About N-Key Rollover (NKRO)
N-key Rollover allows multiple simultaneous key presses to be recognized by your computer. This means that each key is scanned completely independently by the keyboard hardware, so that each key press is correctly detected regardless of how many other keys are being pressed or held down at the time.
True N-key Rollover, as used by Filco works only with the included PS/2 adapter. Using the USB connection will limit the N-key Rollover function to 6 keys, plus 4 modifiers (Ctrl, Alt, Pgup, etc.), at a time.NKRO is similar to but not the same as what some 'gaming keyboards' incorrectly market as "anti-ghosting". You might also hear the term where n is replaced with a number (i.e., 6-key rollover), which means that up to that number of keys can be pressed simultaneously.
Filco use isolation diodes in their keyboard matrix to implement full N-key rollover (sometimes abbreviated NKRO), making them immune to both anti ghosting and key jamming. This sets them apart from most keyboards which, to reduce cost and design complexity, do not isolate all keys in this way. Instead, they use a matrix of key switches, without any isolation diodes, that assumes that only a limited number of keys will be held down at any given time.
A keyboard with "2-key rollover" can reliably detect only any two keys used simultaneously; in other words, a user can hold down any key on the keyboard and press a second key, and be sure that the key press is correctly detected by the computer. However, if the user has two keys depressed and attempts to strike a third key, the third key press may create a "phantom key" by shorting out the switch matrix. This is not acceptable for quality keyboards because there are many cases when more than two keys need to be depressed at the same time, or when more than two keys are depressed because of fast typing ("rolling over" more than two keys).
A simple test can be found at http://rollover.geekhack.org/
Using the edge of a ruler depress the top line keyboard, the software will register the total number of keys registered. If possible, try the same test using both Usb and PS/2 connections.