User's Guide

SCSI Interface Description Page 27
3. When the DAT drive has successfully reselected the Initiator, it sends an
Identify Message to reestablish the path between the drive and the Initiator.
This message is always 80h because the DAT drive is initiating the reselection
and is always LUN 0. A Restore Pointers Message is implied when the DAT
drive sends an Identify Message to the Initiator. The Initiator responds
accordingly.
SCSI Memory Address Pointers
SCSI provides for two sets of three pointers within each Initiator. When a physical
path is established with a host, and this path can accommodate disconnection and
reconnection, the host must ensure that its Current Pointers for the path are equal to
the Saved Pointers in the DAT drive. An implied Restore Pointers Operation occurs
in the host as a result of a connect or reconnect.
Current Data Pointers
Current data pointers, also known as Active Pointers, are used to represent the
state of the interface and point to the next Command, Status or Data byte to be
transferred between the memory of the Initiator and the Target. Each Initiator may
have only one set of Current Pointers. The Current Pointers are used by the Target
currently connected to the Initiator.
Saved Data Pointers
Whether or not a currently active device is currently connected, it has one set of
Saved Data Pointers. This set includes Command, Status and Data Pointers that
point to the Command Descriptor Block, Status Area and Data Area, respectively,
for that device.
The Saved Pointer continues to point to the start of the Data Area until the Target
reconnects to the Initiator. In response to the implied Save Data Pointer message,
the Initiator replaces the Current (active) Data Pointer with the value of the Saved
Data Pointer.
Early Warning Function
Early Warning on the DAT drive is a logical warning given when ten megabytes of
storage space remain on the tape. The position is calculated by the drive. When this
physical position is reached on a tape, the following occurs.
1. Data transfers from the host are terminated at the next block boundary.
2. All data remaining in the drive buffer is written to the tape if the Synchronize at
EW bit is set.
3. The command completes with a Check Condition and a 40h Sense data
meaning EOM and no Sense Key.
4. Subsequent WRITE commands write data and complete with check condition
with EOM Status and No Sense Key until the physical tape end is encountered.