18 Feb
2010
18 Feb
'10
5:47 p.m.
On 19 Feb 2010 at 0:30, Philip Pemberton wrote:
I'm using TI and Fairchild line driver chips --
74LVX14 and 74LVC07.
24mA output drive, series-resistor overcurrent protection (and the
option of fitting transient suppressors as well). They're TSSOP, but
the PCB solder-mask makes them fairly easy to replace with a Stanley
knife and a cheap 25W soldering iron.
Same here, except I'm using a 5v AVR, so it's bog-standard 7407 and
74LS14 DIPs.
Also, what's the issue with "Flippy"
disks? Why can't you just reverse
the data in the buffer to account for the reversed disc rotation?
I suspect it's because the head for side 1 is offset from the side 0
head on a double-sided drive--and the offset doesn't translate to a
track width.
Hm, as I understood it, you couldn't handle
hard-sectored discs with
the Mk1?
Not true--the MK I does have a real-time index status, which works
fine for DOS where the CPU isn't doing anything else and you can wait
for the status change and jump on it. I've done plenty of HS
floppies with a MK I.
Although IIRC it doesn't have any onboard memory
either -- your sample
length is limited by how much base memory you can grab (which would be
somewhere in the region of 64k per segment for plain DOS, or a few
GBytes for DOS+DPMI).
The MK I has 128K of SRAM for its sample buffer.
It would take some effort to write a "good"
GCR encoder/decoder. I'm
having one hell of a time just getting my FM/MFM decoder to behave
itself. For some reason, old Acorn DFS format floppies (FM, 250kbps)
don't read very well in any drive.
I routinely use the MK I to read Brother 3.5" 240K WP floppies and
have used it to read Future Data 8" HS floppies that are also GCR.
There are one or two 5.25" 100 tpi GCR formats that require that the
low-pass filter on the drive electronics have its headroom raised a
bit.
I'm probably going to put the current design (uses two AVRs--one
(ATTiny2313) to handle drive seek, status testing, etc. and one
(ATMega162) to actually do the reading and writing) aside for now.
It was mostly because I needed a few more I/Os using the DIP AVR
packages, but it's turned out very nice. The two communicate via
UART. Output of the gizmo is RS-232; if USB is desired, it's easy
enough to use a cheap converter. Data transfer to the host is plain
old XMODEM.
I'd like to start next on a serial-interface Pertec tape controller.
There doesn't seem to be much out there other than some old ISA
cards and the odd Pertec-to-SCSI controller. Still plenty of Pertec-
interface tape drives, however.
--Chuck