User's Guide

Commands: Log Sense (4Dh) Page 59
Using Page Control Bits
The Page Control (PC) bits specify the type of counts that the initiator is requesting.
For page 0, the PC bits are ignored. For other pages, the PC bits specify the type of
parameter values. The following table lists the Page Control bit values.
Bits
7 6 Type Requested Counts
0 0 0 Threshold values
0 1 1 Accumulated values
1 0 2 Default threshold values
1 1 3 Default accumulated values
Threshold Values
The type 0 indicates the current threshold values. If threshold values are requested,
the drive returns the maximum count that each parameter code can attain (the
default) or the last threshold values set by the last LOG SELECT command.
Threshold values for pages 2 and 3 can by changed by the initiator through the LOG
SELECT command. The drive does not change these values during normal
operation.
Accumulated Values
Type 1 (accumulated vales) is the most common page control bit setting. This type
requests the current counts for the page. These counts reflect the current count of
events since the last power-on cycle, SCSI bus Reset, Bus Device Reset, or the last
LOG SELECT command that cleared or set parameter values.
Note: For pages 30 and 31, the parameter values cannot be changed and
are never reset. These values represent historical data regarding tape
usage and cannot be changed by the LOG SELECT command. Page 30
data is written to the tape log when the tape is ejected either manually or
through the SCSI UNLOAD command.
Accumulated values are incremented by the drive as an event occurs. These page 2
and 3 values can be updated or initialized by the initiator with the LOG SELECT
command. Also, the default accumulated values (all zeros) can be loaded with the
accumulated values through the LOG SELECT command.
Because the drive maintains the accumulated values in volatile memory, the values
may be lost if a power cycle occurs. The values for page 30 are saved on the
cassette itself and are preserved. However, if a power cycle occurs before the
cassette is ejected, the current page 30 data is lost.
The tape log values cannot be updated if the cassette is write protected.
Default Threshold Values
The type 2 page control bit setting requests default threshold values. This request
generally returns the maximum count that each parameter code can attain. For
example, a two-byte field returns two bytes of all 1s; a three-byte field returns three
bytes of all 1s. These default values cannot be changed by the LOG SELECT