20 Jul
2010
20 Jul
'10
2:53 p.m.
That's one issue I recall with the transition from
RK05 to RL01 as
"inexpensive" cartridge media in the DEC world. The RK05 has a
manually-aligned external positioner, and the RL01 (and RL02) uses
embedded servo data. One advantage to the RK05 technique is that you
can format packs that have been bulk-erased (since the platters only
contain data, not positioning info) One advantage to the RL01 is that
you don't have to be quite as precise with head alignment (you have to
be close enough for the embedded servo data to be legible, then, IIRC,
the positioner circuit can lock on the rest of the way).
Actually, it will try to lock no matter how far out the heads are (think
of what hapapnes when you load a pack, it moves the heads in until it
detects the guard band and then servo data on the currently selected head
(nromally head 0). But if the heads are misaligned between the 2 sides,
it may lock to the wrong cylinder when you switch heads.
But the RL's sure are easy to align, and you don't need an alignment pack
which a plus...
There was another method that was acceptable for multi-surface packs -
a servo surface. It's not worth dedicating an entire surface to
positioner data if you only have two surfaces - you get a poor
But that's what the CDC Phoenix drive (9448 IIRC) did. It ws an 80 meg
fixed/16 meg removable drive. 3 fixed platters (5 data surfaces of 16M
each and a servo surface) and a front-loading pack with one platter (one
data surface, one servo surface). Apprently they didn't feel the radial
alignment between the removeable pack and the spindle/fixed pack to rely
on the fixed servo surface when accessing the removeable platter.
space-to-cost ratio. If you have 4-8 surfaces,
it's more practical to
give up a surface (vs a loss of some space for embedded servo info).
E.g the RK06/RK07 (3 data surfaces, one servo surface).
An advantage of a servo-surface pack is that you can
easily format the
data surfaces as long as you leave the servo surface untouched.
I've heard of a field service device for writing servo data on a wiped
RL pack, but I've never seen one in person.
I am sceptical. You would need to have a special positioner with
mechancial feedback (so as to be able to position the heads without using
the servo inforamtion), and I don't believe it could be made rugged
enough to be able to be used in the field.
-tony