Re: Fat40 RAM problem?

From: Gerrit Heitsch <>
Date: Fri, 29 Aug 2014 16:29:26 +0200
Message-ID: <>
On 08/29/2014 12:53 AM, A. Fachat wrote:
> Hi there,
> I've analyzed the RAM problem on my Fat40 a bit more. I've recorded the broken
> addresses - which are stable - and measured the RAM chip in question (UA16 )
> with the scope.
> See
> What looks interesting is that the data I/O pins of the RAM even stays high
> when I write a zero into it!

As someone else mentioned already, unless the CPU itself is broken, this 
means a dead buffer between CPU and RAM. Consult the circuit diagram to 
find the buffer chip in question. It's probably one of the 74LS24x type.

  I.e. either the RAM is broken and pulls the line
> high, or the driver (or something else) is broken.

Pulling a line HIGH against other drivers that want to pull it low takes 
more than a MOS output driver can do and the 4116 is NMOS or PMOS. So 
check the TTLs first.


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Received on 2014-08-29 15:00:03

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