Hi,

I'm building a project which requires 4 pushbuttons to be attached to
a bus interface. I *could* obviously use a tristate buffer, but I'm
trying to keep the chip count down. I do have one spare pin on the
PIC (A2), so my initial solution was to have this pin as the common
connector to 4 switches, and have the other side of the switches
wired to B0-B3, (which is in use as part of a bus interface).
When the bus in in use, I float A2, so pressing a button doesn't
interfere with the bus operation. When I'm not using the bus, I flip
B0-B3 to inputs, set A2 high, read B0-B3, set A2 low, read B0-B3 again
and figure out which switches are closed. (Alternatively, make B0-B3
outputs, A2 input, strobe each in turn, and look for corresponding
strobe on A2).

Of course, a moment's thought demonstrates that if two switches are
closed at the same time, unspeakable confusion will occur, because
two of the bus lines will be shorted together, regardless of whether
or not A2 is floating, and when the bus *is* in use, disaster will
occur.

So, I'm thinking - presumably if I got a transistor for each line,
arranged in an open-collector configuration, such that B0-B3 remain
outputs, each driving a transistor, I could take the second
approach described above (use A2 as input), and it wouldn't matter
if two buttons were pressed simultaneously. (I don't need to detect
simultaneous presses - it's just that they can't interfere with
anything!).

My question is, (and once again, I'm forced to demonstrate my
complete lack of elec eng knowledge...), how exactly do I connect
the transistors? I need a series resistor on the base, yes? If so,
what sort of value should I use?

Alternatively, if anyone can suggest a simpler way to get around
this problem, I'm all ears!

Cheers,
Ben