Re: BASIC and System ROM replacements

From: Gerrit Heitsch <>
Date: Tue, 04 Jun 2013 18:59:36 +0200
Message-ID: <>
On 06/04/2013 04:14 AM, Jim Brain wrote:
> As I look at it, I can;t help but wonder if some juggling of the address
> lines and such would simplify things and remove the requirement for an
> inverter altogether, but I am not seeing an easy way to do it.

You have a few unused inputs on the LS688. Can't you use them to include 
PHI2 into the _SEL signal already? That would mean you could free up 
some inputs on the other logic simplify the circuit there.

Like this: Hook up PHI2 to Q6 and tie P6 high. Then the _SEL signal will 
already include PHI2 and you no longer need it at the UART.

Also why do you hook up CR/_W to the 74LS138? You could use that on G2 
of the 74LS173.

Hm, if you're willing to use _IO2 completly, you could get rid of the 
LS688 and use the LS138 to decode the I/O-Space for the UART and Banking 
register. It has a high active input, meaning you could add PHI2 here as 

Then you should read the datasheet for the Flash since for most RAMs, 
_WE overrides _OE, meaning you can tie _CE and _OE together. If that's 
the same for the flash, you can free up an inverter for use elsewhere.

Maybe some of this is of use to you...


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Received on 2013-06-04 18:00:11

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