Some time ago I constructed a light-pen to interface with my
Nascom 2, and I can declare that it is the cheapest on the market! The
pen has been in use for quite some time and sees to be reasonably
accurate, taking in to account the minimum resolution available with
the standard Nascom Graphics chips.
The design for the light pen evolved because of my reed for a
device that would allow me to select routines from menus of routines.
Some of these routines were to be used to input data by using the
screen. I wanted a routine that would enable me to define the envelope
shape of a particular sound that was to be generated in a
micro-processor controlled synthesiser. Thus a peripheral was needed
that would permit the definition of envelope shapes by drawing the
envelope shapes directly on to the screen. It seemed that a light pen
would be the most obvious device to use and so I looked to various
manufacturers for a suitable light pen. Unfortunately, it transpired
that the majority of pens were either out of my price range. or not
quite the design that I required, and so I decided to build a pen
specifically for the Nascom 2. The design works with my system, and the
pen and its associated hardware is far cheaper than any other pen that
I know of.
Having looked at the various phot0-electric devices that could be
employed in the design of the pen I decided to use a Light Activated
Switch, which as an RS component type 305 434 (5??) as the
photo-sensitive device. This device includes a variable-threshold
switch that not only enables the sensitivity of the pen to be altered,
but also reduces the component count by having the threshold switch as
an integral part of the sensor.
Refering to figure 1 it can be seen that the output from pin 4 of
the sensor is usually buffered by a TTL gate to bring the switched
output to a clean TTL compatible level. The time-constant components,
R1 and C1, set the sensitivity of the device, large values of RC give a
high degree of sensitivity. It would be quite possible to use the
circuit in figure 1 to directly interface with an input port. On the
prototype, a pulse of 20us duration was output from the photosensor
every time the raster struck the active area of the pen. This could be
used to identify an individual pixel, or sub-pixel, or the screen area.
By loading each successive VIDEO RAM location with the data FFH and by
scanning the input ports for a HIGH from the pen, it is possible to
identify the pen location on the video display unit. For a higher
degree of accuracy, each VIDED RAM memory location can be sequentially
loaded with the data C1H, C2H, C4H, C8H, D0H, E0H. This data will then
enable the pixel to be subdivided into 6 sub-pixels, and will ??? ?