Sensors; Read-After-Write - Seagate STD124000N Product Manual

Chapter 8

Sensors

Read-After-Write

Page 78
A number of mechanical and optical sensors are integrated in the drive design. The
cartridge in and cartridge loading sensors are mechanical sensors that determine
the position of the loading mechanism. The other mechanical sensors report specific
information based on detecting the open or closed state of four recognition holes in
the DAT cartridge. The open or closed state of these holes designate tape type,
whether the tape is a cleaning cartridge, whether the tape is prerecorded and
whether the tape cartridge is write-protected. These mechanical sensors and the
sensor for the cartridge in status comply with the DDS-3 standard requirements for
the cartridge.
The beginning-of-tape (BOT) sensor is an optical sensor that uses the light path
transmissivity of leader tape, as specified in the DDS cartridge standard. The sensor
is also designed to recognize media recognition system (MRS) cartridges, which
have a series of alternate opaque and clear stripes at the beginning of the tape.
The reel sensors for the two reels are optical. Also, three optical sensors detect
mechanism position during mode changes.
The capstan sensor is a magnetoresistive Hall sensor that detects a magnetic field.
The cylinder sensors are coil and magnet sensors. Each reel motor contains a high-
resolution, optical speed encoder.
The read-after-write (RAW) technique provides a means of verifying that host data
was written on the tape correctly by applying a read check immediately after writing
the data to tape. The read check is a comparison of the actual signal quality versus
a predetermined acceptable threshold level.
If a frame is identified as bad, it is rewritten later down the tape. The bad frame is
not necessarily rewritten immediately. It can be rewritten after three, four or five
other frames have been written. Any frame can be rewritten multiple times to
provide for skipping over bad areas on the tape.
Excessive consecutive rewrites typically signal a degraded media condition; in these
cases it is best to discontinue use of the tape in question and continue with a piece
of good media.
During a read or restore operation, the threshold level is reduced to maximize the
likelihood that data can be successfully retrieved from tape. The combination of the
elevated read threshold during write and reduced threshold during read ensures that
data is written with the highest possible margin and that recorded data can be read
or retrieved with the highest possible confidence.
Theory of Operations
DAT Drives