I believe the diodes prevent a situation of one output driving a line high
while another drives it low when two switches are closed. Another way to
do this would be in software. Instead of driving the line high, tristate
it. This would result in floating lines when they are not driven low,
increasing  current consumption, though if the lines are polled pretty
quickly, they'd probably just stay low if no switches are closed.

Harold


>
> Hi to all,
>
> I like to thank all of you who replay on my previews post about c#.
> I guess all I have to do is read and read.
> Now I have hardware question about keypad diodes. I attached keypad
> schematic in pdf format. Using standard keypad without diodes most
> system I have seen returns only single key press and I have been told
> if I need multiply key press function I have to add diode per key and
> what I did is used keypad without the diodes and changed the software that
> can
> return multiple key press and it worked fine. When I am looking at the
> schematic I do not see the purpose of diodes unless There are more devices
> connected to same keypad like LCD. In my application I am using 25 keys
> they all
> connected the same way that the schematic shows.
> The way I am looking at the schematic when Row1 is(0) only keys 1,2,3 can
> be scanned
> and with or without diodes I do not see any different. Same thing on Row 2
> and Row3 and so on.
> My question is what is the purpose of diodes?
>
> thank you
>
> any info will appreciate
>
> Andre
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