SWTPC Potentiometer Digitizer (Joystick)

PPG provided by Joe Lang

Assembly Instructions (647K PDF)
6800 Test Source code (1K TXT)

Product description from a 1976 SWTPC Catalog.

With the rash of graphic terminal designs and kits available to the microcomputer hobbyist, such as our GT-61, it only makes sense that there should be a low cost joystick and linear control digitizer kit to make these graphic terminals more useful and entertaining. Just imagine being able to play TANK OR BIPLANE on your own graphic terminal instead of having to sacrifice all of those quarters in the commercial units at the local quarter arcade. Remember also that the commercial units are limited to a specific game while the programmability of the computer and graphics terminal lets you play as many different games as you have programs for. You can even use the digitizer's controls to help you create or change images or pictures on the graphic terminal screen.

The circuit was designed to accommodate either two linear slide potentiometers or a low cost joystick designed for use in four channel sound systems. A joystick, for those not familiar with the term, is simply a vertically positioned handle resembling the type of maneuvering control used in helicopters and some other aircraft. It can be moved forward, backward, left or right, or any degree of any two directions. The joystick has an unrestricted 360 degree movement. On this particular joystick, the handle is interconnected to two potentiometers such that the resistance of one control changes with respect to left-right movement while the resistance of one control changes with respect to forward-backward movement. The digitizer's circuitry then alternately converts each potentiometer's resistance into a seven bit binary count that is fed to your microcomputer thru the input side of a standard eight bit parallel interface. An updated value of each potentiometer's position is generated by the circuitry approximately fifty times a second.

The eighth bit generated by the digitizer tells the computer which potentiometer's data is being presented. A "DATA READY" strobe and its compliment are generated by the circuitry making the board usable in either polling or interrupt driven computer systems.

The linear slide control version of the board works the same way except that instead of having a joystick, there are two linear slide controls in its place. These controls might be used for speed, positioning, skill factor, or whatever you wish. You can of course replace the slide potentiometers with conventional circular potentiometers if the application is more suited to using them.

This product as with all other Southwest Technical Products Corp. peripherals was designed to be as universal and as software independent as possible. All of the counting, timing and latching is done by hardware on the circuit board rather than software in the computer system. This not only makes the digitizer easier to use but saves computer time in an application where the computer needs all the time it can get just to update the graphics display. Interfacing can often be done thru the input side of the same interface that feeds the graphic terminal. That is if the graphic terminal is interfaced thru an eight bit parallel interface as is the SWTPC GT-61. Some applications will require the use of both the joystick and linear controls. In this case it will be necessary to use two separate digitizers with separate interfaces.

The entire unit is constructed on a 3" X 8" single sided circuit board. Since the voltage requirements for the board are a mere +12 to 18 VDC @ 20 ma., power for the board may be drawn from the computer system to which it is interfaced.

The potentiometer digitizer is sold in kit form only and is available in two versions. The PPG-J kit is the joystick version and includes the circuit board, joystick, components and assembly instructions. The PPG-S kit is the slide control version and includes the circuit board, slide controls, components and assembly instructions. A chassis is not offered for either kit.

# PPG-J Joystick/Potentiometer Digitizer kit $39.95
# PPG-S Slide Control/Potentiometer Digitizer kit $35.95

The PPG was introduced in 1976 and in no longer being sold. SWTPC went out of business around 1990. The joystick controller shown is difficult to find but CTS still has a Series 252 Ministick controller. DigiKey carries them as of July 2006.

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This page was last edited July 08, 2006