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SCSI Description

Quantum DLT 8000 Tape System 3-5

3.3 Signal States

The following paragraphs describe signal values and SCSI ID bits.

3.3.1 Signal Values

All signal values are actively driven true (low voltage). Because the signal drivers are

OR-tied, the bus terminator’s bias circuitry pulls false when it is released by the drivers

at every SCSI device. If any device asserts a signal, (e.g., OR-tied signals), the signal is

true. Table 3–2 shows the ANSI-specified and defined signal sources. Any device can

assert RST at any time.

Table 3–2 Signal Sources

Signals

Bus Phase BSY SEL

C/D I/O

MSG REQ ACK ATN

DB(7–0) DB(P) DB(15-8) DB(P1)

BUS FREE None None None None None None

ARBITRATION All Winner None None S ID S ID

SELECTION I&T Init None Init Init Init

RESELECTION I&T Targ Targ Init Targ Targ

COMMAND Targ None Targ Init Init None

DATA IN Targ None Targ Init Targ Targ

DATA OUT Targ None Targ Init Init Init

STATUS Targ None Targ Init Targ None

MESSAGE IN Targ None Targ Init Targ None

MESSAGE OUT Targ None Targ Init Init None

All: The signal is driven by all SCSI devices that are actively arbitrating.

SCSI ID: Each SCSI device that is actively arbitrating asserts its unique SCSI ID bit. The

other seven (or fifteen) data bits are released. The parity bit DB (P or P1) can be

released or driven true, but is never driven false during this phase.

I&T: The signal is driven by the initiator, drive, or both, as specified in the SELECTION

and RESELECTION phase

Init: If driven, this signal is driven only by the active initiator.

None: The signal is released; that is, not driven by any SCSI device. The bias circuitry of

the bus terminators pulls the signal to the false state.

Winner: The signal is driven by the winning SCSI device.

Targ: If the signal is driven, it is driven only by the active drive.

3.3.2 SCSI ID Bits

SCSI permits a maximum of eight SCSI devices on a SCSI bus (16 devices are permitted

when using wide SCSI). Each SCSI device has a unique SCSI ID assigned to it. This

SCSI ID provides an address for identifying the device on the bus. On the drive, the

SCSI ID is assigned by configuring jumpers or connecting remote switches to the option

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Quantum DLT 8000 Manual Página 107