I decide not to modify the BIOS and stick with what every comes with
the mother board so any board will work. Also modifying the BIOS will
damage the board for running DOS PB development software.

I will just  off load the time critical stuff to a bit of hard ware.
  D to A can  output a zero to five volt signal that will control the
pulse width of four 555s driving injector transistors.

The RPM can be measured with a preloaded down counter triggered by
TDC on rotor one. Here is a preliminary schematic. Not shown is the
interface to the slide throttle position yet. None of these chips cost
over $2 bucks with the exception of the D to A.

Two bi di par. port cards are cheap at around $15 each.

What this means is the processor can read or write to these devices
any time it gets around to it.

DOS was designed for 3 par ports right from the get go.
-------------------------------------------------------------

Here is the lastest thinking on the schematic.
The D to A is merely a resistor ladder.
The 555 PWMs I already have PC boards and parts.
I have most of these other CMOS chips in quantity
left over from years ago.

The RPM calculation is done with out any input from
the computer. The computer just reads it when it gets
around to it.

Same goes for throttle position. Generally the computer
reads these ports right after TDC on rotor one.

The ignition will fire right off the R1 TDC and R2
TDC CAS or 20 degrees BTDC.

This means the computer does not need to fire
anything at a precise time and is allowed to do
a lot of other stuff like save data on the hard disk
and display engine info on a screen. One can buy solid
state IDE "hard drives".

I am getting ready to do a pc board. I'll mock up the RPM
counter and simulate the CAS signals which I already have working.

Another neat thing about this approach is you can use two
of these boards and have one running moving map or flight
planning off the shelf PC software under a different OS with
GPS input sharing the same LCD monitor and power supply.

One is an EFIS and the other is nav. Or if you want to go full
airbus or 777 mode use two LCD monitors for wall to wall glass :)

Paul Lamar

The slide throttle control may work fine for ground testing. A provision
for priming would be good, for the mix will tend to be very lean during
starting. The area change of the throttle opening will not be linear but
a resistor network could be constructed that would be proportional to
the area change.
For in-flight, not having any input relating to air (MAP or MAF) will
require the mixture to be tweaked frequently during altitude changes,
and likely will require mixture adjustment for each throttle and/or
airspeed change. It will be much the same as the old Continental fuel
injection system, in which the fuel control is independent of air mass,
dependent only on throttle position and RPM. I am not aware of any
installation of the Continental system with a fixed-pitch prop on a
certificated aircraft, but I have flown it with on an experimental and
it presents a fairly high pilot workload to manage well.

Victor Roberts

Thanks for the tips Victor. Yes I have to get some leaning provisions
in there. It is such a modular design I am thinking of doing a
mass air flow module or a MAP sensor module in place of the
slide throttle sensor. Just unplug it and plug in something else.

Another huge advantage of this approach is the math coprocessors
built into these chips that do floating point math in no time :)

Something you don't find in most EFI single chip computers.

Paul Lamar