SCSI - Computer Busses - page 156

fectively takes over the bus for the time to send a command and the target executes the com-
mand and then contacts the initiator and transfers any data. The bus will then be free for
other transfers.

The main signals are:

•

BSY - indicates that the bus is busy, or not (an OR-tied signal).

•

ACK - activated by the initiator to indicate an acknowledgement for a REQ information
transfer handshake.

•

RST - when active (low) resets all the SCSI devices (an OR-tied signal).

•

ATN - activated by the initiator to indicate the attention state.

•

MSG - activated by the target to indicate the message phase.

•

SEL - activated by the initiator and is used to select a particular target device (an OR-tied
signal).

•

C/D (control/data) - activated by the target to identify if there is data or control on the
SCSI bus.

•

REQ - activated by the target to acknowledge to indicate a request for an ACK informa-
tion transfer handshake.

•

I/O (input/output) - activated by the target to show the direction of the data on the data
bus. Input defines that data is an input to the initiator, else it is an output.

Each of the control signals can be true or false. They can be:

• OR-tied driven, where the driver does not drive the signal to the false state. In this case

the bias circuitry of the bus terminators pulls the signal false whenever it is released by
t he dr iver s at ev ery SCSI device. If any driver is asserted, then the signal is true. The
BSY , SE L , and RST signals are OR-tied. In the ordinary operation of the bus, the BSY
and RST signals may be simultaneously driven true by several drivers.

• Non-OR-tied driven, where the signal may be actively driven false. No signals other than

BSY , RST and D(PARITY) are simultaneously driven by two or more drivers.

SCSI bus

Initiator Target

Function
request

Function
executor

Figure 7.1 Initiator and target in SCSI