Table of Contents
Advertisement
i---IRECORDING
AREA---~
0.03 IN 0.07 CM
HUB END
0.00
0.00
~
t
'EOTI
3.05M
10FT
25 +5 FT 7.62+1.52 M _
·0
-0
1.264 CM ± 0.005
0.498 IN ± 0.002
PHOTO REFLECTIVE MARKERS--.. _ _ _ _ --..
~
ONE INCH
=
2.54 CENTI METRES
ONE FOOT = 0.3048 METRES
0.0048 CM ±.0007
T
0.0019 IN ± .0003
Figure 2·1. Magnetic Tape Layout
MAGNETIC TAPE
The physical layout of half-inch magnetic tape is
illustrated in Figure 2-1. A full reel of tape has a
nominal recording length of 2400 feet (732 m). The
entire length of the tape is oxide·coated. Beginning-
and end-of-tapesensing is controlled by reflective
markers affixed to the Mylar-base side of the tape.
The beginning-of-tape (BOT) spot is attached
approximately 16 feet (4.88 m) from the physical
beginning of the tape, and the end-of-tape (EOT)
spot is attached approximately 25 feet (7.62 m) from
the physical end of the tape.
BEGINNING AND END
OF
TAPE
In order to ensure maximum reliability in the
storage of data, an erased area must be recorded in
the vicinity of the beginning-of-tape (BOT) marker
that is affixed near the reference edge at the start of
every tape, and an unrecorded area must be left in
the vicinity of the end-of-tape (EOT) marker affIXed
on the opposite edge of tape at the trailing end of a
tape reel.
BEGINNING·OF-TAPE GAP
An erased section of tape is required surrounding
the BOT marker. This serves as a defined area
within which reading can start. This section begins
a minimum of 1.3 in. (3.3 cm) before the hub end of
the BOT marker and extends a minimum of 3.0 in.
(7.6 cm) past the hub end of the BOT marker. This
erased section totals about 4.3 in. (10.9 cm).
DATA BLOCKS
The data is formatted and recorded on the tape in
blocks. The exact configuration of a block depends
on whether the tape is in 7 -track or 9-track format.
On 9-track tape, each block consists of the data, a
Cyclic Redundancy Check (CRC) character, and a
Longitudinal Redundancy Check (LRC) character.
The CRC character is positioned four character
times after the final data character, and the LRC
character must occur four character times after the
PROGRAMMING
2-2
CRC. A nominal spacing of 0.6 in. (1.5 cm) is
required between blocks.
On 7-track tape, each block consists of data,
followed by an LRC character four character times
after the final data character. Nominal spacing
between blocks is 0.75 in. (1.9 cm).
Standard block formats used for NRZI recording
are shown in Figures 2-2 and 2-3 (data block and
tape mark block formats respectively).
SKIP3
CHARACTERS
::::L:
I
CRC
INTERBLOCK
GAP 06 IN
I.
NOM.'
.1
)
t
rTTlTl!l-TRACK
4llJ
TAPE
-om
ECTION
OF TAPE
MOTION
LAST C
FIR ST CHARACTER
HAR. LRC
' H AR.
~::AOK
U l l U T A P E
(
CHARACTER
IN BLOCK
)
SKIP3
~I'
·1
CHARACTERS--l
0.75 IN. NOM.
INTERBLOCK
GAP
Figure 2·2. Data Block Formats
INTERBLOCK GAPS
Interblock gaps are areas without data (i.e., all
tracks restored to the dc-erased polarity) placed
between data blocks or records. The length of the
gap is 0.75 in. (1.9 cm) nominal (0.6 in.l1.5 cm
minimum) for 7 -track subsystems and 0.60 in.
(1.5 cm) nominal (0.5 in.l1.3 cm minimum) for 9-
track subsystems. The maximum length should not
exceed 25 feet (7.6 m).
TAPE MARKS
Figure 2-3 shows the format of the Tape Mark for
7-track and 9-track tapes. The distinguishing
feature of the block is that it is a single, specific
character block with a check character.
CFIO
Advertisement
Table of Contents
