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

[DJ Delorie <dj@xxxxxxxxxxx>]

> The gate drive BJT's, being single ended, can't pull the gates to
> "off" actively.

Oops, forgot the pull-down resistors.  Imagine a pair of pull-down
resistors from the MOSFET gates to the other supply ;-)

The reason for the emitter followers is to properly bias the MOSFET
gates when the GPIO is tri-stated.  If you just tie the gates together
(like you would for a CMOS inverter) then you can't get them both out
of their conductive regions at the same time.  Someone suggested the
emitter followers as a type of isolation.

> It's an analog to a software race condition, will both devices turn
> off at the same time?

That is the tricky bit.

> Given that you have a noisy environment.  What's the driving
> impedance when the mos devices are tristated?

For 1wire, there's a 1k pull-up resistor.  All those considerations
are what the four-element filter is all about.

> Have you fired up a demo of this ckt?

Nope, this is the first time I've even done a schematic.