[sebhc] diskette drive

[Dwight K. Elvey]

>From: "Dave Dunfield" <dave04a at dunfield.com>
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>
>If you want to build your own controller, the Nec 765 controller chip
>used on old XT controllers is quite easy to work with - it would not
>be tough to build an H8 controller for "normal" DD floppy drives with
>one of those - In fact, I like the idea someone posted earlier today,
>just adapt the XT card to fit the H8 - that gives me an idea - what
>about an H8 "card" which is just an XT card adapter - Basically a blank
>card with an XT bus socket at right anges, so that you could put an XT
>card on the front of it - the whole thing would still fit in the H8,
>and you could use any XT card you liked.

Hi Dave
 That was me. You'd need to deal with address conversion
and interrupt selection. Still, it is well within what
can be done. The only issue with the 765 is that the
processor needs to be fast enough ( unlike the MFM hard drive
interface that actually buffers at least one sector ). The 8080
at 2MHz isn't going to keep up with much. As I recall, it
takes really tricky code to even do single density.
 In these cases, a DMA controller makes sense. I'm not sure
what the bus signals are for the H8. I wonder if it has
the ability to have DMA. It doesn't need to be too fancy,
just something that can move data on an interrupt. I've worked
with the Intel DMA chip ( forget the number ) but there
may be other choices out there.
 The other thing is that it might be done with a simple
multi-ported arrangement with a RAM on board this interface
card. One would use the processor with dummy writes and holds to
actually transfer data. This would be well within the 8080's
ability to handle double density. Such instructions as PUSH
can be used to automatically increment the address.
 It needs a little thought but it is possible. Even something
as simple as 256 NOP's in a row could be used for a sector
size of 256.
Dwight


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