Top View Of A 2305 Model 2 Disk Surface - IBM System/370 145 Manual

spare tracks in one module. One recording element is positioned over
each track. Each of the twelve surfaces contains 72 tracks, 64
recording and 8 spare. The spare tracks are interspersed among the 72
tracks so that every ninth track is a spare. Data is recorded serially
by bit on each track.
Four nonmovable access mechanisms are positioned around the rotating
disks as shown in Figure 20.15.1. Each access mechanism contains two
groups of 9 recording elements per surface (for a total of 16 recording
and 2 spare elements) and accesses one-quarter of the tracks on each
surface. A group of 8 recording elements accesses every other track.
The outermost element group of the access mechanism at the top of Figure
20.15.1 accesses data tracks 1, 3. 5, ••• , 15, while data tracks 2, 4,
6, ••• , 16 are accessed by the outermost element group of the access
mechanism at the bottom of the diagram.
There are 180 sectors per track on the 2305 Model 2. When RPS is
used, the search time, from sector found to beginning of desired record,
ranges from a minimum of 112 microseconds to a maximum of 167 microseconds.
Access
Access mechanism
per track
Access mechanism
64 recording tracks/surface x 12
=
768 recording tracks/module
8 spare tracks/surface x 12
=
96 spare tracks/module
8 recording elements
1 spare
8 recording elements
1 spare
216 recording elements (read/write heads)/access mechanism x 4 = 864 recording elements
Figure 20.15.1. Top view of a 2305 Model 2 disk surface
Access
mechanism
104 A Guide to the IBM System/370 Model 145