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MFM vs RLL

U

Unknown_K

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I finally got a HD in my XT NEC V20 equipped clone but have a few questions.

The HD controller is an 8 bit Adaptec ST-11M (MFM controller)
Got the correct cables from a member here
Snagged a Seagate ST-238R (RLL) drive from a local recycler who had no idea what it was for a couple dollars as is.

So I hooked it all up expecting nothing to work and booted. The controller found C: drive as a ST-225 20MB HD and it booted to DOS 3.3 (lucky I guess it all worked).

From my understanding the only difference between a RLL and MFM drive is the formatting, RLL tended to cram more sectors (26 in the 238's case) then MFM (17 was standard?). So my 33MB RLL drive shows up as a 20MB MFM drive. RLL drives tended to be the same as MFM but were certified to work ok with a 50% higher format?

My question is can I format the drive to 33MB using my current setup? If so how? If not do I need to modify my card to a ST-11R (RLL roms I asume the rest is the same?)?
 
Obviously, someone had previously formatted it as MFM with a compatible controller, otherwise you would have got "sector not found" errors if it could even boot at all.

I would imagine that, pretty much, any RLL would work when formatted as MFM, but, the reverse is an iffy thing.
 
Hi
You need to get a RLL controller. It isn't just different ROMs, the
chips are different as well. In fact, it is quite likely that the
ROMs are the same for both controllers. The low level software
wouldn't know the difference if the chip interface is the same.
There is little difference to make that part different.
Dwight
 
Just to be a smart*ass, technically, MFM is RLL--(1,3) RLL. Your drive is probably rated for (2,7) RLL There were a few controllers (such as PerStor) that used different encodings to get 2x the MFM capacity.

But Dwight and others are right. You need an RLL controller to realize the capacity increase.
 
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One related question which has been puzzling me for a long time:
Why was MFM so popular, and RLL so rare?
I can't believe there were noticeable differences in controller manufacturing cost.
I guess it had something to do with the drive mechanics cost, but what exactly had to be better to be RLL capable?
 
Marrr said:
I guess it had something to do with the drive mechanics cost, but what exactly had to be better to be RLL capable?
Click to expand...

In a nutshell, a drive used for 2,7 RLL had to be capable of handling bit cells that are only 2/3rds the size of those in MFM (the datarate in RLL is typically 50% greater (7.5MHz) that that in MFM). Not all of the early drives were up to this.

When IDE came in, almost all drives immediately went to RLL 2,7. I suspect some also went to RLL 3,9 (ARLL), though the electronics hid that pretty well.

But RLL and other forms of group coding weren't new then. 1/2" 9 track mag tape used it in the 6250 bpi mode; there were also mainframe hard drives using it.
 
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