A Light Pen For The NAS­COM2

J. Auckland

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.

PEN DESIGN

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 ??? ?