ISA bus throughput

Tony Duell ard at
Thu Feb 2 15:19:45 CST 2006

> >  It would be nice to obtain a schematic of one of the
> > more primitive IDE cards *whistles*. And wouldn't
> > artwork be nice too ;). Would an 8-bit card take the
> > place of a 16-bit card in an AT? Presumably yes,
> > albeit slower I guess. The Acculogic sIDE/16 or
> > something has all discrete logic, save for a pal or
> > gal (or 2).
> Yes, this would be interesting.  I had never realized that IDE was a superset of
> ST506.  Now I'm thinking of all of the old 8 bit machines I have that could use

It's not. IDE  and ST506 are very different interfaces.

IDE ias a formatted interface. You transfer nice 8-bit bytes and 16 bit 
words to the drive. The drive takes care of turning them into a suitable 
pulse train to send ot thehead, etc.

ST506 is a low-level, raw, interface. What you see on the interface 
connectoer is essentially the pulse stream to/from the head. It's up to 
the cotnroller to turn that into the user data bytes/words.

The origianl PC/AT controller talked to an ST506 drive. But of course it 
also had a host-side interface -- it plugged into an ISA slot and 
appeared as a number of I/O ports to the 286 (or whatever) processor. The 
controller card did the translation betweenn the user bytes/words and the 
pulse stream on the ST506 interface connector.

Now you could also split the system up in a different way. Specifically, 
move most of the controller over to the drive. The 'controller card' now 
becomes little more than an address decoder which lets the host processor 
talk to I/O port registers physically located in the drive. To the host, 
it looks the same as the old ST506 controller (you send the same bytes to 
the same ports to read a particular sector). Inside the drive, who knows 
what goes on. 

That's IDE.

> an IDE hard disk.  I've been trying to graft SCSI cards onto them, but 8 bit
> SCSI cards with BIOS aren't exactly common either.

You will have a lot of fun (for suitable values of fun) trying to get 
most old hard disk controllers or their hosts talking to IDE drives. Just 
because an IDE drive looks the same to the host as one particular ST506 
controller doesn't mean that all ST506 controllers look the same. They don't.


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