Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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The Host Adapter/Initiator must be identified by one of the eight SCSI Device Addresses. Make sure that
none of the devices on the SCSI bus have duplicate addresses.
Certain SCSI bus functions are assigned to the initiator and certain SCSI bus functions are assigned to the
target. The initiator will select a particular target. The target will request the transfer of Command, Data,
Status or other information on the data bus.
Information transfers on the data bus are interlocked and follow a defined REQ/ACK Handshake protocol.
One byte of information will be transferred with each handshake. Synchronous data transfers do not require
a one for one interlocking of REQ/ACK signals, but the total number of REQ pulses in a particular data
transfer event must equal the total number of ACK pulses. Synchronous data transfer option is described in
Paragraph 3.5.3.2 and 3.1.5.2.
The disc 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
There are ten control and eighteen data signals, as listed below:
• BSY
• C/D
• MSG
• DIFFSENS
• SEL
• I/O
• REQ
• DB(7-0, P); DB(15-8,P1)
• 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 drive model's Volume 1 Product manual, 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.
