Make-It-Yourself projects Radio Electronics, June 1976

This interesting project allows to build a strange variant of the classic Tennis (PONG) game. The first part published in the June 1976 issue of Radio Electronics allows building the basic game: two paddles, the ball, the top and bottom boundaries, the central line, and the bumper. This last one is unique in its gender and consists in a fixed square in the center of the screen. If the ball strikes its vertical edges, it bounces at random speed and angle. If it strikes its horizontal edges, it enters into the bumper and exists at random angle and speed. The bumper is optional: the classic Tennis game can of course be played.

The next issue, published in July 1976, adds sound and digital on-screen scoring circuits. As we don't have this issue, we couldn't include it here, but the most important part is there. What is most interesting is the presence of digital on-screen scoring instead of the simple "follow me" scoring where two squares shift horizontaly each time a point is marked. Here, real numbers are displayed using a character generator (mainly a ROM memory chip containing the data forming the digits). These memories, altough very small in size (typicaly less than a kilobyte) contain the sufficient space required to store the digits' data. Here are two examples of how same digit can be stored in memory depending on its size. The 0's are replaced by dots for better reading.

                       .1111111 (7F)
    ...11111 (1F)      .......1 (01)
    .......1 (01)      .......1 (01)
    ...11111 (1F)      .1111111 (7F)
    ...1.... (10)      .1...... (40)
    ...11111 (1F)      .1...... (40)
                       .1111111 (7F)
    
The smaller character requires five bytes. To store all digits (0 to 9), 50 bytes would be required, so a 64-bytes PROM memory could be used.
The larger character requires seven bytes, so 70 bytes would be required to store all digits. A 128-byte PROM would therefore be required.
Special character generators contain not only the space needed to store the data, but also the logic to address the data according what should be displayed, and in some cases, specific signals such as horizontal and vertical video synchronization and X/Y locators in order to locate the character at a specific place on th screen.

In the simplest form of character generator, the memory chip is not driven like in modern computers. Instead of being driven by a microprocessor or another advanced component, it is driven by simple logic gates.

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Cover	Page 35	Page 36
Page 37	Page 79	Pages 80 and 81

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