Re: ROMs replacement

From: Gerrit Heitsch <>
Date: Sun, 07 Oct 2012 14:34:22 +0200
Message-ID: <>
On 10/07/2012 01:31 PM, wrote:
> >> Something is really strangely off / on the verge of specs here. Any help ideas?
> Thinking aloud as I don't have a scope here at hand to verify.. could it be that PLA's outputs are not fully compatible with the LS inputs? When sinking off the original ROMs there are no pullups nor anything else on the way. Maybe we need something when connecting those outputs to the LS?

No, then there is a 74LS139 connected to one of the PLA outputs and that 
doesn't give any trouble.

The vertical bars are indeed strange. Especially since they only happen 
in the border area where the Char-ROM is not involved in any way.

Did you remember to put the pullups where they belong so that no input 
of the LS00 or LS11 is ever floating, no matter what your switches are 
set to? Also, did you remember to include 100nF ceramic capacitors 
between GND and Vcc for the LS?


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Received on 2012-10-07 13:00:05

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