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4.1 Introduction
CHAPTER 4
DRIVE OPERATION
DRIVE OPERA TlON
This chapter describes important operational procedures for the Python DDS-DC drives. It covers the
following topics:
o
Data compression operation
o
Operation of the front panel LEDs
o
Loading and unloading a cassette
o
Using a blank cassette
o
Using a cassette with data on it
o
Using a prerecorded audio cassette
o
Loading revised SCSI firmware (updating flash EEPROM)
NOTE:
Required drive maintenance including using a cleaning cassette is explained in
Chapter 9.
4.2 Data Compression Operation
Default operation for the Python DDS-DC drives is with data compression enabled---the drive automatically
compresses all data written to tape and decompresses all compressed data read from tape, in DDS-DC
format. Data with high degrees of redundancy, such as structured database fIles, can be compressed most
efficiently, often at a ratio of 4:1 or more. Data with little redundancy, such as executable programs, can be
compressed least.
A unique feature of the Python DDS-DC drives is expansion minimization. To provide this feature, the drive
continuously monitors the compression ratio. If the "compressed" version becomes larger than the original
version, data compression would be automatically turned OFF. Although this situation is very rare, it could
occur, for example, if the drive is trying to compress data that has already been compressed.
To switch between compressed mode (writing compressed data, DDS-DC format) and noncompressed mode
(writing uncompressed data, DDS format), the host computer executes a SCSI MODE SELECT command.
On internal Python DDS-DC drives, a hardware DIP switch (S6) can also be used to enable or disable data
compression.
NOTE:
This switch is designated ''DDS pass-through mode" and leaving this switch OFF
enables DDS-DC compression. External Python DDS-DC drives are factory set with
DDS-DC enabled. See Chapter 3 for more information about this switch.
4-1
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