On Mon, 30 Mar 1998, John Wilson wrote:

> >From: Johnny Billquist <bqt@Update.UU.SE>
> 
> >But for RX02, the
> >drive switches to double density for the data part of the sector. So,
> >changing between RX01 and RX02 format on the disk really is just a
> >question of noting somewhere which it actually is, and this is done on
> >track 1, sector 0.
> 
> Hmm, I thought it was done on every sector.  The drive certainly seems to step
> through the whole disk during a "set density" command.

Sorry, you're most likely correct. I haven't done that much research on
the RX02... :-)

> Anyway naturally it's not even quite as simple as a DD data field with SD
> headers, apparently a certain bit string encoded as regular MFM looks like
> one of the SD header values, so they had to hack things so that particular
> bit string is encoded/decoded using a different series of MFM transitions.
> You'd think that would have been DEC's clue to stop futzing around with SD
> and use S/34 DD format like everyone else!

One could have wished. :-)

> >RM02/03/05, RP02/03/04/05/06 are all removable. RM80 and RP07 are fixed.
> >(I'm not sure if there ever was an RP01, but that was before my time in
> >that case...)
> >The difference between them? Hmmm. Good question. All RM-drives I've seen
> >has been physically smaller than RP-drives.
> 
> The main difference I notice is that all the RM drives use SMD internally
> (modified enough to be incompatible with anyone else's SMD, of course), with
> the "RM Adapter" converting between that and Massbus.  The Massbus-compatible
> RPs used other kinds of drive-specific DCL boxes.  Also the RMs have a slightly
> different register set than the RPs (except the RP07 which looks RMish to
> software).

The RP07 is undeniably "special" (although nice...). When I wrote me
answer, I had the passing notion that the MEMOREX drives (RP06) also were
SMD, but when I start thinking about it, I suddenly remember that the
cables from the MASSBUS box definitely don't look SMDish to me...

RP07 normally use the RM stuff in the operating systems. I believe the
RM80 also were pretty special, which kindof made the two "related" by
being odd.

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt@update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol