User's Manual

DAT 72/DDS-4 Product Manual

factor. These design features are also important contributors to the overall reliability,

durability and performance of the drive.

The mechanism is designed for minimum tape wear and prevention of damage to the

tape. The modes or operational states, such as stop, rewind and play, reduce

mechanism and tape wear. Fewer mechanical mode changes result in less wear on

key drive components. In some cases, the need for a mode change is circumvented

using the Pause mode, which stops the tape without activating the mechanism. All

mode selection is performed by the controller firmware. The host computer does not

directly control mode selection.

A custom timing tracking design, combined with the four-head cylinder design,

implements the specifics of the DAT 72 and DDS-4 recording format standards and

provides the precision required to perform seamless appends, or the ability to add

subsequent recorded data frames immediately adjacent to the last data frames

written on the tape.

A bank of jumpers is available at the rear of the drive. These jumpers allow you to

set the SCSI ID for the drive and to change configuration choices. Refer to Chapter 3

for information about setting these jumpers.

By using the jumpers, you can also enable terminator power if needed. (The default

for internal models is with terminator power disabled. For external drives, the default

is with terminator power enabled.)

Note:

The DAT 72 and DDS-4 drives come with a terminator power fuse to provide

protection from component damage in case the SCSI cable is connected

incorrectly.

Three rectangular front-panel LEDs indicate a drive busy status and tape cartridge in

place status. When blinking, these LEDs also function as fault indicators. (Refer to

Chapter 4 for a summary of the function of these LEDs.) The external subsystem

also provides a round, green LED on the front panel to indicate that the power is on.

Helical Scan Recording—Four-Head Design

In helical scan recording, the heads are positioned opposite one another on a

cylinder, which is tilted approximately 6 degrees from the vertical plane and rotates

counterclockwise at 10,000 revolutions per minute (rpm). At the same time, the tape

moves slowly (20.375 mm per second in DDS-4 mode) in a horizontal path around

part of the cylinder. This simultaneous motion of cylinder and tape results in the head

traveling across the width of the tape in a helix-shaped motion.

The cylinder is designed with four, long-life heads—two read and two write heads.

These heads are set opposite one another with a rotation sequence of: write A, read

B, write B, read A (or write A new, read B old, write B new, read A old). The

advantage of this design is that a RAW check is performed immediately after the

data is written.

As mentioned earlier, the cylinder rotates rapidly (10,000 rpm) in the same direction

that the tape moves. The wrap angle of the tape on the cylinder is approximately 102

degrees. The combined movement of the tape and cylinder results in a relative head-

tape speed of 20.4 inches per second (ips).