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Each Q250/Q280 bus driver has the following output characteristics:
True (Signal Asserted)
=
0.0 to 0.4 V dc at 48
mAo
False (Signal Non-Asserted)
=
2.5 to 5.25 V dc.
Each signal received by the Q250 or Q280 must have the following input
characteristics:
True (Signal Asserted)
=
0.0 to 0.8 V dc.
Maximum total input load
=
-0.4
mA
at 0.4 V dc.
False (Signal Non-Asserted)
=
2.0 to 5.25 V dc.
Minimum input hysteresis
=
0.2 V dc.
SCSI Bus Signals
Up to eight devices can be supported on the SCSI bus, but communica-
tion is allowed between only two SCSI devices at any given time.
Information transfers on the SCSI DATA BUS are asynchronous and fol-
low a defined -REQ/-ACK (request/acknowledge) handshake protocol.
One
byte of information can be transferred with each handshake.
There are
a total of 18 logic signals: nine control signals and nine signals
that comprise an 8-bit DATA BUS with parity.
Each of the signals is
described briefly below.
-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/D (CONTROL/DATA).
A signal driven by a target that indicates
whether CONTROL or DATA information is on the DATA BUS.
Assertion
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 ini-
tiator.
Assertion indicates input to the initiator.
This signal is
also used to distinguish 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
a -REQ/-ACK data transfer handshake.
-ACK (ACKNOWLEDGE).
A signal driven by an initiator to indicate an
acknowledgement for a -REQ/-ACK data transfer handshake.
2-11
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