21 Sep
2014
21 Sep
'14
6:37 p.m.
On 09/21/2014 05:10 PM, Mark J. Blair wrote:
I think the software for decoding and encoding the
flux transitions
is the hard part, so compatibility with the Disk-Utilities software
would be a Good Thing. I wonder if Keir would be interested in
branching out into hard disk formats if somebody donates suitable
hardware to him?
Not all that hard for garden-variety FM and MFM. You can do an
acceptable job with a lowly Atmel AVRmega 8-bit controller with external
memory. Heck, you can do an acceptable floppy emulator with one--that's
pretty much what the old FlashPath gizmos used.
Consider that on FM there are basically 2 intervals between flux
reversals and on MFM there are three. That's it. My floppy emulator
used 2 bits to record flux transitions(3), which left one combination
unused for future use with GCR.
Those Gotek floppy emulators that go for cheap on eBay are nothing more
than a Cortex M3 STM332F105 MCU. 32K of RAM, half-a meg of program
flash, USB OTG interface, plenty of timers, uarts, etc on chip--and
5V-tolderant inputs. About a third of the pins on the 64 pin package are
unused. Most of what little is on the board is for driving the outputs
(discrete MOSFETs).
Given that, a PIC32 or Cortex M3 running at 72+ MHz should be able to do
ST506 with not much more than the differential drivers added. An ST506
has roughly the same unformatted capacity per track as a 1.44M
floppy--it just spins faster.
But if you wanted to do an IMD-style image file, you'd probably want to
record exact address marks--although the data rate's the same on ST506
MFM drives, address marks and CRC/ECC details are different from vendor
to vendor.
--Chuck