IDE hard drive on original IBM AT?

[Tony Duell]

> 
> Hello all,
> 
> I have two "true-blue" IBM ATs, both the early motherboard revision. 
> I have the following questions:
> 
> - Is it possible to use an IDE hard drive instead of MFM?  I have
> tried several different drive/controller combos, and nothing works so
> far :-( .  The drives are small ( < 100 MB), so it's not a geometry
> issue (at least, I don't think so :-) )
> 
> - Is anyone running an early rev. AT with an IDE drive?  How did you do it? :-)

Alas I am running a later motherboard version here (8MHz clock), but I 
had no problems plugging in a combined floppy and IDE hard disk controller 
card and linking it to an IDE drive.

There were 2 things I did to get it 'right' although neither were 
essential. One was to overlay the top 8K of the system ROMs with EPROMs 
(this was a minor motherboard mod) so I could have a more suitable 
geometry. The other was to add a kludgeboard (a few TTL chips) to the IDE 
controller to operate the drive-in-use LED on the front of the machine. 
The IDE drive I used didn't have an obvious output for this.

The IDE interface was designed to be software-compatible with the IBM 
PC/AT hard disk controller card, so there shouldn't be problems. Note I 
said 'shouldn't', not 'won't'. What exactly happens when you replace the 
MFM controller with the IDE one?

-tony