Thought I'd initiate a (sort of at least) on-topic thread for a change... :)

I've been using a PIC 16F84 in applications where I've needed many easily
addressable I/O's. Though I've been mostly needing outputs (to drive many
LEDs in a display panel, or twenty relays, or both at the same time), there
have been times when I've needed extra digital inputs as well.

For one system with 48 LEDs (actually 24 tri-color ones, so 48 outputs), 19
relays and 3 other outputs, I only need to dedicate two output pins on the
PIC - the rest can be shared with other devices. I'm using RA3 as clock to
a 74HC574 latch driving the I/O-bus, and RA1 as a sort of "bus strobe" -
apparently with a longish distance between the latch and target ICs, the
signals won't change simultaneously. So, I have seven address lines and a
data line, and the strobe.

The lowest address lines (A0-A2) go straight to as many 74HC259 addressable
latches as needed; likewise for the data. The next logical decoding level
is A3-A5, these go to two 74HC138 decoders. And the decoders, in turn, are
selected by a combination of the strobe and A6 line. So, for example, the
panel display only needs a 10-lead flat cable for data and power.

A very good thing about this is that logic chips like 74HC259, 74HC138 etc.
are very cheap, so it beats using I2C port expanders which seem to cost a lot.
I can also do inputs by using a 74HC151 8-to-1 multiplexer, though I haven't
tested it much yet.

I can also do 8 extra outputs by using only one dedicated output on the PIC,
with a single 74HC259.

So... anyone see possible errors arising from this system? I've also heard
about different methods, like shift registers; what else is available?