Asteroids pinout, "DIAG STEP", what is it?

joemagiera

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On the pinouts for Asteroids, pin K is "DIAG STEP" (for "diagnostic step"). It goes to pin 14 of the 74LS251 @ L10 (board revisions -01 through -04) or L11 (board revisions -05 through -06). Anyone know what this is or what it does or what it can be used for? I believe Asteroids Deluxe has the same item. It's definitely not for self-test, there is a separate pin for that (also going to that same 74LS251).

I looked through the manual cover to cover and all the schematics and it's not covered at all (other than showing it going to the 74LS251).
 
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I messed with it a while back.. It draws a vector pattern one vector at a time. It was a good way to keep the noise down while tracing signals.
 
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