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CHARACTERS---r--l
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DiRECTION
CRC
LRC
CHAR. CHAR.
LRC
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LAST
CHARACTER
MOTION
CHARACTER
IN BLOCK
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CHARACTERS
Figure 2-3. Tape Mark Block Formats
NRZI DATA RECORDING FORMAT
In NRZI coding, a logic One bit appears on the
interface lines as a low voltage level and a logic Zero
as a high voltage level. However, on the tape, a logic
One bit is recorded as a flux change and a logic Zero
bit as no change. The direction of the change is
immaterial; however, the initial flux change direc-
tion after BOT conforms to ANSI standards.
CHECK CHARACTERS
The NRZI format provides for both vertical and
longitudinal parity checks. In the 9-track system an
additional check called the cyclic redundancy check
character is used. Refer to Figures 2-2 and 2-3 for
the location of the check characters.
• Vertical Parity -
7-track and 9-track tape
subsystems use 6 and 8 tracks, respectively,
for recording data. The remaining track car-
ries parity information. When performing a
Write operation, one parity bit is generated,
either odd parity or even parity based on a
previously stored Configuration Word, to
accompany each character written on tape.
The 9-track subsystems read and write odd
parity only while 7-track subsystems allow
the selection of odd or even parity to be
recorded or read. .
When performing a Read (including Read-
After-Write) operation, the parity read is
checked against the parity created from the
data portion of the character read. A mis-
match causes a Vertical Redundancy Check
(VRC) error condition to be set in Status.
Note, however, if an even number of bits in the
data character are "dropped" or "picked," a
VRC error will not be detected; thus addi-
tional checking facilities are necessary.
• Longitudinal Parity -
A Longitudinal
Redundancy Check (LRC) character is writ-
ten following the data portion of each block. It
is separated from the end of the data or CRC in
each block. This character is made up on a
PROGRAMMING
2-3
per-track basis. The LRC character written is
one calculated so that an even number of
"one" bits, including those of the data and
LRC character, is recorded in each track of the
block. On reading, this is checked and an error
is detected if the count is odd in any track.
This possibility of detecting an erroneous
block still exists if an even number of bits in a
given track of a block is dropped or picked.
However, when this test is combined with the
vertical parity test, the probability of not
detecting an error is reduced.
• Cyclic Redundancy Character Check (CRC) -
In the 9-track system another check character
is written. This character is derived with
relatively complex logic and, along with the
LRC character and vertical parity, minimizes
the possibility of undetected errors. The CRC
character bits of a Tape Mark Block are all
zeros. This check character follows the last
data byte of the block by four cell positions.
DATA FORMATS
7-TRACK
Information can be written or read in two modes:
• Byte Mode - Transfers 12 bits
(0£16)
of a data
word
to
or from the tape as shown in Figure 2-4
(the four nonsignificant bits - bits 0, 1,8, and
9 of the memory word - are ignored on writes
and zero-filled on reads from tape).
• Pack Mode -
Transfers three characters
between memory and the tape. On writing
tape, the controller generates two bits (zeros)
to complete the six bits of the third character.
On reading tape, the controller strips off these
two bits (Figure 2-4).
Even parity is used in the BCD (byte) mode and
odd parity is used in the binary modes. In either
mode, longitudinal even parity is indicated for each
track at the end of a record. Since the file mark (17
8 )
is a single-frame record, the file mark longitudinal
parity frame is identical to the file mark itself.
To accomplish IBM tape code compatibility in the
BCD mode, the octal value 00 is converted to the
octal value 12 when writing. Conversely, when
reading, the octal value 12 is converted to the octal
value 00.
9-TRACK
Data being written on or read from tape is
handled on a byte basis. All 16 bits of a data word
are transferred to or from the tape as shown in
Figure 2-4. Odd parity (vertical redundancy check)
is written on tape and is checked when read.
INSTRUCTIONS
Table 2-1 lists the I/O commands to which the
MTClMagnetic Tape Device-Pac/tape units respond.
A detailed description of each command follows this
table.
CFIO
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