Re: 1551 Repair revisited

From: Jim Brain <>
Date: Tue, 5 Apr 2016 18:54:29 -0500
Message-ID: <>
On 4/5/2016 5:31 PM, Rob Clarke wrote:
> Hello gentlemen,
> Raising this two year old thread from the dead as I have 3 broken 1551 
> paddles and Jim has offered to prototype a 6523T replacement. I've 
> been doing some investigations into their operation and there were a 
> few loose ends from the discussion that I thought were worth 
> commenting on.
Yep.  I apologize.  I wrote all the code, and then it seems I got onto 
something else and never came back to it.
> 1) The core of the 6523T is indeed a 6525. Using a plus/4, all bits of 
> all three ports can be set as outputs and will store a written 8-bit 
> value. Additionally, the control registers R6 and R7 from a 6525 are 
> present and can be read/written to at FEF6 and FEF7 (assuming set as 
> device 8). Setting the mode control bit in R6 appears to affect the 
> usage of port C as expected in the 6525 datasheet. The defined 
> behaviour of the 6525 on /reset is to set mode control to 0 and 
> neither FEF6 or FEF7 are accessed by the 264 series ROM, so I would 
> see no reason to implement them.
Good, because adding all that Priority Encoder code will cost a lot of 


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Received on 2016-04-06 00:00:02

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