Diablo Series 30 removable disk drives & sector sensor

pbirkel at gmail.com pbirkel at gmail.com
Sun May 22 01:26:06 CDT 2022

I've been researching uses of the IBM 2315 disk pack design in various


There appear to be two styles of sensors used for sector identification in
IBM 2315 disk pack designs.  The IBM 2310 disk drive used an optical sensor
and notched-ring integrated into the spindle so that a U-shaped assembly of
light+sensor is appropriately positioned when the pack is mounted (see:
https://www.manualslib.com/products/Ibm-2310-10271354.html ).  DEC also used
this style in their RK05 drives (see:
Figure 1-3 and Section 4.2.6 on page 4-16 (56-of-135) "an optical device
that contains a light emitting diode and a photosensor".


The other style utilizes a magnetic sensor and the hub (perimeter of the
flat surface to which the pack is affixed when mounted) itself is notched.
A flat sensor including a magnet is suitably positioned with respect to that
face when the pack is mounted.  HP used this style in their HP7906 drive,
which combined a removable disk with a fixed disk (the fixed disk actually
provided the sector-timing information for the removeable disk pack; the
removeable disk itself only had a single index notch on the hub).


I've become confused about the one used with Diablo Series 30 drives,
typically employed by OEMs (e.g., Data General).  I assume that it was
one-or-the-other, but perhaps either might be used based on the requirement
of a particular OEM?


In D3140-171_Maintanance_Manual_For_Series-30_Disk_Drive_Jan72.pdf, on page
7-17 (92-of-103) Figure 7-9 SECTOR TRANSDUCER ASSEMBLY clearly illustrates
the mechanism of a magnetic transducer, although nowhere can I find Series
30 technical documentation that specifically discusses the nature of its
sector transducer.  All schematics that I have checked do not seem to
include any provisions for powering a light source, which is consistent with
the use of a magnetic transducer.


from Ken Shirriff shows a Diablo drive where Ken clearly
75xg0Mcm6eeytILHMyUGZ1TFAUzkwCHMYBhgL/w9999/disk-opened.jpg) illustrates a
disk pack that is hard-sectored based on slots-in-ring which AFAIK implies
an optical sensor.  He references
81503-02_Series_30_Disk_Drive_Maintenance_Nov75.pdf which is non-specific
regarding the sector transducer, but on page 5-6 (41-of-63) it refers to
there being "two interconnecting wires at the transducer" and Figure 5-7
SECTOR TRANSDUCER ADJUSTMENT appears to me to be consistent with the earlier
maintenance manual Figure 7-9 SECTOR TRANSDUCER ASSEMBLY - a magnetic


So is the opened disk pack not actually one for a Diablo Series 30 drive?
Or did the Diablo Series 30 include an OEM-specific (?) variant that used an
optical sensor?  Or perhaps there's something else going on here with the
various IBM 2315 disk pack designs regarding sector sensing that I fail to












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