Scsi Interface Signal Descriptions - Black Box 2 x 2 SCSI Matrix Switch SC121A User Manual

2 X 2 SCSI MATRIX SWITCH
• If the last device on the chain has two SCSI connectors, attach the cable to one and a
terminator to the other. Otherwise, you'll have an open connector that may cause
noise on the SCSI chain.
• Always turn off the power to your computer and SCSI devices before swapping cables or
moving devices around. SCSI cables contain sensitive data transmission lines and
one or more live power wires.
• Turn on your SCSI devices before you turn on the computer. Some SCSI devices will
not mount if they are not running when you power up your computer.
Shutting down your computer first and then the attached SCSI devices allows
your system to completely "flush" itself.

6.3 SCSI Interface Signal Descriptions

A total of 18 signals are required for the SCSI interface. These signals are described
as follows:
• BSY (BUSY): An "OR-tied" signal indicating that the bus is being used.
• SEL (SELECT): An "OR-tied" signal used either 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. 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 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
acknowledgment for a REQ/ACK data-transfer handshake.
• ATN (ATTENTION): A signal driven by an initiator to indicate the Attention
condition.
• RST (RESET): An "OR-tied" signal that indicates the Reset condition.
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