Date   : Mon, 29 Nov 2010 12:35:07 +0000
From   : afra@... (Phill Harvey-Smith)
Subject: Harddisc fakery using a microcontroller

Jim Hearne wrote:
>   I repaired a lot of PC's in the ST506 era and i can't ever remember a 
> disc interface built into a motherboard, 

I have on a 286 board, but I have only ever seen one, also the board was 
one of those with a single slot that a card with more slots plugged 
into, so may well have been somthing proprietory.

>                                   it was always on a ISA card.
> The other problem is though that almost every MFM and RLL control card 
> disc format was incompatible with any other.
> You had to have the same make and model of control card that the drive 
> was low level formated with, otherwise you couldn't read the drive.
> This was a major pain if the control card failed and you didn't have one 
> the same.

I have generally found cirtainly with Western Digital chipsets, that as 
long as the chipset was the same e.g. both mfm or rll then the drive 
would work fine. For example I once formatted an MFM drive for my Altos 
unix box which used a WD chipset, on a PC 8 bit MFM controler and it worked.

However between manufacturers was as you say almost garanteed not to work :(

> I guess it was because the lower level format code was in the Bios for 
> the control card and there wasn't a well defined spec for it at the time.

May well have been the presise way that the MFM chip drove the signals 
to the drive, rather than the bios.

Cheers.

Phill.