Tape Motion; Forward Tape Motion; Reverse Tape Motion; Operations - IBM 5100 Maintenance Information Manual

TAPE MOTION
Forward Tape Motion
Tape moves in a forward direction (counterclockwise)
during mark, find, read, and write operations.
When forward tape motion stops, the momentum of the
tape unit carries the tape into the 251 bytes of hex 55s
in the format record. These 251 bytes provide a time
delay when the tape stops; they are not read as useful
information. Refer to Format Record.
The next three bytes (two bytes 00 and E7) provide
position information. The control unit reads these bytes
one bit at a time until the hex E7 byte is detected. After
hex E7 is detected, the controller is synchronized (it
reads 8 bits at a time rather than 1 bit at a time) at the
start of the format record.
The record type byte indicates whether the data track
associated with this format record has a header record
or a data record written on it.
Following the record type byte are the record sequence
bytes. These bytes contain the record number of the
next record on the data track. A header record always
has a record sequence number of hex 0000.
Another hex E7 byte is expected 33 bytes after the first
hex E7 byte is read. This hex E7 byte indicates that the
tape is now positioned correctly for reading or writing
the data track.
When the tape is moving forward (counterclockwise), it
passes the erase coils before the read/write coils. If the
data track is to be written, its erase coil is activated
after the first hex E7 is read from the format track and
the write coil is activated after the second hex E7 is
read from the format track.
The 29 hex 00 bytes (read data) in the format track
provide a time delay between erasing the data track and
writing on the data track. This time delay causes the
data track to be erased well in advance of any data and
ensures that all previously written information is erased.
The last 4 bytes in the format track establish
synchronization and determine the record type when
reading in the reverse direction.
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Reverse Tape Motion
Tape moves in a reverse direction (clockwise) during the
search, rewind, and backspace operations.
When searching the format track in a reverse direction,
the tape unit reads two bytes of 00, the E7 (sync) byte,
then the record type byte.
If the record type byte contains a hex 18 (data record),
the tape unit continues searching in the reverse direction
until it findS a header record to determine if this is the
file the microprogram is looking for'.
If the record type byte contains a hex 81 (header
record), the tape unit stops searching the format record
and reads the header record in the forward direction. If
the header record is the one the microprogram is·
looking for, the entire file is read in the forward
direction. (The microprogram compares the file number
from the tape to the file number the microprogram is
searching for.)
If the header record does not contain the correct file
number, the tape unit resumes moving tape backward
until it finds the correct file number.
OPERATIONS
The tape unit can be used for APL, BASIC, and
communications because it is language independent.
Error detection and correction, and several operations
such as formatting new files and finding old ones is
performed by the controller.
Operations provided by the controller and tape unit are
mark, find, read, write, rewind and backspace.
Mark
The mark operation formats new files on a tape, or
reformats existing ones. In a mark operation, the file
header record is written and checked for CRC errors,
and the format track information is written for the
requested number of data records. During this
operation, the format track is never checked after
writing and the data track is always checked after
writing.
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