[net.micro] 3D imager timing output.

gnome@olivee.UUCP (Gary Traveis) (04/27/84)

Some people with 3D cartridges were wondering how the
imager acts when plugged in.  Well here goes...




Here is what I found when scoping out the Vectrex
3D imager pins.
First off, there are only four pins used -

Pin#    what
7	+5vdc
8	GND
4	Index hole (from imager)
3	Pulse out (from system)

This is what I saw ---

On startup the program sends a "high torque" sequence consisting
of a high freqency pulse train --

1 ----- (160us) --------------------- (160us) ----------
0      _________     (520us)         _________            <PIN 3>

This frequency is sent out until the wheel gets up to speed, which
is sensed by index pulses sent from the imager to the system.  When
the wheel reaches the correct 23rps (rev's per Sec), the system
backs down the pulse frequency to the following --

1 --------  (2ms) --------------------------  (2ms) -----------
0         ________        (40ms)            ________            <PIN 4>

1 ----------------  (2/20ms) ----------------------- (same) ---
0                 ______...............             ________....<PIN 3>


The Vectrex is then acting as a software phase-locked-loop.  As the
wheel slows slightly, the timing loop (or interrupt?) detects a
longer pulse-to-pulse time and boosts the wheel speed by sending
one or two 20ms pulses to the motor control input (pin 3).
The index pulse is sent at the time just before the left eye
is "opened" or entering the red-green-blue sequence.  It also
is the "closing" time for the right eye.

Even though I haven't tried to simulate the index pulse yet,
in order to run a 3D program without the imager, I intend to
see just how picky the program is.  Unfortunately, wiring pin 3
to pin 4 doesn't work because the "high torque" sequence never
satisfies the criteria for being "up to speed", never
terminates, and the screen stays blank.

I hope to follow up with an index simulation circuit as   
soon as my spare time reaches the critical level to get
myself into gear...

More to come...
	Gary

(hplabs,fortune,allegra)oliveb!olivee!gnome(@BERKELEY.ARPA)