User's Guide

Page 14 DDS-4 SCSI Interface Manual, Rev. B
The minimum value fo the Transfer Period (25 nsec) is determined by the maximum
burst transfer rate of the SCSI interface hardware of the drive and is 80 Mbytes per
second (06h). The host must specify a Transfer Period that allows it to successfully
receive data from the DAT drive during the Data In Phase. Transfer Period
represents the actual transfer period, in nanoseconds, divided by 4.
The REQ/ACK Offset is the maximum number of pulses that can be sent by the DAT
drive in advance of the number of ACK pulses received from the host, establishing a
pacing mechanism. If the number of REQ pulses is greater than the number of ACK
pulses by the REQ/ACK Offset, the DAT drive stops sending data until after the
leading edge of the next ACK is received.
The ACK/REQ Offset is used to prevent an overflow condition in the host's reception
buffer during the Data In Phase. The REQ/ACK Offset should be set to the size of
the host's reception buffer minus one.
To set up a new synchronous data transfer agreement, the host asserts the ATN
signal and sends an SDTR message. The Transfer Period must be set to one of a
set of specific values equal to or greater than 0Ah and less than the maximum rate
of the host’s reception buffer. The ACK/REQ Offset will be set to a nonzero value
between 4 and the size of the host’s reception buffer minus 1, but it must not exceed
32.
If the Transfer Period and the ACK/REQ Offset are within the ranges described
above, the DAT drive goes to the Message In Phase and returns an SDTR message
with the same Transfer Period and ACK/REQ Offset. This return indicates a
successful completion of the SDTR message exchange. The implied synchronous
data transfer agreement remains in effect until:
• A Bus Device Reset message is received.
• A hard reset condition occurs.
• The successful completion of the next SDTR message exchange.
If the Transfer Period is less than 0Ah or the REQ/ACK Offset less than 4 the drive
returns either a Transfer Period of zero or a REQ/ACK Offset of zero to indicate the
request cannot be supported in a synchronous manner and use of asynchronous
transfer is requested.
If the host specifies a REQ/ACK offset of zero, the DAT drive operates in the
asynchronous data transfer mode.
Command Phase
During the Command Phase, the Target requests command information from the
Initiator. The Target asserts the C/D signal and de-asserts the I/O and MSG signals
thus denoting the Command Phase. The REQ/ACK then handshakes the command
bytes across the SCSI bus (Figure 2-5). The command bytes are also called the
Command Descriptor Block (CDB).