Re: gEDA-user: furnace controller II: I/O driver

[DJ Delorie <dj@xxxxxxxxxxx>]

> I've talked about this before, and this is probably the weirdest
> part of the new furnace controller, so I'm open for input.

Second try.  Still no optos, but used the 2.5V idea to bias the first
stage MOSFETS, so that a "floating" output makes Vgs zero for both
MOSFETS, and an active signal drives them at +2.5V or -2.5V, thus
turning one on and the other off.  This also means this is now a
non-inverting driver, so I added a second stage on the input tap to
maintain polarity symmetry.

I used an op-amp as a 2.5V source.  I thought of using a 2.5V
regulator, but I thought that an op-amp could sink or source relative
to either +5v or ground, and accuracy wasn't a problem as the gate
reference and the common source were driven by the same voltage
anyway.  Op-amps are also small and cheap, needing only a pair of
resistors to set the input voltage.  For eight I/O drivers, I think
worst case is about 2 mA drive needed, to get 2.5V across the 10k
resistors to turn the second stage on, for eight drivers all in the
same logic state.