Re: Logic levels question 6502A (Missile command)

On Sat, 5 Nov 2011 11:33:13 -0700, you wrote:

>I am trying to trouble shoot a board. I am getting some odd responses
>reading the roms. I noticed that the address lines coming from the 6502
>seem to be going high at 3.5 - 3.7 vdc. I suspect this is causing some of
>the ttl to not latch high.
>
>This is checked through a meter using the fluke to read a particular address
>on a loop and using my scope for the same and while running the game.
>
>Is this a normal value? I don't see any pull-ups on the address lines on
>the schematic.

        While it is true that, as John & Rodger pointed out, 2.4Vdc is the
minimum logic-high level for TTL, personally I would still consider those
values to be somewhat suspect, as I can't recall ever working on a TTL
board where the outputs didn't swing from ~0V Low to ~5V High fairly
consistently... (Not saying it *can't* happen that way, I suppose; just
that I've never encountered it myself.)

        To me, that would generally be a sign of either two outputs trying to
drive the same line to opposite states at the same time, or one of the
chips further down the line having an input that's shorted down to ground.
Are *all* of the address lines doing this, or just some of them?

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Received on Sat Nov 5 20:25:58 2011