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C156-E097-01EN 7 - 25
(2) Synchronous mode
Information is transferred through offset-interlock of REQ/ACK handshake. Operation in this
mode is only available for the DATA phase.
IMPORTANT
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 90 ns (Assertion Period) [30 ns (Fast Assertion Period) in
FAST SCSI mode].
• The minimum period from the trailing edge of a pulse to the leading edge of the next pulse is
90 ns (Negation Period) [30 ns (Fast Negation Period) in FAST SCSI mode].
• 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.