Figure 1.3 Data Bus And Scsi Id - Fujitsu MBA3073 SERIES Technical Manual

(1) DB15 to DB00, P1, P_CRCA (Data buses)
The 16-bit SCSI system uses a bidirectional data bus consisting of two-byte data and two odd
parity bits.
15
MSB (2 ): DB15, LSB (2
The data bus is used to transfer a command, data, a status, or a message in the INFORMATION
TRANSFER phase. However, DB15 to DB08 and P1 are used for data transfer only. The data is
transferred only after the WIDE DATA TRANSFER REQUEST or PARALLEL PROTOCOL
REQUEST message has been exchanged and the 16-bit data transfer mode has been established
between the INIT and TARG.
In the ARBITRATION phase, the data bus is used to send a SCSI ID to determine the bus
arbitration priority. In the SELECTION or RESELECTION phase, the data bus is used to send a
SCSI ID of the INIT and TARG. Figure 1.3 shows the relationship between the data buses and
SCSI IDs.
DB15 DB14
SCSI ID #14
SCSI ID #15
(a) DB15 to 0
Sixteen data-bit signals that form the 16-bit DATA BUS.
(b) DB7 to 0
Eight data-bit signals that form the 16-bit DATA BUS.
(c) P1 (ST DATA phase)
A signal sourced by the SCSI device driving the data bus during ST DATA phases. This signal is
associated with the DB(15-8) signals and is used to detect the presence of an odd number of bit
errors within the byte. The parity bit is driven such that the number of logical ones in the byte plus
the parity bit is odd.
16
0
): DB00
(16 bit SCSI)
DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
SCSI ID #5
SCSI ID #6
SCSI ID #7
SCSI ID #8
SCSI ID #9
Figure 1.3 DATA BUS and SCSI ID
SCSI ID #0
SCSI ID #1
SCSI ID #2
SCSI ID #3
SCSI ID #4
Data bus
C141-C015