Scsi Bus Signals - Seagate SCSI Interface Product Manual

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2.1

SCSI bus signals

There are ten control and eighteen data signals, as listed below:
· BSY · C/D · MSG
· SEL · I/O · REQ
· ACK · ATN · RST
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, but not here.
The 28 SCSI standard signals are described as follows:
BSY (Busy)—An "OR-tied" signal to indicate the bus is being used.
SEL (Select)—A signal used by an initiator to select a target, or by a target to reselect an initiator.
C/D (Control/Data)—A signal driven by a target to indicate whether Control or Data information is on the Data
Bus. Assertion (see Paragraph 2.1.2) indicates Control.
I/O (Input/Output)—A signal driven by a target to control the direction of data movement on the Data Bus with
respect to an initiator. Assertion indicates input to the initiator. This signal also distinguishes between Selection
and Reselection phases.
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 REQ/ACK data transfer handshake.
ACK (Acknowledge)—A signal driven by an initiator to indicate an acknowledgment for a REQ/ACK data
transfer handshake.
ATN (Attention)—A signal driven by an initiator to indicate the Attention condition. It is used to request to send
a message out to the target. See Paragraph 3.2.1. If an initiator asserts ATN while asserting SEL it indicates to
the target that the initiator supports messages other than command complete.
RST (Reset)—An "OR-tied" signal that indicates the Reset condition.
DIFFSNS (Differential Sense)—When the drive has high voltage differential SCSI I/O circuits (HVD), the
DIFFSNS signal disables the drive's differential driver/receiver circuits if the SCSI I/O cable is plugged in
upside down, or if a single-ended SCSI I/O cable is plugged into a differential I/O drive. Disabling the differen-
tial I/O drivers/receivers is necessary to prevent burning them out if a grounded I/O line is connected to any of
the differential circuit outputs, which are at a positive voltage (+2 V or +3 V) when not disabled.
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 mode (LVD). When the interface "DIFFSNS" line is between 0 V and 0.6
V, the drive interface circuits operate single-ended. When "DIFFSNS" 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 chang-
ing 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 cir-
cuits used by "LC" and "LW" devices do not operate at high voltage differential levels and should never be
exposed to high voltage differential environments unless the command mode voltages in the environment are
controlled to safe levels for single-ended and low voltage differential devices (see the ANSI SPI-3 specification,
T10/1302D).
DB(7-0,P) and DB(15-8,P1) (Data Bus)—Sixteen data bit signals, plus parity bit signals form a Data Bus.
DB(7) is the most significant bit and has the highest priority during the Arbitration phase (on both eight and six-
teen device systems). Bit number significance, and priority decrease downward to DB(0), and then from DB15
down to DB8 (DB0 is higher than DB15). A data bit is defined as one when the signal is asserted and is defined
as zero when the signal is negated.
Data parity DB(P) and DB(P1) is odd—The use of parity is a system option. The drive always checks parity
on the data bits, but has the capability to enable/disable parity error reporting to the host. See configuration
selection in the individual drive's Product Manual, Volume 1. Parity checking is not valid during the Arbitration
phase.
Greater detail on each of the SCSI Bus signals is found in the following sections.
· DIFFSNS (Multimode) (may sometimes be designated "DIFFSENS")
· DB(7-0, P); DB(15-8,P1)
SCSI Interface Product Manual, Rev. J