18 Oct
2008
18 Oct
'08
7:02 p.m.
Christian Corti wrote:
...
This reminds me of the CRT display of a Cogar C4 (or
ICL/Friden 1501).
It is a standard 5" CRT with an almost ordinary deflection system. But
the deflection itself is rotated/mirrored, i.e. the "vertical" sweep is
from left to right and the "horizontal sweep" from top to bottom. So the
display controller writes all characters from the first text column
first (and then IIRC only the text lines 1,3,5 and 7). Then comes the
next columns. After all 32 columns come the even text lines (there's a
display mode which only displays lines 1,2,3 and 4; the display has a 32
x 8 character cells). But that's not all. There is a second horizontal
deflection coil that interferes with the first main deflection coil (I
think this is called twiggle sweep or something like that). This second
coil deflects the beam according to the width of one character cell, and
one raster line of a character is written on the screen during this time.
The Datapoint 2200 used a serial mos memory in its first incarnation.
Sweeping each row of text implies scanning the same characters seven
times in a row. They couldn't afford to add a line buffer, so they
invented "diddle scan".
They actually scan to the upper left corner of each character, then
sweep out the 5x7 dot matrix for that character, before adjusting the
x,y deflection to the upper left corner of the next character.