Synchronous Data Transfer Request Message (I T) - Fujitsu MAN3184 SERIES Specifications

2.3.19

SYNCHRONOUS DATA TRANSFER REQUEST message (I T)

Byte
The synchronous data transfer parameters are defined by exchange of the SYNCHRONOUS
DATA TRANSFER REQUEST message between two SCSI devices.
When an SCSI device having the synchronous transfer functions is first connected to another SCSI
device after its power-on, a RESET condition (a hardware reset), or after reception of TARGET
RESET message, these two devices exchange the SYNCHRONOUS DATA TRANSFER
REQUEST message by each other and determine the synchronous data transfer.
Each SCSI device must always respond to this message which is issued by another SCSI device.
(The SCSI devices can exchange this message to select or change the data transfer mode any time
other than the first connection.)
(1) Data transfer mode parameter
Two SCSI devices exchange the SYNCHRONOUS DATA TRANSFER REQUEST message and
determine the Transfer Period and REQ/ACK Offset allowance to be set during data transfer
between them. These values apply to all logical units (LUNs) assigned to the two SCSI devices.
The Transfer Period is the minimum repeat cycle of REQ and ACK pulses which are allowed for
data reception by SCSI devices. (It is the minimum time between the leading edge of an REQ
pulse and the leading edge of the next REQ pulse, or between the leading edge of an ACK pulse
and the leading edge of the next ACK pulse.)
The REQ/ACK Offset is the maximum number of REQ pulses (an offset) which the TARG can
send before receiving an ACK response (the leading edge of ACK signal) from the INIT. This
offset must be set to the enough value not to cause an overflow of data receive buffer and offset
counter. When the REQ/ACK Offset is X'00', data is transferred in the asynchronous transfer
mode. When it is X'FF', this offset is unlimited.
The SCSI device that sends the SYNCHRONOUS DATA TRANSFER REQUEST message at
first sends this message with specifying the Transfer Period and REQ/ACK Offset values within
the appropriate range so that the SCSI device itself can receive data normally. The SCSI device
that received the SYNCHRONOUS DATA TRANSFER REQUEST message returns the
SYNCHRONOUS DATA TRANSFER REQUEST message with setting same Transfer Period and
REQ/ACK Offset values as received ones if it can receive data using specified parameters. If the
SCSI device requires a larger Transfer Period or a smaller REQ/ACK Offset to receive data
correctly, it sets the appropriate values (which can satisfy the SCSI device requirements) in the
SYNCHRONOUS DATA TRANSFER REQUEST message and returns it to the sender unit.
2 - 16
0 1 2
X'01' X'03' X'01'
C141-E123-01EN
3 4
m x
REQ/ACK offset
Transfer Period [4 m (ns)]