DiscFerret: First working hardware, firmware and microcode!

Tony Duell ard at p850ug1.demon.co.uk
Fri Nov 5 17:05:31 CDT 2010

> > > 20 years would have to include "RLL" and ESDI  (RLL has same cabling as
> > > ST412, but different encoding and data-rates? , ESDI cabling looks the
> On Thu, 4 Nov 2010, Tony Duell wrote:
> > Eh? RLL is a date encoding scheme, ST412 is a de-facto stadnard for the
> > signals on the drive interface connectors. They are not the same thing at
> > all.
> Thanks.  I got sloppy.  This is one situation where I should have said

A lot of people do, and most of the time it doesn't matter (even I once 
asked for an 'MFM' drive in a second-hand PC shop years ago. I knew it 
was wrong, but I also knew that's what they were often called by PC 
types). But in this case, when we're talking aobut using the drive with 
essentially a non-stnadard controller, I think it helps to be prceise.

> "MFM", or clarified the comparison as "ST412 MFM", since I was trying to
> say that the RLL drives were cabled the same, but the data transfer rates
> and encoding may be different.

IIRC, the pulse rate at the interface connector is much the same, it's 
just a more efficient encoding scheme, using more of the possible patterns.

> > > ESDI cabling looks the same but has signal differences)
> > It's very different... For one thing the data separator is in the drive
> > (meaning the encoding method doesn't matter to the end user). FOr
> > another, head position, etc, is done by sending a command bit-serially to
> > the drive.
> What I was trying to convey (albeit poorly worded), is that the CABLES
> used for ESDI are the same as the CABLES used for ST412 (from a
> purchasing viewpoint), but the interface is different.

Even the cambles are not always the same. ST412 uses a 1-of-n drive 
select scheme, like a floppy drive. Which means you can (and IBM did) use 
a cable with a twist in it to link 2 drives, indentically jumpered, to 
the controller (It's not the same twist as the floppy cable). ESDI uses a 
3-bit binary select scheme (IIRC 0 means 'no drive selected'), and a 
twisted cable si not normally useful.


More information about the cctalk mailing list