Videx Videoterm 80

[mressl@macgui.com (Marc S Ressl)]

Following the format of the previous posts, I'll continue describing old
Apple II peripherals. And as before, this will soon be available in
OpenEmulator :-).

Today we'll be describing the Videx Videoterm 80. The schematics are quite
straightforward, nevertheless there are some important details that must not
be left aside.

The card is composed of:

* 1K of ROM memory
* 2K of static RAM memory
* An MC6845 CRT display controller
* Logic to control the whole thing

Memory is mapped as follows:

* $C300-$C3FF is mapped to $0300-$03FF of the ROM (the latest firmware only
supports slot 3)
* $C800-$CBFF is mapped to $0000-$03FF of the ROM
* $CC00-$CDFF is mapped to RAM. It is banked, meaning that what is visible
in this range can be either $0000-$01FF, $0200-$03FF, $0400-$05FF or
$0600-$07FF of the RAM
* $CE00-$CFFF is used to disable $C800 memory

The I/O range is mapped as follows:

* All I/O accesses talk directly to the MC6845, but also change some
flip-flops
* Address bit 0 selects the address or data register of the MC6845
* Address Bit 1 does something undocumented that will be explained later :-)
* Address bits 2 and 3 define the RAM bank that is visible in $CC00-$CDFF

Now to the more exotic facts:

* The MC6845 uses a 17.430 MHz crystal for generating the video clock
source. This is a bit faster than the Apple II one (14.31818 MHz), so the
video quality is a bit higher.
* The 17.430 MHz video clock is usually divided by 9 to derive the character
clock. Now to the undocumented part mentioned previously: by accessing I/O
memory with bit 1 set, the character clock is divided by 8 instead, which
allows for denser characters. Most likely nobody ever used this, but it is
available on the card.
* The cursor is generated by simply inverting the font's bits.
* There are two font EPROMs that control the font for characters $00-$7F and
$80-$FF. It seems that it was usual to have an ASCII font in the lower range
and the inverse ASCII font in the higher (Visicalc 80 supports this).
* The 9th font bit is replicated from the 8th font bit. This is important
for inverse character sets.
* Video RAM is accessed continuously from the MC6845 in order to show video,
but the Apple II can also read/write from it. There is a circuit that
multiplexes these accesses and gives priority to the accesses from the Apple
II bus. When that occurs, the MC6845 does not display valid video
information.
* And now to the most interesting part. There is a *REALLY AWFUL AND LAME*
(imho) circuit implemented with a diode and a 100 pf capacitor. It's
function is to disable the video output for a short time after whatever RAM
access from the Apple II bus. I find this so awful because it was done with
an RC circuit :-), as it takes advantage of the TTL input-resistance. I
checked the specs, and by taking an input resistance of 100 kOhm for U22
(check schematics), one gets a time constant of 0.01 ms, which is approx.
174 video dots.
* This last circuit is important because it disables video output while
scrolling. Otherwise you see shadows of previous lines at the bottom.

So that's it! It really took some time to grasp the last part about video
disabling, but I think it was worthwhile for truthfully emulating the legacy
Videx Videoterm 80 card!

With the best wishes,

Marc.-