Re: VECTREX

>As I recall (reading on net, not exploring for myself) the vectrex uses
>an A/D that my be multi-channel, but I seem to recall only one channel with
>multiple sample and hold circuits.

Hi guys--

I used to be *really* familiar with the Vectrex (I wrote "Moon Lander"
lander for it), so I'm impart what I remember.

Just as a general comment, the Vectrex is very cool. :-) It has a REALLY
limited set of hardware that is uses for all sorts of neat functions...

Anyway. The Vectrex doesn't have a dedicated A/D per-se. The vectrex
actually uses a single DAC to provide a variety of functions. The DAC is
used for choosing the brightness of the beam (Z), the X/Y drawing (using an
integrator), reading the analog joystick (using the integrator and a
comparitor or sample/hold, I *think*-- a little fuzzy there), and generating
digitized sound (direct digital output).

>Anyway, the analog signals can be
>fed into integrators to move the beam around the screen. I think the
>integrators can be discharged to bring the beam back to center, but that
>it can not be positioned directly (on this I may be wrong).

Yep, Paul's been doing his homework. ;-) To a programmer it actually looks
rather a lot like the Atari AVG. You need to center the beam (by zero-ing
out the integrators) every now and then to keep the drawing precision good.
You don't get direct positioning (also like the Atari AVG), but rather you
"draw" to a position with the beam off). I think there are provisions for a
"fast" draw and a "slow" draw, but I might be mis-remembering that.

>Text display
>is done by scanning several horizontal lines in a raster pattern, the
>beam ON/OFF signal (can?) comes from a shift register that runs rather
>fast.
>This allows a single row of pixels for a character to be shifted out as the
>beam is scanning across. There is code for this in ROM. I think a complete
>line of text is drawn using the same 6-8 vectors for all the characters
>rather than generating a raster pattern for each character. There is
>a vectrex FAQ somewhere, and it has been emulated so there is information
>out there :-)

Yeah, this was another cool "hack" implementation in the Vectrex. The PIA's
shifter provides the beam gating while the firmware sweeps a "raster scan".
It works quite well, although your beam-width is fixed which means that
larger text sizes can be pretty hard to see. I liked the fact that I could
scale the text up and the letters would "blow apart" like Grunts in
Robotron. :-)

> BTW, for you hardware geeks out there. I found that a PIC processor
>running at 20MHz can use this same technique to generate a character based
>video text display.

<tangent> Yep, that's similar to how my PIC raster-monitor tester works. I
actually over-clock a 16C54 to 14.318180MHz to dead-on timing. I think a
shift instruction can go directly to a PIC I/O port which saves a cycle
compared to something like an AVR that has to move from a register to the
I/O port... </tangent>

>Oh, BTW I don't remember where the vectrex shift register is, but I don't
>think it's a discrete component (this may be wrong too :)

It's from the PIA...

-Clay

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Received on Thu Sep 14 16:51:33 2000