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The net

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Now that we have covered the horizontal and vertical counters we can start talking about how the graphics for the game are actually generated. We will start with the simplest part, the net.





At the start of each horizontal line, /256 is high which gets clocked through F3 to one of the inputs of G3. Since the other input of G3 is connected to 256H (which is low), the output of G3 will be high thus turning the net off. When the horizontal counter reaches 256, 256H goes high, and the output of G3 will go low. On the next clock pulse the low on /256H will get clocked through F3, thus turning off the net. The result of this is that the net will be one pixel wide and positioned at column 256 on the screen. Since the horizontal counter counts up to 455, this might not seem centered, but 80 of the counts are during the horizontal blank period so most of these won’t appear on the screen which brings the net to roughly center.


The output of G2 will go high, displaying the net, whenever all of its inputs are low. The 4V input to G2 turns the net off and on every 4 vertical lines, turning the net into a dotted line. Finally the VBLANK input turns the net off during the vertical retrace.

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To summarize: blank is hcount 0-79, sync is 32-63 and net is 256


So, if we say that end of blank (hcount=80) is pixel #0, then the net is at pixel #176, and the end of the active screen is pixel #375 (hcount=455). So the net is shifted slightly to the left (pixel #176 vs pixel #188) than the center of the active screen. Not a really big deal, and probably tweakable with the CRT trim pots. And since everything else (paddles & ball(s)) will be relative to the net (which is nicely at hcount=256), the difference is immaterial.

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