> Im new to PIC and electronics in general, having come from a software
> background. Im looking to design something which can sample 90 wires for
> the presence or absence of a DC voltage (50v, but can be taken down to
> say 5v with a voltage divider if needed). It then needs to store a bit
> in memory for each wire indicating if the wire was live or not and then
> interface to a computer over USB so the computer can read the bits.

What  they said.
Plus:

ICs to sample 100 lines are not too costly, but you MAY be able to use
an addressed  XY or even XYZ matrix to use many less active devices,
or to use many diodes and fewer ICs.

Using XY you need 10 x 10, or in multiples of 4 say = 8x12 = 96.

If you can work an XYZ addressing scheme you can get say 5x5x5 = 125
or 4x4x8 = 128 .

In an XY system an X line drives a row (say) of inputs with matrix
points being held down (or up) by input voltages. The y colums read
the result. This may end up with more R's and / o r diodes than are
worthwhile.

_____________

Using a number of serial shft registers makes it all very easy.

The CD4021B parallel to serial  is under 40 cents at Digikey in 25's.
        http://www.fairchildsemi.com/ds/CD/CD4021BC.pdf

Each  allows 8 inputs and they can be chained in series indefinitely.

12 x 8 = 96 inputs.

There are various ways to get 50V to logic level input - but it should
not take more than 2 x resistors and 1 x diode per input and just
maybe a small capacitor too, but  could take less. They can run at up
to 15 V supply and it MAY be worth running them at near that level and
translating the very few control and data signals to PIC level.

Depending on the tolerance and noise spikes on your 50V signals you
MAY be able to use just 2 x resistors.

What's the application?
Sounds like telephone line sampling ? :-).
Dim and not wholly pleasant memories from long ago stir :-)


 Russell
.
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