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SCSI Interface Description Page 11

1 The host supports disconnects. The drive disconnects from the SCSI bus

appropriately for the command in progress to relinquish the bus for other units.

Note: LUN = 1 is legal for Autoloader drives only.

Information Transfer Phases

The Command, Data, Status, and Message Phases are grouped together as the

Information Transfer Phases because they are all used to transfer data or control

information on the SCSI bus.

The following table shows the Control Data (C/D), 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.

Signals -MSG -C/D -I/O Phases Direction of Transfer

-I/O

1 1 1 Data Initiator to Target

1 1 0 Data Target to Initiator

-C/D

1 0 1 Command Initiator to Target

1 0 0 Status Target to Initiator

-MSG

0 0 1 Message Initiator to Target

0 0 0 Message Target to Initiator

Key: 1 = False, 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/D, 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.

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 shows the data transfer signals for information transfer from Target to

Initiator, and Figure 2-5 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.

Transfer from Target to Initiator

The transfer from Target to Initiator is described in the following table and illustrated

in Figure 2-4.

Timing Point Action

The I/O signal is asserted; the Target drives the data (DB7-0)

and parity signals to their desired values.

The Target then asserts the REQ signal.

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 ... 180 181

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