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Parallel SCSI Interface Product Manual, Rev. A ) 15
2.1 SCSI bus signals overview
Information transfer on the SCSI bus is allowed between only two SCSI devices at any given time except dur-
ing MESSAGE IN PHASE when QAS is enabled. All SCSI devices that have QAS enabled are required to
monitor messages during a MESSAGE IN PHASE for a QAS REQUEST MESSAGE. The maximum number of
SCSI devices is determined by the width of the data path implemented. The SCSI devices may be any combi
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nation of SCSI initiator ports (commonly called “initiators”) and SCSI target ports (commonly called “targets”),
provided there is at least one of each.
Each SCSI device has a SCSI address and a corresponding SCSI ID bit assigned to it. When two SCSI
devices communicate on the SCSI bus, one acts as the initiator and the other acts as the target. The initiator
originates an I/O process and the target receives the I/O process.
Some drive models have a single 80-pin I/O connector that contains additional interface lines that carry drive
configuration select signals. These are peculiar to certain drives and are not SCSI standard signals. These are
described in the individual drive’s product manual, Volume 1.
The 28 SCSI standard signals are described as follows:
BSY (Busy)—An “OR-tied” signal to indicate the bus is being used.
SEL (Select)—An “OR-tied” signal used by a SCSI initiator port to select a SCSI target port, or by a SCSI tar-
get port to reselect a SCSI initiator port.
RST (Reset)—An “OR-tied” signal that indicates the bus reset condition (see Section 5.2).
C/D (Control/Data)—A signal sourced by a SCSI target port that indicates whether CONTROL or DATA
PHASE information is on the data bus. Assertion indicates Control (i.e., COMMAND, STATUS, and MESSAGE
phases).
I/O (Input/Output)—A signal sourced by a SCSI target port to control the direction of data movement on the
Data Bus with respect to a SCSI initiator port. Assertion indicates input to the initiator. This signal also distin
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guishes between SELECTION and RESELECTION phases.
MSG (Message)—A signal sourced by a SCSI target port to indicate the MESSAGE phase or a DT DATA
phase depending on whether C/D is true or false. Asserted indicates MESSAGE or DT DATA.
REQ (Request)—A signal sourced by a SCSI target port to indicate a request for an information transfer on the
SCSI bus.
ACK (Acknowledge)—A signal sourced by a SCSI initiator port to respond with an acknowledgment of an
information transfer on the SCSI bus.
ATN (Attention)—A signal sourced by a SCSI initiator port to indicate the Attention condition.
DIFFSENS (Differential Sense)/Multimode—SE or LVD alternative— “LW” and “LC” models have I/O cir-
cuits that can operate either in single-ended (SE) or low voltage differential (LVD) mode. When the interface
DIFFSENS line is between -0.35 V and +0.5 V, the drive interface circuits operate single-ended. When DIFFS
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ENS is between +0.7 V and +1.9 V, the drive interface circuits operate low voltage differential. This arrange-
ment is not intended to allow dynamically changing transmission modes, but rather to prevent incompatible
devices from attempting to interoperate. Drives must operate only in the mode for which the installation and
interface cabling is designed. Multimode I/O circuits used by “LW” and “LC” devices do not operate at high volt
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age differential levels and should never be exposed to high voltage differential environments unless the com-
mand mode voltages in the environment are controlled to safe levels for single-ended and low voltage
differential devices (see the ANSI SPI-5 specification). High Voltage Differential (HVD) is now an obsolete
ANSI standard.
P_CRCA (Parity/CRC Available)—A signal identifying either parity or CRC available based on bus phase and
negotiated settings.