is not capable of nkro?
If you don't have a keyboard that's capable of nkro, but still want to give Plover a try, you can arpeggiate/roll the keyboard chords. More info can be found at the bottom of this post: http://plover.stenoknight.com/2011/02/plover-211-released.html
Known supported keyboards
The Sidewinder X4 and Filco Majestouch are confirmed to have at least 16-key rollover over USB. Some of the other keyboards are known not to have usable rollover over USB, though they will probably have sufficient rollover over PS2.
Commercial PS2 to USB adapters may exist that support NKRO, but at least one DIY solution definitely exists. Geekhack has more details about why USB keyboards and adapters are commonly limited to 6KRO.
|Qwerty||Microsoft||Sidewinder X4||NKRO over USB||$45|
|Qwerty||Noppoo||Choc||NKRO over USB||$130|
|Qwerty||Das Keyboard||Model S *||NKRO||$129 - $135|
|Qwerty (tenykeyless)||Corsair||K65 (CH-9000040-UK tested)||NKRO over USB||$90|
|Qwerty||Filco||Majestouch-2||NKRO over USB||$150|
|Qwerty||Cooler Master||CM Storm Quickfire TK||NKRO over USB||$92|
|Alternative||DataHand Systems Inc.||DataHand PS/2 version||NKRO||Find used ~$500-1000|
|Qwerty||Cherry||Multiboard V2 G80-8200HPDUS-2||NKRO||Find used ~$40|
|Qwerty||Kinesis||Maxim PS/2 version (KB200PC)||NKRO||Find used ~$30-100. Modern USB version is only 6KRO|
|Qwerty||Logitech||G710+||NKRO over USB||~$150|
|Qwerty||Various||Laptop Keyboard||Not NKRO, but can arpeggiate keys|
Confirmed working steno machines
Plover supports several protocols that are in use by various machines. This means that, in theory, many machines work with Plover. But these machines have been confirmed by users to work with Plover after actually trying it.
|Neutrino Group||Revolution Grand|
|Neutrino Group||Infinity Ergonomic|
|Stenograph||Stentura 400 SRT|
|Stenograph||Elan Cybra Student||Confirmed on windows, Mac and linux|
|Word technologies||Tréal||Confirmed on windows and mac but not on linux|
|ProCAT||Flash Writer||Confirmed on windows and Mac|