gEDA-user: basic anti-EMI design q

[DJ Delorie <dj@xxxxxxxxxxx>]

Ok, you all know about the furnace project.  I'm starting to think
about the next rev of the board (cpu, ethernet, etc onboard).  Mostly,
I want to eliminate the beige box on the floor and redesign the system
to be a lot more EMI tolerant.

Keep in mind, this board is mounted inside a five-sided metal box,
which is mounted on the metal air duct coming out of our furnace.
This is the same furnace that includes a couple of induction motors,
an electrostatic air cleaner, fire, water, refrigerant, and 18 gauge
wires running all over the house.

Needless to say, the existing board suffers from interference.  On the
current board, the latches occasionally change state on their own, so
I continuously set them to limit the fault time, but I can't reset
them often enough to keep the furnace from clicking on-off
occasionally.

For the next board, I've planned a number of changes to protect the
board.  The power supply is onboard (it's got to have 24vac anyway, I
get +5 from that).  All I/O lines running around the house are
isolated behind FETs (read and write), with LCL filters, shunt diodes,
and small resistors.  Yes, even +5/gnd going out to the thermostats is
LCL filtered.  There will be an opto signalling the CPU at 120Hz from
the AC line (for master RTC and re-setting the ports).

The design has 10baseT (20MHz) and a 32MHz CPU, plus the 150KHz or so
switching power supply.  Looks like 8/8 rules suffice, which fit the
0.5mm pitch nicely.  I/O lines are 1wire and 9600 baud serial.  Power
switching is 24vac alternistors with opto isolation (no relays).
Board is 5.5 x 3.5 inches, and the four standoffs are metal, so
chassis ground is available (if attaching yourself to a spark
generator is a good idea).

So, the big question is - ground/power planes.  I could probably
squeeze the design onto two layers, but power and ground traces would
be going all over the place.  Going to four layers gives me power and
ground planes, with easier signal routing.  An auxiliary question is,
should I split the planes?  I'm thinking, isolating the power/gnd
going to the I/O drive FETS and thermo power (i.e. the stuff going
around the house) back to a common point near the power supply, and
maybe splitting the 10baseT analog power similarly.

Based on a goal of "minimize the effect of external EMI interference",
what makes sense?