Section 4
SCSI Interface
4.3.2.1
Terminator power
ProDrive supplies tenninator power through a schottky diode and a I-Amp fuse with the following
characteristics:
VTERM
= 4.25 VOC to 5.25 VOC
800
milliamps minimum source drive capability when supplying both sets
of
tenninators
1.0 milliamp maximum sink capability. except for the purposes
of
providing
power
to
an internal tenninator
1.0
Amp
current
li~iting
(fuse)
4.3.3
BUS SIGNALS
There are a total
of
18
SCSI bus signals. Nine are control signals and nine are used for data. (The parity
signal option is included as a data signal.) Definitions
of
these signals are given below:
Control Signals:
BSY
(BUSY) - An "OR-tied" signal that indicates that 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/O
(CONTROL/OATA)
- A signal driven by a target that indicates whether CONTROL
or
OAT
A infonnation is on the
OAT
A BUS. True indicates CONTROL.
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.
This signal is also used to distinguish between
SELECTION and RES ELECTION 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 a REQ/ ACK data transfer
handshake.
ACK
(ACKNOWLEDGE)
- A signal driven by an initiator to indicate an acknowledgment for a
REQ/ACK data transfer handshake.
A
TN
(ATTENTION)
- A signal driven by an initiator to indicate the ATTENTION condition.
RST
(RESET)
- An "OR-tied" signal that indicates the RESET condition.
Data Signals:
DB(7·0,P)
(DATA
BUS)
- Eight data-bit signals. plus a parity-bit signal that
fonn
a
DATA
BUS. OB(7) is the most significant bit and has the highest priority during the ARBITRATION
phase. Bit number. significance. and priority decrease downward to
DB(O). A data bit is defined as
one
when the signal value is ·true and zero when the signal value is false.
DB(P) is set to either zero
or
one so that data parity is odd. The use
of
parity is a system option.
but
parity generation and checking must be consistent for all SCSI devices on the bus: either all
SCSI devices on the bus must generate and check parity,
or
all SCSI devices on the bus must not
check parity. Parity is not valid during the ARBITRATION phase
..
SCSI Interface 4 • 3
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