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SCSI BUS
7-30 C156-E228-02EN
(2) Synchronous mode
Information is transferred through offset-interlock of REQ/ACK handshake.
Operation in this mode is only available for the DATA phase.
The default data transfer mode is asynchronous mode. When power
is first switched on, a RESET condition develops or a BUS
DEVICE RESET message is exchanged, data transfer is performed
in asynchronous mode until the message described below is
exchanged, even if synchronous mode transfer is permitted with the
setting terminal.
Before synchronous mode transfer can be used, a SYNCHRONOUS DATA
TRANSFER REQUEST message must be exchanged between the INIT and
TARG to define synchronous mode transfer between them. Then, the following
transfer parameters are determined to define a range of possible transfer rates
between the SCSI devices.
• REQ/ACK Offset: Number of REQ signals that the TARG can send before
receiving the ACK signal.
• Transfer Period: Minimum repetition cycle of REQ and ACK signals.
The TARG can send more than one REQ pulse before receiving the ACK pulse
from the INIT if the number of REQ pulses is within the range defined by the
REQ/ACK Offset parameter. When the difference between the number of REQ
pulses sent by the TARG and the number of ACK pulses received by the TARG
reaches the number assigned to the REQ/ACK Offset parameter, the TARG must
not send an REQ pulse until it receives the leading edge of the next ACK pulse.
For normal termination of transfer in a DATA phase, the number of REQ pulses
and the number of ACK pulses must be equal.
The TARG must satisfy the following timing requirements concerning the
transmission of the REQ signal at the connector pin on the TARG:
• The minimum pulse width is Assertion Period.
• The minimum period from the trailing edge of a pulse to the leading edge of
the next pulse is Negation Period.
• The period between the leading edges of a pulse and the next pulse is equal to
or greater than the time defined by the Transfer Period parameter.
The INIT must respond to the TARG by sending as the same number of ACK
pulses as the REQ pulses received from the TARG. It can send an ACK signal
when it receives the leading edge of the corresponding REQ signal. The INIT
must satisfy the following timing requirements concerning the transmission of the
ACK signal at the SCSI connector pin on the INIT: