Computer Interfacing

Computer interfacing

Keyboard problems

—

rollover

and

lockout

It

is

virtually impossible

to

type without,

at

sometime

pressing

two or

more keys

in

such rapid succession that

they

are

almost instantaneous. Without some effective

method

of

guarding against

the

consequences, this could

result

in the

wrong codes being generated. There

are

three available techniques

to

prevent such errors. They

are known

as,

n-key lockout, 2-key rollover

and

n-key

rollover.

The simplest

and

cheapest

is

n-key lockout.

In

this

method,

the

first

key

down generates

a

pulse known

as a

keyboard strobe. This causes

the

code

for

that

key to be

fetched. During this perio ...