Viper 60s Theory Of Operation And Maintenance Manual page 41

VIPER SCSI 60. 125 & 150 THEORY OF OPERATION AND MAINTENANCE
4.8 Information Transfer Phases
The Command, Data, Status, and Message Phases are grouped together as the Informa-
tion Transfer Phases (Table 4-6) because they are all used to transfer data or control
information on the SCSI bus.
Table 4-6 shows the Control Data (C/O), Input/Output (I/O), and Message (MSG) signals
used to distinguish between the various Information Transfer Phases. The Target drives
these three signals and thereby controls all changes from one phase to another.
Table 4-6 Information Transfer Phases
I
· · _ _ ··--··--l
SIGNALS! ·MSG ·C/O ·1/0
PHASES
I
DIRECTION OF TRANSFER
1
I
·1/0 1 1 Data Initiator to Target
1 0
Data to Initiator
0 1
Command Initiator to Target
1
0 0 Status Target to Initiator
0 0 1
Message Initiator to Target
0
0
0
Message Target to Initiator
0= True
Information Transfer Phases use one or more REQ/ ACK handshakes to control the
information transfer. Each REQ/ ACK handshake allows the transfer of one byte of
information. During the information transfer phases, BSY remains true and SEL remains
false. C/O, I/O, and MSG control signals are valid for a bus settle delay of 400 nSec before
assertion of REQ signal at the first handshake and remain valid until negation of ACK at
the end of the last handshake.
4.8.1 Asynchronous Data Transfer
The Target controls the direction of information transfer with the I/O signaL When I/O
is
asserted (low), information
is
transferred from the Target to the Initiator. When I/O is de-
asserted (high), information is transferred from the Initiator to the Target. Figure 4-4
shows the data transfer signals for information transfer from Target to Initiator, and
Figure 4-5 shows the data transfer signals for information transfer from Initiator to Target.
Each direction of information transfer is discussed in detail in the following paragraphs.
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