[sebhc] Re: Re-creating actual floppies from archive

[Lee Hart]

Robin England wrote:
> The cause of the lack of serial comms was actually that the 8250
> itself was faulty. Talk about bad luck; I had originally discounted
> the serial card having a hardware fault because I tried swapped
> serial cards between my two H89s and this had not fixed the fault.
> As it turns out, BOTH the serial cards had duff 8250s in the LP
> port circuit!

By any chance, were the serial cards that held these 8250's the original
H89 H88-3 cards? The original cards had a timing problem; they worked
with old parts, but then National "improved" the 8250 design to be
faster, fix some bugs, etc. The newer chips didn't work reliably in the
old card.

Heath added a 74LS74 flip-flop to the newer H89-A cards to fix the
timing problem. The H88-3A cards are recognizable because they have
large copper ground plane areas, and LC filters on all the serial I/O
pins.

Or, were you running your H89 at 4 MHz? You also needed this 74LS74
circuit if you tried to run at 4 MHz.
-- 
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
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