SCSI - Computer Busses - page 157

7.4 SCSI operation

The SCSI bus allows any unit to talk to any other unit, or the host to talk to any unit. Thus
there must be some way of arbitration where units capture the bus. The main phases that the
bus goes through are:

• Free-bus . In this state there are no units whi ch either transfer data or have control of the

bus. It is identified by disactive SEL and BSY (both will be high). Thus, any unit can cap-
ture the bus.

• Arbitration . In this stat e a unit can take control of the bus and become an initiator. To do

this it activates the BSY signal and puts its own ID address on the data bus. Next, after a
delay, it tests the data bus to determine if a high-priority unit has put its own address on
the bus. If it has then it w ill a llow the other unit access to the bus. If its address is still on
the bus then it asserts the SEL line. After a delay, it then has control of the bus.

• Selection . In this state the initiator selects a target unit and gets the target to carry out a

given function, such as reading or writing data. The initiator outputs the OR value of its
SCSI-ID and the SCSI-ID of the target onto the data bus (for example, if the initiator is 2
and the target is 5 then the OR-ed ID on the bus will be 00100100.). The target then de-
termines that its ID is on the data bus and set the BSY line active. If this does not happen
within a given time then the initiator deactivates the S EL signal, and the bus will be free.
The target determines that it is selected when the SEL signal and its SCSI ID bit are ac-
tive and the BSY and I/O signals are false. It then asserts the BSY signal within a selec-
tion abort time.

• Rese lectio n . When the arbitration phase is complete, the winning SCSI device asserts the

BSY and SEL signals and has delayed at least a bus clear delay plus a bus settle delay.
The winning SCSI device sets the DATA BUS to a value that is the logical OR of its
SCSI ID bit and the initiator's SCSI ID bit. Sometimes the target takes some time to reply
to the initiators request. The initiator determines that it is reselected when the SEL and
I/O signals and its SCSI ID bit are true and the BSY signal is false. The reselected initia-
tor then asserts the BSY signal within a selection abort time of its most recent detection
of being reselected. An initiator does not respond to a reselection p hase if other than two
SCSI ID bits are on the data bus. After the target detects that the BSY signal is true, it
also asserts the BSY signal and waits a given time delay and then releases the SEL signal.
The target may then change the I/O signal and the data bus. After the reselected initiator
detects the SEL signal is false, it releases the BSY signal. The target continues to assert
the BSY signal until it gives up the SCSI bus.

• Command . The command phase is used by the target to request command inf orm ation

from the initiator. The target asserts the C/D signal and negates the I/O and MSG sig-
nals during the REQ / ACK handshake(s) of this phase.

• Data . The data phase covers both the data in and data out phases. In the data in phase the

target requests that data be sent to the initiator from the target. For this purpose the target
asserts the I/O signal and negates the C/D and MSG signals during the REQ / ACK
handshake(s) of this phase. In the data out phase, the target reques ts th at data be sent
from the initiator to the target. The target negates the C/D , I/O and MSG signals during
the REQ / ACK handshake(s) of this phase.