Z
ILOG
Bus and Status Register.
Address 5
(Read Only). The
Bus and Status Register (Figure 12) is a read-only register
which can be used to monitor the remaining SCSI control
signals not found in the Current SCSI Bus Status Registers
(/ATN and /ACK), as well as six other status bits. The
following describes each bit of the Bus and Status Register
individually.
Address: 5
(Read Only)
Z5380 SCSI
Bit 5.
Parity Error.
Bit 5 is set if a parity error occurs during
a data receive or a device selection. The Parity Error bit can
only be set (1) if the Enable Parity Check bit (Mode
Register, bit 5) is active (1). This bit may be cleared by
reading the Reset Parity/Interrupt Register.
Bit 6.
DMA Request.
The DMA Request bit allows the CPU
to sample the output pin DRQ. DRQ can be cleared by
asserting /DACK or by resetting the DMA Mode bit (bit 1)
in the Mode Register. The DRQ signal does not reset when
a phase-mismatch interrupt occurs.
Bit 7.
End of DMA Transfer.
The End of DMA Transfer bit
is set if /EOP, /DACK, and either /IOR or /IOW are simulta-
neously active for at least 100 ns. Since the /EOP signal
can occur during the last byte sent to the Output Data
Register, the /REQ and /ACK signals should be monitored
to ensure that the last byte has been transferred. This bit is
reset when the DMA Mode bit is reset (0) in the Mode
Register.
Input Data Register.
Address 6
(Read Only). The input
Data Register (Figure 13) is a read-only register that is
used to read latched data from the SCSI Bus. Data is
latched either during a DMA Target receive operation
when /ACK goes active or during a DMA Initiator receive
when /REQ goes active. The DMA Mode bit (bit 1) must be
set before data can be latched in the Input Data Register.
This register is read under DMA control using /IOR and
/DACK. Parity is optionally checked when the Input Data
Register is loaded.
Address: 6
(Read Only)
D7 D6 D5 D4 D3 D2 D1 D0
/ACK
/ATN
Busy Error
Phase Match
Interrupt Request Active
Parity Error
DMA Request
End of DMA
Figure 12. Bus and Status Register
Bit 0.
/ACK. Bit 0 reflects the condition of the SCSI Bus
control signal /ACK. This signal is normally monitored by
the Target device.
Bit 1.
/ATN.
Bit 1 reflects the condition of the SCSI Bus
control signal /ATN. This signal is normally monitored by
the Target device.
Bit 2.
Busy Error.
The Busy Error bit is active if an unex-
pected loss of the /BSY signal has occurred. This latch is
set whenever the Monitor Busy bit (Mode Register, bit 2) is
True and /BSY is False. An unexpected loss of /BSY
disables any SCSI outputs and resets the DMA Mode bit
(Mode Register, bit 1).
Bit 3.
Phase Match.
The SCSI signals /MSG, C//D, and
I//O, represent the current information Transfer phase. The
Phase Match bit indicates whether the current SCSI Bus
phase matches the lower 3 bits of the Target Command
Register. Phase Match is continuously updated and is only
significant when operating as a Bus Initiator. A phase
match is required for data transfers to occur on the SCSI
Bus.
Bit 4.
Interrupt Request ACTIVE.
Bit 4 is set if an enabled
interrupt condition occurs. It reflects the current state of the
IRQ output and can be cleared by reading the Reset Parity/
Interrupt Register.
D7 D6 D5 D4 D3 D2 D1 D0
/DB0
/DB1
/DB2
/DB3
/DB4
/DB5
/DB6
/DB7
Figure 13. Input Data Register
PS97SCC0100
PS009101-0201
9
ZILOG [ ZILOG, INC. ]
