Earlier microcomputer systems such as the MITS Altair used a series of switches on
the front of the machine to enter data. In order to do anything useful, the user had
to enter a small program known as the "bootstrap loader" into the machine using these
switches, a process known as booting. Once loaded, the loader would be used to load a
larger program off a storage device like a paper tape reader. It would often take
upwards of five minutes to load the tiny program into memory, and a single error
while flipping the switches meant that the bootstrap loader would crash the machine.
This could render some of the bootstrap code garbled, in which case the programmer
had to reenter the whole thing and start all over again.

What is a monitor? A monitor is hard wired software in  ROM that allows one to do
certain useful things like load a program, read, or write a block of RAM. The Apple
II had a Monitor as did the Kim 1 and SuperKim. The monitor ROM receives and executes
commands from a host computer. Simple monitor commands can access any memory address.
In monitor mode, the MCU can execute code downloaded into RAM by a host computer
while most MCU pins retain normal operating mode functions.

You send a command to the monitor using a keyboard and the monitor will display a
block of memory for one example. Therein lies the real beauty of the MC68HC908 and
the Megasquirt. This is precisely what we need for an EFIS and data logging in an
aircraft. Certain engine parameters are stored in RAM and are changing as the engine
runs. Injector pulse widths for an excellent example. The fuel burn is directly
proportional to the injector pulse widths. The RPM is stored in RAM as well. The so
called VE (Volumetric Efficiency) table that we need to change when we tune the
engine is another example. The monitor also allows us to write the VE table again and
again  when we tune the engine.

The keyboard used and the display are in any notebook computer. The notebook computer
communicates with the monitor by way of a serial interface. RS232, USB or Blue Tooth
wireless can be used. You pay big bucks when you buy a dedicated EFIS for an aircraft
engine. Comes free with megasquirt.

BTW the rotary VE table is  mostly a function of your intake manifold design and gear
box ratio. All P-ports and 2.85:1 gear boxes will have similar VE tables.
That means they can be stored on the notebook and merely emailed around among users.


Paul Lamar
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