>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.

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!

>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).

John Wilson
D Bit