# Maximum Number of Keys on Keyboard



## Rich18144 (Jul 8, 2005)

Im attempting to find out how many keys it is possible to depress at any given point using a Trust - 'Wireless Optical Deskjet' keyboard. So far, I can depress 3 (e.g. Ctrl-Alt-Del) as a max, or so i believe, but I need to be able to put in 4 (e.g. Ctrl-Alt-Shift-C).

Any ideas? Is it possible to ammend this or is it wired into the keyboard?

Thanks for your Time!

Rich18144


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## ebackhus (Apr 21, 2005)

I think that even wired keyboards max at 4 keys at a time. Personally, I prefer them because I hate replacing batteries and the cord makes a good tether should my cats invade my desk and knock things down.


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## InspectorGadget (May 14, 2005)

Keyboards operate on a key-down/key-up message system (much like MIDI). There's no limitation on the number of keys down at one time in the communication protocol or the front-end interpretation by the computer. However, keys on the keyboard are multiplexed, and in order to support 4 (or more) keys down, the MPU in the keyboard must be able to uniquely detect all 4 keys' activities.

From what I know about modern multiplexing, there should be no problem. Typically, the MPU asserts (usually pulling low) one line at a time in one "axis" (the columns or rows), and then reads the set of lines in the opposite axis and sees which are low (if any). With this scheme, you can detect activity in ANY number of keys uniquely, regardless of the state of the other keys.

So in the nuts and bolts, there should be no problem. From my work with BIOS and low-level keyboard access from "C", Ctrl, Alt, and Shift are all just status bits (even left-Alt and right-Alt) in a status word. So I would think a program could detect Ctrl-Alt-Shift-<letter key> if it wanted to.

You might be able to find a utility somewhere that monitors the low-level keyboard activity and displays it. You could even probably write one in VB (I know you could write one in "C").

What's the application? What are you trying to push all these keys down for?

- The Inspector


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