Z5380 Datasheet, PDF (8/37 Pages) Zilog, Inc. – SMALL COMPUTER SYSTEM INTERFACE (SCSI)

ZILOG
Z5380 SCSI
FUNCTIONAL DESCRIPTION (Continued)
Target Command Register. Address 3 (Read/Write). When
connected as a target device, the Target Command Reg-
ister (Figure 9) allows the CPU to control the SCSI Bus
Information Transfer phase and/or to assert /REQ by writ-
ing this register. The Target Mode bit (Mode Register, bit
6) must be True (1) for bus assertion to occur. The SCSI
Bus phases are described in Table 2.
Address: 3
(Read/Write)
D7 D6 D5 D4 D3 D2 D1 D0
Assert I//O
Assert C//D
Assert /MSG
Assert /REQ
"X"
Figure 9. Target Command Register
Table 2. SCSI Information Transfer Phases
Bus Phase
Assert
I//O
Assert
C//D
Assert
/MSG
Data Out
0
0
0
Unspecified
0
0
1
Command
0
1
0
Message Out
0
1
1
Data In
Unspecified
Status
Message In
1
0
0
1
0
1
1
1
0
1
1
1
Initiator device can use this register to determine the
current bus phase and to poll /REQ for pending data
transfers. This register may also be used to determine why
a particular interrupt occurred. Figure 10 describes the
Current SCSI Bus Status Register.
Address: 4
(Read Only)
D7 D6 D5 D4 D3 D2 D1 D0
/DBP
/SEL
I//O
C//D
/MSG
/REQ
/BSY
/RST
Figure 10. Current SCSI Bus Status Register
Select Enable Register. Address 4 (Write Only). The
Select Enable Register (Figure 11) is a write-only register
which is used as a mask to monitor a signal ID during a
selection attempt. The simultaneous occurrence of the
correct ID bit, /BSY False, and /SEL True causes an
interrupt. This interrupt can be disabled by resetting all bits
in this register. If the Enable Parity Checking bit (Mode
Register, bit 5) is active (1), parity is checked during
selection.
Address: 4
(Write Only)
D7 D6 D5 D4 D3 D2 D1 D0
When connected as an Initiator with DMA Mode bit True, if
the phase lines (I//O, C//D, and /MSG) do not match the
phase bits in the Target Command Register, a phase
mismatch interrupt is generated when /REQ goes active.
To send data as an Initiator, the Assert I//O, Assert C//D,
and Assert /MSG bits must match the corresponding bits
in the Current SCSI Bus Status Register. The Assert /REQ
bit (bit 3) has no meaning when operating as an Initiator.
Bits 4, 5, 6, and 7 are not used.
Current SCSI Bus Status Register. Address 4 (Read
Only). The Current SCSI Bus Register is a read-only
register which is used to monitor seven SCSI Bus control
signals, plus the Data Bus parity bit. For example, an
/DB0
/DB1
/DB2
/DB3
/DB4
/DB5
/DB6
/DB7
Figure 11. Select Enable Register
8
PS009101-0201
PS97SCC0100