Scsi Bus Signals Overview - Seagate Ultra160 Product Manual

14 SCSI Interface Product Manual, Rev. B
ticular data transfer event must equal the total number of ACK pulses. The synchronous data transfer option is
described in Paragraphs 3.1.5.2 and 3.5.3.2 of the Ultra2 SCSI Interface Product Manual, Volume 2, part num-
ber 77738479.
This Ultra160/Ultra320 SCSI Interface Product Manual, Volume 2, discusses SCSI-3 protocol (now called only
"SCSI"). There are now different ways of latching data from the REQ and ACK signals, depending on whether
ST DATA phases, DT DATA phases, or paced transfers are being used for information transfers. This is
described in Section 2.1.2.
The drive supports single initiator, single target; single initiator, multiple target; multiple initiator, single target; or
multiple initiator, multiple target bus configurations.
2.1

SCSI bus signals overview

Information transfer on the SCSI bus is allowed between only two SCSI devices at any given time except during
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 combination
of SCSI initiator ports (commonly called "initiators") and SCSI target ports (commonly called "targets"), pro-
vided 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-
guishes between SELECTION and RESELECTION phases.
MSG (Message)—A signal sourced by a SCSI target port during the MESSAGE PHASE.
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 circuits
that can operate either in single-ended (SE) or low voltage differential (LVD) mode. When the interface DIFFS-
ENS line is between 0 V and 0.6 V, the drive interface circuits operate single-ended. When DIFFSENS is
between +0.7 V and +1.9 V, the drive interface circuits operate low voltage differential. This arrangement 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 voltage differential
levels and should never be exposed to high voltage differential environments unless the command mode volt-
ages in the environment are controlled to safe levels for single-ended and low voltage differential devices (see
the ANSI SPI-4 specification). High Voltage Differential (HVD) is now an obsolete ANSI standard.