IBM System/370 145 Manual page 122

Like 2400-series tape units, the 3420 utilizes a two-gap read/write
head that performs readback checking during write operations. The 3420
also has a separate erase head that erases the entire width of the tape
during any write operation before writing OC~lrs. Full-width erasure
reduces the likelihood of leaving extraneous bits in interblock gaps or
skip areas and minimizes the interchangeability problems that can occur
when tape is written on one tape unit and read on another.
Advanced engineering design. The tape path in the 3420 tape unit is
designed for "soft handling- of tape volumes to minimize tape wear and
thus improve tape reliability. Other features, such as the single-
capstan drive and optical tachometers, result in faster data access and
rewind times than those of the 2401.
On a 3420 tape unit, the tape reel is mounted on the right side of
the tape transport, instead of on the left as on a 2401 tape unit, so
that an inverted tape path exists.
As a result, when the tape is loaded
in the columns, the recording side touches only the tape cleaner and
read/write head. Friction and tape "wear are also reduced by the
presence of air bearings in the tape transport that provide a thin film
of air between the nonrecording surface and each metal bearing.
Use of a single-capstan drive transport for tape movement and optical
tachometers for control of motor speed result in several advantages.
First, faster access times than those of 2401 tape unit models are
achieved. Access time is defined as the time interval from initiation
of a write or forward read command (given when the tape is not at load
point) until the first data byte is read or written, assuming the tape
is brought up to speed from stopped status. Nominal access times for
3420 Models 3, 5, and 7 are 4.0 ms, 2.9
IDS,
and 2.0 ms~ respectively.
second, the single-capstan drive can
be
made to operate faster than
normal read/write speed, and in-column rewind is thus implemented. Full
reel rewind speeds average 410, 480, and 640 inches per second for
Models 3, 5, and 7, respectively. In addition, less time is required to
rewind less than a full reel
OD
a 3420 as compared to a 2401 because of
faster rewind times achieved by in-column rewinding.
Last, three optical· tachometers that monitor motor speed are used to
achieve precise control of the speed of both the capstan motor and the
tape reel motorsa The capstan tachometer measures the size of the
interblock gaps (IBG's) created during tape writing~ The result is a
more consistent IBG size (.6 inches) than is created by 2400-series tape
units, which enables more accurate calculation of tape passing time.
IBG passing times are 8.0 ms,
.4.8
ms, and 3.0 ms for 3420 Models 3, 5,
and 7, respectively. These times wou1d be used in calculations for
command-chained tape operations (reading or writing more than one tape
block with a single START I/O instruction). More precise capstan motor
speed also resu1ts in smoother starts and stops, thereby minimizing tape
stretching and breaking.
The two tape reel tachometers measure tape speed as the tape enters
and leaves the vacuum columns, and tape speed is adjusted when
necessary. The 3420 tape unit is, therefore, less sensitive to voltage
changes. More precise control of tape reel motor speed improves rewind
speed and minimizes erratic tape stacking during rewinds so that there
is less chance of damaging tape edges.
Automatic threading and cartridge loading. These advanced features
are standard on all 3420 models and significantly reduce tape mounting
and demounting time. Tape "threading is automatic for tape reels not
enclosed in a wraparound cartridge once the reel (10.5-inch, 8.5-inch,
or minireel) is mounted on "the tape unit with the tape end placed in the
threading chute and the load-rewind button is depressed. The power
window is closed, the tape is threaded on the takeup reel, and the tape
112 A Guide to the IBM System/370 Model 145