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SCSI BUS
7-20 C156-E228-02EN
7.6.2 ARBITRATION phase
The ARBITRATION phase allows one SCSI device to gain control of the SCSI
bus so that an INIT starts the SELECTION phase or a TARG starts the
RESELECTION phase.
Implementation of the ARBITRATION phase is a system option. This phase is
required for system that has two or more INITs or uses the RESELECTION phase.
The procedure to obtain control of the SCSI bus is as follows (see Figure 7.10):
1) The SCSI device shall wait for a BUS FREE phase (see Subsection 7.6.1).
2) The SCSI device shall wait at least Bus Free Delay after Bus Free phase
detection.
3) Then the SCSI device that arbitrates the bus asserts the DATA BUS bit
corresponding to its own SCSI ID and BSY signal within Bus Set Delay after
last observation of the BUS FREE phase (*1).
4) After waiting at least Arbitration Delay since the SCSI device asserted the
BSY signal, the SCSI device shall examine the value on the DATA BUS to
determine the priority of the bus arbitration. (The priority of the bus
arbitration is in the descending order of data bus bit numbers; the highest
priority is DB7 (ID#7) and the lowest priority is DB0 (ID#0)).
− When the SCSI device detects any ID bit which is assigned higher
priority than its own SCSI ID, the SCSI device shall release its signals
(BSY and its SCSI ID), then may return to step 1). (The SCSI device #1
in Figure 7.10 has lost the arbitration.)
− The SCSI device which detects no higher SCSI ID bit on the DATA
BUS can obtain the bus control, then it shall assert SEL signal. (The
SCSI device #7 in Figure 7.10 has won the arbitration.)
− Any other SCSI device that is participating in the ARBITRATION
phase shall release its signals within Bus Clear Delay after the SEL
signal becomes true, then may return to step 1). (The SCSI device #3 in
Figure 7.10 has lost the arbitration.)
5) The SCSI device which wins arbitration (SCSI device #7 in Figure 7.10) shall
wait at least Bus Clear Delay + Bus Settle Delay after asserting the SEL
signal before changing any signal state.
*1: When an SCSI device sends its SCSI ID to the DATA BUS, it asserts only the
bit at the position corresponding to its own ID and leaves the other seven bits
false. The parity bit (DBP signal) is not driven or is driven true, rather than
false. The parity bit on the DATA BUS is unpredictable during an
ARBITRATION phase.