Quantum 10K II - Manual (Page 312)

Feature Descriptions
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives 6-11
6.17.2 DIFFSENS SWITCHING
After establishing its initial operating mode, the Atlas 10K II disk drive continues to
monitor the DIFFSENS signal. If a change in level is detected, such as might occur
if a device is added to or removed from the bus, the DIFFSENS circuit detects the
change and presents a mode change interrupt to the drive firmware. The operating
mode of the interface switches to the new mode 100 msec after the change is
detected, providing the new level has remained constant. This delay is required to
prevent spurious mode switches due to noise on the DIFFSENS line.
6.17.3 DT AND ST CLOCKING
With 160/m interface, Quantum has introduced a new method of clocking data on
the SCSI bus. This method, called Double Transition (DT) clocking, uses both the
rising and falling edges of the REQ and ACK signals to clock data into the receiving
device. This has the advantage of allowing the REQ and ACK signals to run at half
the frequency that would be required under the traditional Single Transition (ST)
SCSI clocking scheme for the same data rate. Lower frequency clocks result in a
more reliable bus. Figure 6-1 illustrates the difference between DT and ST clocking.
Figure 6-1 ST Clocking and DT Clocking
The Atlas 10K II disk drive can operate at speeds up to 80 MB/sec in either ST or
DT modes, depending on the capabilities of the host adapter. Transfer rates above 80
MB/sec are only supported in DT mode, as required by the SPI-3 SCSI standard.
D
RE
Single-
D
RE
Double-
Maximum data
Maximum data frequency is
Single Transition (ST) Clocking
REQ/ACK
Data
Double Transition (DT) Clocking
REQ/ACK
Data
Maximum data frequency is ½ the clock frequency
Maximum data frequency is the same as the clock frequency