Re: I: STAR CASTLE PCB with a "little" problem

From: William Boucher <>
Date: Tue Jun 07 2011 - 23:00:35 EDT

I thought that your video was very interesting and it certainly demonstrated
the problem. The rotation of a ship or ring is a math intensive and
relatively complex procedure that involves EPROM lookup tables (for both the
basic object drawing data and Sin/Cos data), the ram chips (L14, M14, N14),
the '181 ALU chips (L6, M6, N6), and many other ICs (assorted gates,
latches, data selectors, etc.). Since so much of the general game operation
also depends on many of the same chips that deal with all of the rotation
calculations, I would not expect the game to run properly if one of them was
failing. I would expect the game to crash. Since you say that you tried
several chips (EPROMS, ALUs, RAMs) in another board and they worked then
we'll consider them proven okay. This also eliminates the 34-conductor
ribbon cable and the monitor as a source of trouble. The condition of the
chip sockets on the problem board may remain an issue but like I said
already, I would expect a crash if this were the case because such a fault
would mess up a lot of things, not just rotation. If I were to assume that
the sockets are okay, and since the game play is okay and the problem
appears to only effect the picture, then I might suspect the video data
output latch chips (74LS377 at M2, P2, S2).

When I watched the video, I got the impression that the misplaced vertex was
shifted mainly (or entirely) in the "Y" axis. I could be wrong about this
since I only watched it a couple of times however for this moment I'll go
with it. The 12-bit "Y" data is output as 3 nibbles (4 bits each) where
each nibble is latched by one of the 74LS377 chips M2, P2, S2 where M2 has
the least significant nibble and S2 has the most significant nibble. Since
the positional error is very small, I'd suspect a very low order magnitude
bit, so I'd replace M2 first. You might want to check the condition of the
J2 pins 1 through 13, the traces leading to the pins, and the solder on the

William Boucher

----- Original Message -----
From: "vernimark" <>
To: <>
Sent: Tuesday, June 07, 2011 8:47 PM
Subject: VECTOR: I: STAR CASTLE PCB with a "little" problem

> (sorry for the bad quality)
> In some starship positions vecrots are drawn with one wrong vertex. The
> same
> happens with the castle and other starships. Any Idea?
> 74181: ok
> ROMs: ok
> (on another PCB RAMs and ROMs work well)
> Thank you,
> vernimark
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Received on Tue Jun 7 23:00:27 2011

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