IBM System/370 Guide

the alternate track when the defective recording track is addressed.
This is called alternate track sparing. Switching to an assigned
alternate track must be a programmed action for currently announced
System/360 direct access devices.
The 2835 control unit contains new error correction facilities
similar to those of the 3830 control unit. Recorded data areas within
self-formatting records have correction code bytes (16 for the Model
1
and 10 for the Model 2) appended to them instead of a two-byte cyclic
check code. When certain types of data errors occur during the reading
of the data portion of a record, the control unit can determine the
bits in error and generate correction data. This recovery information
is presented to the error routine via the sense bytes and can be used
to correct the invalid record in processor storage (as described for
the 3330 facility).
A command retry feature is implemented in the 2835 that permits
certain types of failing commands to be reissued automatically by the
channel, when requested by the control unit, without an intervening
I/O interrupt. For example, when a count or key area is read
erroneously, the control unit retries the command once. If the error
is not corrected by the retry, the control unit corrects the data in
its own buffer, reexecutes the failing read, and presents the corrected
data from the buffer instead of reading it from the track.
Like the 3830 control unit, the 2835 contains a device that reads
magnetic disk cartridges containing the control unit microprogram and
diagnostic routines.
DATA RECORDING ON THE MODEL 2
Data tracks on the Model 2 are formatted in the same manner as on
System/360 direct access devices except for the absence of a home
address on each track. There are 768 recording tracks and 96 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 90 sectors per track on the 2305 Model 1 and 180 sectors
per track on the Mod~l 2. When RPS is used~ search time. from sector
found to beginning of desired record, ranges from a minimum of 114
microseconds to a maximum of 198 microseconds.
42

IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English