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SCSI Description

Quantum DLT 8000 Tape System 3-7

3.4 SCSI Signals

The following paragraphs define SCSI signals and bus timing values.

3.4.1 SCSI Signal Definitions

Table 3–3 defines the SCSI bus signals.

Table 3–3 SCSI-2 Bus Signal Definitions

Signal Definition

ACK (acknowledge) A signal driven by the initiator as an acknowledgment of receipt

of data from a target or as a signal to a target indicating when

the target should read the data (out) lines.

ATN (attention) A signal driven by an initiator to indicate that it has a message

to send.

BSY (busy) An OR-tied signal that indicates that the bus is in use.

C/D (control/data) A signal driven by a target that indicates whether CONTROL or

DATA information is on the DATA BUS. True (low voltage)

indicates CONTROL.

DB(7–0,P) (data bus) Eight data-bit signals, plus a parity-bit signal that form a DATA

BUS. DB (7) is the most significant bit and has the highest

priority (8 or 16-bit) during ARBITRATION. Bit number,

significance, and priority decrease downward to DB(0). A data

bit is defined as 1 when the signal value is true (low voltage)

and 0 when the signal value is false (high voltage). Data parity

DB(P) is odd. Parity is undefined during ARBITRATION.

DB(15–8,P1) (data bus) Eight data-bit signals, plus one parity-bit signal, that forms an

extension to the DATA BUS. They are used for 16-bit (wide)

interfaces. DB(15) is the most significant bit and has the higher

priority (but below bit DB(0) during ARBITRATION. Bit number,

significance, and priority decrease downward to DB(8). Data

Parity DB (P1) is odd.

I/O (input/output) A signal driven by a target that controls the direction of data

movement on the DATA BUS with respect to an initiator. True

indicates input to the initiator.

Also used to distinguish between SELECTION and RESELECTION

modes.

MSG (message) A signal driven by a target during the MESSAGE phase.

REQ (request) A signal driven by a target to indicate a request for an

information transfer to or from the initiator. Each byte of data

transferred is accompanied with a REQ/ACK “handshake”. See

also, ACK.

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Quantum DLT 8000 Manual Página 109