It can be seen that the output to the ports is now held at a LOW state. If
the pen is now activated a 20us pulse will be sent to both the OUTPUT1 and the
RESET input of the counter. Q3 now goes LOW, and thus IC3(b) goes HIGH, enabling
the clock input and allowing the count to begin again. The output2 has now gone
HIGH indicating a hit. As long as the frequency from the clock generator is
lower than 20ms x 16 ie about 800Hz, then IC2 can not count up to 14 before the
receipt of another reset pulse. Therefore, if the pen is struck by the raster
every 20ms, IC2 will be reset and Q3 will remain LOW and the output2 HIGH.
Even though the pen has moved from a white area to a black area, it will
not register the change until 16 clock pulses have been received, and RV1 can be
used to slow down the clock and increase the time taken to register a change in
state. This can be used to effectively slow down the speed of the pen.
Similarly, the frequency can be increased to a point where the reset pulse has
no effect. The two LEDs indicate the pen’s status. LED1 indicates whether the
pen has registered a hit, and LED2 shows if the data has been enabled into the
The prototype was built on a small piece of veroboard and housed in a small
diecast aluminium box, and seems to have tolerated an immense amount of
knocking. The pen was constructed using a small length of plastic tube with a
5-pin plug and socket to allow it to be disconnected from the system. I found
that it was easier to assemble the pen in three pieces than to try and poke
everything down the tube. If the sensor is glued into a smaller tube, (I used a
drilled, solid piece of plastic for this) and then that inserted into a larger
tube, the pen can be easily separated for modificaction. Similarly, the switch
can be set at the junction of two tube halves to facilitate re-wiring if
It should be possible for most people to construct a light pen using the
above design, though I am quite sure that most readers will be aware that this is
only one way of obtaining information as to the whereabouts of the raster.
Another method could be to count the blanking pulses, or indeed build a
circuit on the lines of the NASCOM VIDEO RAM scanning circuitry, or buy a GEMINI
video board and have done with the interface problems!
Now that ARFON have gone into receivership, it may be that this
do-it-yourself light pen will be the cheapest on the market, and I see no reason
why it can not be used for the GEMINI video board.
PLUTO Colour Graphics Card with Extended Command ROM.
Suitable for any 80-BUS System.
Retail price including VAT over £500.
Will accept £320. Ring Bob on __-___-____.