Viper 60s Theory Of Operation And Maintenance Manual page 44

INTERFACE
4.9 Command Phase
During Command Phase, the Target requests command information from the Initiator.
The Target asserts the C/O signal and deasserts the I/O and MSG signals thus denoting
the Command Phase. The REQ/ ACK then handshakes the command bytes across the
SCSI bus (Figure 4-5). The command bytes are also called the Command Descriptor Block
(COB).
4.10 Data Phase
The Data Phase is subdivided into the Data-In and Data-Out Phases.
4.10.1 Data-In Phase
During Data-In phase, the Target requests that data be sent to the Initiator from the Target.
The Target asserts the I/O signal and deasserts the C/O and MSG signals thus denoting
the Data-In Phase. The REQ/ ACK handshakes (Figure 4-4) then transfers the requested
byte count.
4.10.2 Data-Out Phase
During a Data-Out phase, the Target requests that data be sent from the Initiator to the
Target. The Target deasserts the C/O, I/O, and MSG signals thus denoting the Data-Out
Phase. The REQ/ ACK handshakes (Figure 4-5) then transfers the requested byte count
across the SCSI bus.
4.11 Status Phase
During a Status Phase, the Target requests that status information be sent to the Initiator
from the Target. The Target asserts the C/O and I/O signals and deasserts the MSG signal
thus denoting the Status Phase. The REQ/ ACK handshakes the one byte status code
across the SCSI bus (refer to Figure 4-4).
4.12 Message Phase
The Message Phase consists of either the Message-In or Message-Out Phases. The mes-
sages the Viper drive supports are all one byte messages.
4.12.1 Message-In Phase
During Message-In Phase, the Target requests that messages be sent to the Initiator from
the Target. The Target asserts the C/O, I/O, MSG signals thus denoting the Message-In
Phase. The REQ/ ACK handshakes the one byte message across the SCSI bus.
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