80-Bus News

  

May-June 1983, Volume 2, Issue 3











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10

It can be seen that the output to the ports is now held at a LOW state. If the pen is now activated a 26us pulse will be sent to both the OUTFUT1 and the RESET input of the counter. @3 now goes LOW, and thus ICZ(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 2@ms » 16 ie about S@@Hz, then ICZ 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 03 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 RVi can be used ta slow down the clock and increase the time taken to register a change in state. This can be used to effectively slaw 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. LEDI 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 ta have tolerated an immense amount af knocking. The pen was constructed using a small length of plastic tube with a S-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 inta 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 peaple toa 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.

CLASSIFIED ADS. 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 01-542-0873.


This is an OCR’d version of the scanned page and likely contains recognition errors.











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