Re: 1551 Repair - 28 pin 6523 Wanted

From: Gerrit Heitsch <>
Date: Sat, 08 Mar 2014 18:42:00 +0100
Message-ID: <>
On 03/07/2014 09:55 AM, John McKenna wrote:
> Excellent, thank you.  That makes a lot more sense. I'll see if I can
> make it fit a 64/32.

Here's something else I remembered today.

All the 1551 I have seen use a 6525A as the TIA, but I remember seeing 
an Image of a 1551 PCB that used a 6523A. Looking at the schematics of 
the drive (*) confirms that the drive does not use the IRQ capability of 
the 6525, the only IRQ source in a 1551 is a 555 rigged as an oscillator 
and hooked up to _IRQ of the 6510T. The code seems to be doing 
everything else by polling (in or out of the IRQ routine).


So, to me it looks like what needs to be implemented to keep our 1551 
running is an CPLD implementation of the 6523 which is a bit simpler 
than the 6525.

Also, the Gate array used in the 1551 should be the same as was used in 
the 1541-II and 1570/1571.

I can't read it, but it looks interesting:


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Received on 2014-03-08 18:00:07

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