Information Transfer Phases; Information Transfer Phase (Phase Control) - Fujitsu MAN3184 SERIES Specifications

1.6.5

INFORMATION TRANSFER phases

The COMMAND, DATA, STATUS and MESSAGE phases are generally called the
INFORMATION TRANSFER phase. This phase can transfer the control information and data
between the INIT and TARG via the data bus.
The type of INFORMATION TRANSFER phase is determined by the combination of C/D, I/O,
and MSG signals (see Table 1.1). Since these three signals are specified by the TARG, phase
transition is controlled by the SCSI device operating as a TARG. The INIT can request the TARG
to initiate a MESSAGE OUT phase by sending an ATN signal. Besides, the TARG can change the
bus phase to BUS FREE by ceasing the transmission of BSY signal.
During INFORMATION TRANSFER phase, the information transfer is controlled by the REQ
and ACK signals. The TARG sends the REQ signal to request for information transfer, and the
INIT responds to it with the ACK signal. A pair of REQ and ACK signals is used to transfer a
single-byte information on the 8-bit SCSI bus or two-byte information on the 16-bit SCSI bus.
There are two types of information transfer modes: synchronous and asynchronous transfer modes.
They differ from each other by their REQ signal transmission and ACK signal response methods
(called the REQ/ACK handshaking). Also, the 16-bit SCSI bus can transfer 16-bit wide data only
in the DATA phase.
The 16-bit SCSI bus can transfer 16-bit wide data only in the DATA phase except alternate error
detection for the asynchronous information phase (COMMAND, MESSAGE and STATUS). The
detail of these phase is described below section.
The target shall not transition into an information transfer phase unless the REQ/ACK signals are
negated. The target shall not transition from an information transfer phase into another information
transfer phase unless the REQ/ACK signals are negated. During INFORMATION TRANSFER
phase, the TARG shall keep the BSY signal true but keep the SEL signal false. The TARG shall
establish the status of C/D, I/O and MSG signals (which determine the phase type) at least Bus
Settle Delay before the leading edge of REQ signal which requests to transfer the first byte. The
TARG shall keep the status until it detects the trailing edge of the ACK signal which corresponds
to the last byte in that phase (see Figure 1.25).
Figure 1.25 INFORMATION TRANSFER phase (phase control)
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