Noob question: why diodes?

[blueangel2323]

Sorry if this is a stupid question, but what is the purpose of adding diodes at each switch? I know that the function of a diode is to only allow current to travel in one direction, but why is this important when building a mechanical keyboard matrix? And why are diodes NOT required for the membranes of rubber dome keyboards? Can anyone explain this in non-technical terms? Thanks.

[jdcarpe]

Here is a pretty detailed explanation on the subject by komar007:

http://blog.komar.be/how-to-make-a-keyboard-the-matrix/

[blueangel2323]

Thanks. The page you linked to was a little too technical for me, but this page answered my question quite clearly: http://deskthority.net/wiki/Rollover,_blocking_and_ghosting

Keyboards without a PCB solve this problem using blocking: the controller will limit the number of keys pressed concurrently to a safe limit. Since ghosting occurs when three out of four keys are pressed within a square section of the matrix (which typically does not correspond to a group of four physical keys), the safe limit is considered to be two keys: any third key pressed after that is simply not registered, to protect against the third key causing ghosting to occur. Such keyboards are referred to as two-key rollover, or 2KRO. Keyboards with diode protection that can detect an unlimited number of simultaneous keypresses are termed N-key rollover, or NKRO.