14 Jan
2011
14 Jan
'11
12:13 p.m.
On 13/01/11 21:29, Tony Duell wrote:
Not really... other than "read data is a 1us pulse every time there's a
transition" and similar stuff like that.
Is that 1us guaranteed? IIRC it comes from an RC one-shot in the drive,
and there's considerable tolerance there.
Is there
a defined standard as to when the READY output on a
Shugart-type disc drive should go active?
Not that I a maware of. Are there any standards for anything on that
interface :-)
It's the minutiae that cause the trouble.
Indeed yes. As I said in another message, at one time you could get OEM
and service manuals for floppy drives, at least one of which would
docuemnt that drive's interpretation fo the interface. Not any more :-(
That is nto
too suprising. Amstrad may have done a lot of odd things, but
these drives did turn at 300 rpm and use the standard data rate.
Never a bad thing :) I asusme you've looked at some old drive
schematics. The write circuit is
normally pretty simple, a divide-by-2 D-type clocked from the WD signal
and reset by WG (to get a consistent starring phase when writing),
That's the easy part :)
a couple of traisnstors to drive the heads and a
current sink. And a
bit of
enable logic. Often the transistors were part of
a tranistor array chip.
Hm, I don't think I have any discrete transistor array chips (unless the ULN2003 and 2803 High Current Darlington Arrays
count).. and the only
transistors I have any great number of are BC857s (BC547s in SOT23
surface mount packages).
You mean you don't keep 2N3904s and 2N3908s in stock? I am seriously
suprised, I use those by the dozen...
The problem I'm trying to figure out is: how much current do I need to
put through the head, and how much current did the Amstrad board put
through it?
I thought we estimated this some time ago from the resistors on the
Amstrad board and a reasonable assumption that the write current was
passed through one of the resistors.
Alternatively, look at th shcemaitcs for some normal 5.25" and 3.5"
drives that use discrete components in the write amplifier (I'll have a
go if you can't find any..) Work out the write curtrent from those. It
won't be exactly right, but I;ll be it'll be close...
I'd rather like to avoid overloading the head --
permanently magnetising
or frying it would not be a good start.
Frying it would be a darn bad idea, I susepct the only sourve of spare
heads comes witrh logic boards attached :-)
However, you can demagnetise disk drive heads with a tape hard
demagnetiser ('defluxer' in some books). Basically a coil with a soft
iron cor that's connected to the mains through a push switch. Put the tip
of th core on the head, press the switch, slowly move it away to arm's
length and then (only then) release the button. I have no idea if you can
still buy such things, but they must turn up on E-bay, or I guess you
could make one.
-tony