IBM System/370 Guide

Page of GC20-1730-0
Added
11/20/70
By TNL GN20-2277
On a 3420 tape unit, the tape reel is mounted on the right side
of the tape transport as on a 2420, 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-drive capstan transport for tape movement and optical
tachometers for control of motor speed result in several advantages.
First, faster access times than those of 2400-series 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 ms, and 2.0
ms, respectively.
Second, the single-drive capstan 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. (The 2420 rewinds at about 500
inches per second except for the last 100 feet, which are rewound at
the model's rated read speed.) In addition, less time is required
to rewind less than a full reel on 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 motors. 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 would be used in
calculations for command chained tape operations (reading or wri·ting
more than one tape block with a single START I/O instruction). More
precise capstan motor speed also results 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. T'"~ese 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 cartridqe once the reel (lO.s-inch. 8.s-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 is loaded in the columns and positioned at load point within ten
seconds after the button is depressed for Models 3 and 5. On the
Model 7, only seven seconds are required. In addition, unload and
rewind/unload operations cause the tape to be completely rewound on
the tape reel and the power window to be lowered so that the reel is
ready for immediate demounting.
51.3
I

IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English