Josh,

Adam said:
> 2) a Schmitt Trigger input is a nice way of saying, "this part will always turn
> on and off athe the specified voltages despite process variation, etc, etc."
> Thus you can put an analog signal in and it will always trigger on and off at
> the same voltage levels.  Other inputs will vary due to other conditions on the
> chip

It's cooler even than that.  Yes, the schmitt trigger inputs have
specified threshold levels that are more closely controlled, but
there is also hysteresis.  This is useful when the input signal is
slowly varying  (slow compared to the input buffers ability to
respond) or is close to the threshold value.  Without hysteresis, the
small noise always riding on the signal can cause the input to
"chatter" back and forth between 0 and 1 when you really want a
single transition.  With hysteresis, once the input changes state,
the threshold shifts so that it takes a significant voltage swing to
change the input back.

An example would be an AC line zero-cross detector.  You want ONE
transition as the AC line voltage passes through zero.  Without
hysteresis, you would likely get a burst of pulses around the time of
the zero crossing.

> 3) I've not heard of a normally closed quad bilateral chip, but I bet someone
> somewhere has them...

But most analog switches (and certainly the 4066) act strangely when
the switched signals approach or exceed the power supply voltage on
the switch chip.  If you need a signal path to be valid with power
off, you are probably going to have to use a reed or other small
mechanical relay.

Adam said:
> 4) There are all sorts of 8, 16 and 24 bit data latches made.

Tons.  The garden variety 8-bit latch is the 74xxx373.  xxx can be
HCT, HC, etc, depending on you needs for output drive current,
voltage compliance and chip current consumption.  The 74HCT373 is a
very common tradeoff.  Its a CMOS part, but intended to mate with
bipolar TTL chips.  It has a seperate output enable which tri-states
the outputs while holding the last latch state valid.  That might be
useful.  If not, you can wire the output enable to ground to set the
outputs always on.

Adam suggested:
> A suggestion:  If you need 24 output controlled with an 8 bit port, look up the
> 8255.  It is an i/o chip where you can set three 8-bit ports to either input or
> output.  It is obsolete, but they are all over the place still.

I disagree.  The 8255 has the right functional specs but I;ve found
it to be damn fragile.  Very suceptable to latchup and to false
configuration register writes (i.e. scrambled outputs).  Also, they
have a stupid power-on-reset default state.  I've had to program
around them on several old hardware designs.  I'd stick with MSI
latches like the 74HCT373 for new designs unless you need huge or
specialized I/O spaces, then there are other LSI solutions.

Good Luck, Josh.

------------
Barry King, KA1NLH
NRG Systems "Measuring the Wind's Energy"
http://www.nrgsystems.com
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