Tandberg Data
Hardware Interface
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8-8
Tandberg SLR Reference Manual
8.4.2. Bus Free Phase
The Bus Free Phase, indicating that the bus is free for use, is invoked by all signal lines being
deasserted. All devices must release their bus signals (within a bus-clear delay of maximum
800 ns) after deassertion of BSY and SEL. To recognize the Bus Free Phase, devices have
to test that both BSY and SEL are not asserted (simultaneously within a deskew delay), and
that the Reset condition is not active.
8.4.3. Arbitration Phase
The SCSI-bus allows multiple host configurations. To avoid bus-crash situations in a
configuration like this, the devices have to arbitrate for the bus. If more than one device is
requesting the bus simultaneously, the one with the highest priority (highest SCSI ID no.) will
win the bus. To arbitrate, the device has to test that the bus is in the Bus Free Phase. (See
Section 8.4.2). Then it waits for a bus-free delay of minimum 800 ns before it asserts BSY,
and the data-bit corresponding to the SCSI ID number, on the bus. The device will then wait
for the arbitration delay (min. 2.4 µs) before testing if the arbitration was won. If so, it asserts
SEL to claim the bus, and enters the Selection Phase. All devices that have lost the
arbitration (devices that recognize a higher address on the bus than their own), should
immediately deassert BSY and their bus address bit. Parity is not valid during the arbitration
phase.