SCSI BUS
7-4 C156-E228-02EN
(1) DB 7 to 0, P (DATA BUS)
These signals form a bidirectional data bus consisting eight data bits and an odd
parity bit.
MSB (27): DB7, LSB (20): DB0
The DATA BUS is used to transfer commands, data, status, or messages in the
INFORMATION TRANSFER phase. SCSI IDs are sent to the DATA BUS in the
ARBITRATION phase for determining the priority of bus arbitration. In the
SELECTION or RESELECTION phase, SCSI IDs of the INIT and the TARG are
indicated on the DATA BUS. Figure 7.3 shows the correspondence between
DATA BUS bits and SCSI IDs.
Figure 7.3 DATA BUS and SCSI ID
When the DB(n) signal is true, the data bit (n) is 1, and when false, it is 0.
The use of a parity bit is a system option. The ODD handles parity as shown
below:
• The ODD implements the bus parity check feature, which can be enabled or
disabled by a setting terminal on the ODD. For the setting terminal, refer to
Item (2) in Subsection 4.3.1 in OEM Manual -Specifications & Installation-.
• Parity values are always guaranteed when valid data is transferred to the data
bus from the ODD in a phase other than the ARBITRATION phase.
(2) BSY (BUSY)
This signal indicates that the SCSI bus is being used. In the ARBITRATION
phase, it indicates an arbitration request.