80-Bus News

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 ports.

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 necessary.

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.