Scsi Signals; Scsi Signal Definitions - Quantum DLT 8000 Product Manual

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.
Signal
ACK (acknowledge)
ATN (attention)
BSY (busy)
C/D (control/data)
DB(7–0,P) (data bus)
DB(15–8,P1) (data bus)
I/O (input/output)
MSG (message)
REQ (request)
Table 3–3 SCSI-2 Bus Signal Definitions
Definition
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.
A signal driven by an initiator to indicate that it has a message
to send.
An OR-tied signal that indicates that the bus is in use.
A signal driven by a target that indicates whether CONTROL or
DATA information is on the DATA BUS. True (low voltage)
indicates CONTROL.
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.
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.
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.
A signal driven by a target during the MESSAGE phase.
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.
Quantum DLT 8000 Tape System
SCSI Description
3-7