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 Zalman ZM-K600S 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.
The USB HID drivers in most computers' BIOS (and in the Linux GRUB bootloader) can not read the NKRO HID reports, and only work when the keyboard is in 6KRO mode. NKRO USB keyboards have switches for changing from BIOS compatible 6KRO mode to full NKRO mode. Some have toggle switches on the keyboard housing, and others have hot-key combinations for switching modes. For example, the Noppoo Choc-84 / N-Key-USB powers on in 6KRO mode, and pressing Fn-F12 switches it to NKRO. It will remember that setting as long as it has power to the USB port while the computer is off. If it does not function correctly after switching it, try pressing Caps_Lock twice after changing to NKRO mode. It presents a different HID ID for each mode. Some keyboards change modes with Fn-Del-6 and Fn-Del-n.
|Qwerty||Zalman||ZM-K600S||NKRO over USB||$40|
|Qwerty||Cooler Master||CM Storm Quickfire TK||NKRO over USB||$85|
|Qwerty||Corsair||Vengeance K65||NKRO over USB||$90|
|Qwerty||Noppoo||Choc||NKRO over USB||$95|
|Qwerty||Logitech||G710+||NKRO over USB||$130|
|Qwerty||Filco||Majestouch-2||NKRO over USB||$167|
|Qwerty||Das Keyboard||Model 4||NKRO over USB||$169|
|Qwerty||SteelSeries||Apex m-800||NKRO over USB||$199|
|Qwerty||Matias||Ergo Pro||NKRO over USB||$200|
|Qwerty||Ultimate Gadget Labs||Ultimate Hacking Keyboard||NKRO over USB||$250|
|Qwerty||Ergodox||Ergodox||NKRO over USB||$295|
|Qwerty||Keyboardio||Keyboardio||NKRO over USB||$329|
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.
|Utopen||Stenoboard||Open source hardware|
|Scott Urueta||SOFT/HRUF||Open source hardware (see Google Groups thread for updates)|
|Neutrino Group||Revolution Grand|
|Neutrino Group||Infinity Ergonomic|
|Neutrino Group||Infinity Genesis||On linux, /dev/ttyUSB1 is a good bet. Set the protocol to Gemini PR.|
|Stenograph||Stentura 400 SRT|
|Stenograph||Elan Cybra Student||Instructions|
|ProCAT||Flash Writer||Confirmed on Windows and Mac|
See also Steno Machine Notes