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Repair of two 1050 floppy disk drives

1050 floppy

26 replies to this topic

#26 1050 OFFLINE  

1050

    Stargunner

  • 1,117 posts

Posted Sun Feb 17, 2019 7:55 PM

Same pinout but different functionalities at those
pinouts. 2332 has postive enable, 2532 has negative
enable or as we found out the hard way, it's actually
a negative chip select signal and not the output
enable.

And that caused the 2532 to mirror it's data to a
non selected address range, a very bad habit for a
eprom to have. The 2332 was better behaved
in this regard because there the chip select was
enabled all the time and OE was toggled by address
range properly. When they made the 2532 they made it
with those two functions reversed on those two pins
and then just buffaloed everybody with lingo speak to
hide the actual functionality. And why I can't know.

But we proved it as far as we are able, thanks to
Nezgar's efforts.

On paper you can say it's the same duck.
But it don't walk like the same duck and it don't
quack like the same duck. So not a drop in
replacement even if it does have the same pinout.

JP3 would have sent it the positive enable and 2532
is an eprom with negative enable instead. Certainly
can't send it both positive and negative enable at
the same time either so no to JP3 for 2532. Keep in
mind the 2532 was never meant to be here in the first
place.

#27 Nezgar OFFLINE  

Nezgar

    Stargunner

  • 1,978 posts
  • Location:Saskatchewan Canada

Posted Mon Feb 18, 2019 2:50 AM

Scratches head..

Isn't the 2332/2532 the same pinout as the masked ROM, so you would leave the four jumpers set to ROM: Only short JP1 and JP3.
2732 requires the flipping of two lines, hence the jumpers: only short JP2 and JP4


Yep, I was very interested in the prospect of a solderless 'drop in EPROM' that didn't require moving the jumpers. But, I do have a solution with the adapter that only requires one solder point connection for a drive with a JP1/3 configuration. Could be attractive for some people wishing to upgrade to a US Doubler, with minimal soldering in the drive itself.

 

1050 summarized it pretty well above. To recap, if I understand it correctly, The 'Power Down' function of the 2532 on pin 20 doesn't match up with the 'Chip Select' used on Atari's 2332's on pin 21, and on top of that the signal needs to be inverted.
 
I've also confirmed more things/simplifications since my last post:
 
- Since pins 20/21 are not connected to the mainboard with my adapter, JP3 can remain in place, and lifting leg of R83 is not necessary for easy return to stock 2332.
- 2532 Pin 20 (High=Power Down) is the only one that needs to connected to the inverted A12 signal for a negative chip select (Top of JP4, or U16 pin 6).
- Pin 21 (only used for power for programming) can be left floating. OR:
- Since 2532 Pin 21 is only used for programming, there's no harm in leaving it in place, receiving the positive enable. 1 less pin to snip off of the socket 'adapter'. I've just tested this, and it still works fine.
 
pin20.jpg
 
A permanent LOW is normally sent to pin 20 (Ground via R81), causing a 2532 to never DISABLE, causing it to clobber the lower 4K of RAM & Registers in the 1050 memory map. Disconnecting that, and instead feeding it it the inverted A12 from the Top of JP4, or U16 pin 6.
 
The 2532 datasheet that I'm reading actually describes this behaviour accurately. (pin 21 for Vpp 25V programming voltage, otherwise not used), and Pin 20 for +5V Powerdown... which is consistent for EPROMS, but unfortunately breaks the idea of a 'drop in replacement' for Atari's 2332's, which uses pin 21 for chip enable, and opposite LOW means powerdown...

I notice now that atariry also observed this in one of his first posts on this thread:
 

Also when using the 2532 EPROM, the logic high level on pin 20 (the program pin) was really low. This pin is used as a chip select and connects to A12 (if I recall correctly). Again this appears to be normal and the system still runs (it doesn't lock up as I originally observed). I assume that the program pin is a little unusual and just heavily loads whichever IC is driving it.


It's interesting, and I learned a lot...







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