address lines, but this would mean looking at more than 1 input port, and I was
already using that for something else.
Address lines A4 to A? can be used ta identify which of the 16 character
lines is being addressed. This brings the pen location resolutien to 1 in 14. To
increase the resolution-we can lock at either all 14 raster lines, or, as the
minimum sub-pixel is only 4 raster lines, we can look at the address lines that
relate to the selection of each group of 4 raster lines. The RS lines, RSO® to
RSS, are used to select the appropriate raster lines for each pixel, and soa by
looking at RS2 and RSS it becomes possible to identify on which of the
sub-pixels the pen is resting.
A& to AP can, therefore, be treated as the Most Significant Bits, and R52
and RSS as the Least Significant Bits, of a six-bit address that can identify a
location in a column of 48 locations.
To tap off these address lines, I soldered 14-pin header-plugs directly on
to IC6@ and ICSS. This method allowed me to solder wires to the plug with ease,
and gave test-points to see if I had blown the chip by soldering too close ta
it! The six address lines were taken along the board and put on to the bus at
pins 59 ta 64. On the Gemini 86-BUS, these pins are now allocated as interrupt
request lines, powerfail warning, and backup power, so it might be advisable not
to put the video address lines on to the bus if you can help it. The Video
Blanking signal, VBLANK, was also taken to the bus so that the screen data would
be updated only in the screen blanking period. this prevents screen flicker when
drawing images on to the screen.
For certain routines, I wanted to know if the pen was in the same place, or
whether it had moved from an area of white to an area of black. Unfortunately,
you can not merely test the pen status. This 15 because the sub-pixel consists
of 4 raster lines, each being 64us in duration, and each raster-strike on the
active area of the pen produces a pulse of 20us duration. As the picture frame
is re-displayed every 20ms, the sub-pixel will produce a train of four 20us
pulses every 26ms. It is only through the persistance of vision that the image
appears to be constantly displayed.
It now becomes necessary to build a circuit that will output two pulses.
One pulse will be of 2@us duration, to indicate that the pen has been struck by
a raster, the other pulse will go high when the pen has been struck, and remain
high for period of time that is greater than 26ms and will also be kept high
every time that the pen is struck. By doing this, a high state is output as long
as the pen remains over the displayed area, even though the area is being
refreshed every 2ems and being displayed for only &@us.
Figure S is the circuit diagram for the pen interface. Ici is an NESSS
timer configured as an astable multivibrator, the frequency of which is adjusted
by RVi. ICP is a 7493 Binary counter and ICS and 4 are 7400 NAND gates and
74LS5124 TRI-STATE buffers.
On power-up, the 7493 0 outputs are in a low state and so I€3ta), in
conjunction with the inverted output of IC2 and the output from the clock
generator, begins‘*to clock the counter through 16 counts until @2 goes high. At
this point, one of the inputs ta IC3(a) becames low and sa the clock pulses are
inhibited and the 749% ceases ta count. The HIGH at @3 is inverted by ICS(b) and
buffered by the TRI-STATE buffer IC4(a), the output of which will be enabled
only when SWI is closed.