20 May
2006
20 May
'06
10:24 a.m.
Here is the situation. I have many older systems that
only support the
ST-412 standard drives. Most use a WD-1005HDO style controller to talk
This is a problem that many of us have. And the HDAs are not the sort of
thing that can easily be repaired in a home workshop.
to the machine's bus. I also have several that use
a custom controller
to interface the system bus directly to ST-412 (without the WD-1005HDO).
I haev a couple of PERQs that use ST412 drives. The controller is part of
a large (200+ chip) I/O board, so can't easily be repiaced. It consists
of a 2910 sequencer + microcode PROMs + FIFOs + ... Yes, I have
schematics, but I don't feel like redesigning that lot.
The second problem is that the PERQ puts filesystem pointers in 4 words
of the sector header (it reduces the inter-sector gaps to compensate, I
guess). Which makes using a 'fromatted intereface' drive, like SCSI or
IDE a little more difficult, since they expect to store the data in each
sector only.
I'd like to continue using these systems, but
I'm quickly running out of
working drives.
I do have the specifications for the system bus, but most of the
software interface is not documented. I would need to dissasemble the
executables to figure out what is going on.
What I interpret what you said above to mean is that the interface
between the controller card and the ST-412 drive is not simply a digital
interface. It is acutally the MFM stream bi-directionally, and that the
Well, it is a digital interface in the sense that all the signals are
2-state. You don't actually get the head connections on the interface
connector, the read amplifier chain, write drivers, etc are part of the
drive's electronics.
There are restrictions on the pulse rate too, but apart from that,
anything goes. It doesn't have to be MFM encoded, for example. The drive
has no concept of sectors, it's up to the cotnroller to detect and decode
the sector headers.
You step the heads to a particular cylinder, you select a head, and then
uou get the bitstream from that track. That's it.
controller is what converts this to the system
required digital stream,
and the systems digital stream to the MFM stream.
Yes. That's why the ST412 controllers are more complicated than, say, an
IDE interface (which is just an address decoder and buffers most of the
time).
-tony