IBM 3590 - Manual (Page 83)

Random and Random 2-LUN Modes
Random mode is designed to act as a small, self-contained library of up to 10
cartridges, controlled by the attaching host. The attaching host uses the SCSI
Medium Mover commands to select a cartridge from a given cell and move it
to its destination cell. Control of the source and destination cells are left
entirely to the attaching host.
Random 2-LUN mode behaves the same as Random mode. The 2-LUN allows
software to control the ACF in a different manner. To determine if 2-LUN
mode is required, refer to your software documentation.
Random mode is supported only on Model B1A. Random 2-LUN mode is
supported by Models E1A and B1A.
Being in Random mode does not preclude attaching host requests for specific
mounts. Requests for specific cartridges to be mounted can be satisfied by
inserting the specific cartridge in the priority cell. In Random mode, the
priority cell is simply another addressable cell. The ACF notifies the attaching
host of the presence of a cartridge in the priority cell. However, the attaching
host must specifically address that cell to access the cartridge.
To ensure the integrity of the library, the magazine has a lock on the
magazine. The magazine lock prevents the addition or removal of cartridges
from the magazine after the magazine is installed in the ACF and locked. The
ACF senses the state of the magazine lock. It does not permit Random mode
to be enabled unless the magazine lock is set to the locked position. A sensor
on the ACF allows the unit to determine that the magazine is not installed or
that it was removed. This disables Random mode.
Because the magazine must be locked in Random mode, the priority cell is
used as an I/O port to the library. In this mode, it will add or remove
cartridges from the magazine without having to violate the integrity of the
locked magazine. The attaching host can move cartridges from the priority
cell to any empty magazine cell (or the device). It can also move cartridges
from a magazine cell (or the device) to the priority cell. Library inventory and
the control of the inventory are entirely the responsibility of the attaching
host.
If a Move Medium or Read Element Status command is issued, with no
magazine present, or with the magazine unlocked, the command fails with a
permanent error.
When the ACF is in Random mode, the following applies (see also “ACF
Notes” on page 56):
v Initial conditions
Chapter 4. Automatic Cartridge Facility (ACF) and Magazine 63