Technical data

2.2 Density Selection
The density at which the drive is reading or writing, is termed the operating density. When
the drive is active, it communicates the operating density to the alphanumeric display. The
operating density is set in one of four ways:
• At power-up by the drive
• Commanded by the system through the interface
• Found by the drive automatically when it reads the identification burst from a recorded
tape
• Commanded by an operator at the front panel
2.2.1 Power-Up Density Selection
At power-up the drive reads its non-volatile memory to determine the default density and
sets this as the operating density. The default density is a configuration option that can be
changed with Service Aid 142 as described in Chapter 4.
2.2.2 Host Density Selection
Selecting the operating density through the interface is a function of the host system software
and the interface itself.
2.2.3 Automatic Density Selection
The drive automatically selects the operating density when it reads an identification burst
from a tape. When a load is completed with the drive off line, or after a rewind with the
drive off line, the density detected (found density) by the drive appears in the alphanumeric
display for approximately five seconds. The message displayed is either, ID 6250 or ID 1600.
If the drive does not detect an allowable density, it displays either the message BLANK or
the message ID UNKWN. The BLANK message indicates that the tape is blank. The ID
UNKWN message indicates that the tape was written in a format that the drive cannot read.
The drive can prevent writing in a density commanded by an operator or the host under one
condition. If a read from BOT is followed immediately by a write command in a different
density, the drive will only write in the density determined by the identification burst found
on the tape (found density); in other words, it ignores the command to append data in a
different density. However, if a rewind precedes the write command, or if a write is the first
tape motion command after a tape is loaded, the commanded density becomes the operating
density and the drive ignores the found density. This feature ensures that an operator or the
system cannot write a mixed density tape.
2.2.4 Operator Density Selection
The operating density may be selected by an operator at the front panel as previously
described in Table 2–1.
2–6 Operation