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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

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.

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.

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Quantum DLT 4000 Handbuch Seite 72