IBM System/370 145 Manual page 121

• A tape transport design that minimizes tape wear and increases
reliability, a single-capstan drive to control tape movement that
provides faster data access times and rewinds, and more precise
control of motor speed to help minimize damage to tape media
• Cartridge loading of tape, automatic tape threading, and a new
automatic tape reel hub latch, all to reduce tape setup time
• Dual Density and seven Track (mutually exclusive) features to enable
a 3420 tape unit to handle either nine-track 800-BPI NRZI and 1600-
BPI PE tape or seven-track 556/800-BPI NRZI (BCD or binary) tape
• Flexible, lower cost tape switching implemented in a new compact
physical design. A two-channel switch is available also.
• Features such as new technology to improve subsystem reliability and
new diagnostic facilities to aid serviceability and thereby increase
subsystem availability
Phase-encoded recording. The phase-encoded (PE) recording technique
offers superior error detection and reliability as compared with the
non-return-to-zero (NRZI) technique. In both cases, magnetic recording
of one and zero bi t s i s accomplil:;hed by means of flux reversals or
changes in polarity. In NRZI .rec~ording, only one bits are recorded as
magnetized spots, and a flux rev.~rsal occurs only for one bits,. In PE
recording both zero and one bits are recorded (the zero bit and one bit
being opposite in polarity), and a flux reversal is required in every
bit positionu Thus, the PE dual flux recording technique differentiates
between no recording and the preBence of a zero bit, and the absence of
any signal is detected as an error.
The positive recording of all zero and one bits .in PE eliminates the
need for horizontal parity bits I[longitudinal redundancy check used in
NRZI recording), and vertical pal::-ity bits are used to correct single-bit
read errors in flight. During r4::!ading, if a single track fails to
respond with a suitable pulse in any bit position, reading of the rest
of that track is immediately disabled for the remainder of the data
block, and the remaining bits fo]~ that track are automatically generated
by use of the vertical pari,ty bit:s. In-flight singl~-track error
correction eliminates the time normally lost in backspacing and
rereading NRZI tape for correction of single-track dropouts or defects.
Phase encoding offers other advantages. If a string of zeros is
recorded on tape, successful reading in NRZI requires close
synchronization to "count" the ,correct number of zeros. With PE, this
synchronization is' provided by tt~ flux reversal in every bit position;
hence, PE recording (and reading) is self-clocking. In addition, each
block written on a PE tape is prE~eded and followed by a coded burst of
bits in all tracks to set up the individual track-clocking rates. The
read circuitry is designed to recognize these bursts and thereby
minimize the effect of noise in
t~he
gap.
Tihe critical nature of verticClll skew (alignment of bits within a
byte) that is imposed by NRZI recording is minimized by this individual
track-clocking scheme (one clock per track versus one clock for the
entire tape subsystem), and by the use of one-byte (nine-bit) capacity
skew buffers that can be in the process of collecting up to four data
bytes at the same time, as the tape passes the read head. Because of
the posi ti ve recording of all bit.s, once a skew buffer contains nine
bits, one from each horizontal da.ta track, it is an indication that a
byte has been read. Thus, the 3L1·20 can handle the situation in which
the tape is not exactly aligned, and bits from up to four adjacent bytes
can be read concurrently.
A Guide to the IBM System/310 Model 145 111