IBM System/370 145 Manual page 113

SUMMARY
The 3330-series offers more than large capacity, faster access, and
attractive price performance. A 3330-series string and its control unit
are actually a subsystem. A 3830 control unit or integrated storage
control can control the concurrent execution of one RPS channel program
on each of its drives and can handle certain er~or correction and
logging functions, which normally must be programmed, thereby relieving
the control program of these activities.- In addition, the availability
and serviceability of the 3330-series are improved by the implementation
of error correction features:inhardware, by use of inline diagnostics,
and
by
the speed and ease of engineering change installation,. These
factors add to the improvement of total system availability.
20: 15 THE 2305 FIXED HEAD STORAG:E MODULE AND 2835 STORAGE CONTROL MODEL
1
One or two 2305 Fixed Head Sto:['age Modules can
be
attached to 2835
Storage Control. Each module con-tains six nonremovable rotating disks
on which data is recorded. Read/li1irite heads, called recording elements,
are fixed in position to access eiach track on the twelve recording
surfaces so that no arm motion is required. (See Tables 20.15.1 through
20.15.3 at the end of this subsec1tion for a comparison of 2305 Model 2
and 2303 Drum Storage characteris·tics and capacities.)
spare, or alternate, tracksar4e provided in 2305 modules and must be
wired in by a customer engineer to replace defective recording tracks.
However, one spare track is availa.ble for assignment by the alternate
track assignment utility program .~hen a permanent track error occurs on
a recording track during processing. Once a spare has been assigned as
an alternate track, the hardware automatically accesses the alternate
track when the defective recordin9 track is addressed. This is called
alternate track sparing. switch~lg to an assigned alternate track
during processing is a programmed action for currently announced direct
access devices.
The 2835 control unit provides new error correction facilities
similar to those of the 3830 contlrol unit. Recorded data areas within
self-formatting records have ten correction code bytes appended to them
instead of a two-byte cyclic check code. When certain types of data
errors occur during the reading oj: the data portion of a record, the
control unit can determine the bi1:s 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 3330-series disk storag~).
A command retry feat~e if.» implemented in the 2835. This feature
permits certain types of f·ailing (:ommands to be reissued automatically
by the channel, when requeste:d by the control unit, without an
intervening I/O -interruption. FOJ~ 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 reb~y, 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 i t from the track.
Like the 3830 -control unit, thE~ 2835 contains a device that reads
disk cartridges containing the
cO~ltrol
unit microprogram and diagnostic
routines.
DATA RECORDING
Data tracks on the 2305 Model 2 facility are formatted in the same
manner as on System/360 direct access devices, except for the absence of
a home address on each track. ThE~re are 768 recording tracks and 96
A Guide to the IBM System/370
ModE~1
145
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