SDLT 220 and SDLT 320 Product Manual
CHAPTER 6: SCSI Description
81-85002-01
March 2004
6-9
6.6.1 SCSI Signal Definitions
Table 6-4
lists the SCSI bus signals.
Table 6-4.
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”.
RST (reset)
An OR-tied signal that initiates a RESET condition.
SEL (select)
An OR-tied signal used by an initiator to select a target or by a target to
reselect an initiator.
Summary of Contents for DLTtape SDLT 220
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