Quantum DLTtape DLT 4000, Product Manual

Page: 72 / 300

SCSI Description
3-10 Quantum DLT 4000 Tape Drive
Table 3–4 SCSI Bus Timing Values (continued)
Timing Description Value Description
Selection Time-Out
Delay 250
ms
1
Recommended minimum time a SCSI device should
wait for a BSY response during SELECTION or
RESELECTION before starting the time-out procedure.
Transfer Period
(set during an SDTR
message.) Minimum time allowed between the leading edges of
successive REQ pulses and of successive ACK pulses
while using standard or fast synchronous data
transfers. The period range is 200 to 500ns minimum,
standard, or 100 to 500ns minimum, fast-
synchronous.
1
Recommended Time.
3.5 SCSI BUS PHASES
The SCSI architecture includes eight distinct phases:
1. BUS FREE phase
2. ARBITRATION phase
3. SELECTION phase
4. RESELECTION phase
5. COMMAND phase
6. DATA phases (In/Out)
7. STATUS phase
8. MESSAGE phases (In/Out)
The last four phases are called the “information transfer phases”.
The SCSI bus can never be in more than one phase at any given time. In the
following descriptions, signals that are not mentioned are not asserted.
3.5.1 BUS FREE Phase
The BUS FREE phase indicates that there is no current I/O process and that the
SCSI bus is available for a connection.
SCSI devices detect the BUS FREE phase after the SEL and BSY signals are
both false for at least one bus settle delay.