Z53C80 Datasheet, PDF (14/40 Pages) Zilog, Inc. – SMALL COMPUTER SYSTEM INTERFACE (SCSI)

ZILOG
FUNCTIONAL DESCRIPTION (Continued)
D7
D0
0 0 0XXX 0X
/DBP
/SEL
I//O
C//D
/MSG
/REQ
/BSY
/RST
Z53C80 SCSI
D7
D0
0 1 1XXX 0X
/DBP
/SEL
I//O
C//D
/MSQ
/REQ
/BSY
/RST
Figure 16. Current SCSI Bus Status Register
Figure 18. Current SCSI Bus Status Register
End of Process (EOP) Interrupt. An End Of Process
signal (EOP) which occurs during a DMA transfer (DMA
Mode True) will set the End of DMA Status bit (Bus and
Status Register bit 7) and will optionally generate an
interrupt if Enable EOP Interrupt bit (Mode Register, bit 3)
is True. The /EOP pulse will not be recognized (End of DMA
bit set) unless /EOP, /DACK, and either /RD or /WR are
concurrently active for at least 50 ns. DMA transfers can
still occur if /EOP was not asserted at the correct time. This
interrupt is disabled by resetting the Enable EOP Interrupt
bit.
The proper values for the Bus and Status Register and the
Current SCSI Bus Status Register for this interrupt are
shown in Figures 17 and 18.
D7
D0
1 0 0 1 0 0 0X
/ACK
/ATN
Busy Error
Phase Match
Interrupt Request Active
Parity Error
DMA Request
End of DMA
The End of DMA bit is used to determine when a block
transfer is complete. Receive operations are complete
when there is no data left in the chip and no additional
handshakes occurring. The only exception to this is
receiving data as an Initiator and the Target opts to send
additional data for the same phase. In this /REQ goes
active and the new data is present in the Input Data
Register. Since a phase-mismatch interrupt will not occur,
/REQ and /ACK need to be sampled to determine that the
Target is attempting to send more data.
For send operations, the End of DMA bit is set when the
DMA finishes its transfers, but the SCSI transfer may still be
in progress. If connected as a Target, /REQ and /ACK
should be sampled until both are False. If connected as an
Initiator, a phase change interrupt is used to signal the
completion of the previous phase. It is possible for the
Target to request additional data for the same phase. In
this case, a phase change will not occur and both /REQ
and /ACK are sampled to determine when the last byte was
transferred.
SCSI Bus Reset. The SCSI generates an interrupt when
the /RST signal transitions to True. The device releases all
bus signals within a bus-clear delay of this transition. This
interrupt also occurs after setting the Assert /RST bit
(Initiator Command Register, bit 7). This interrupt cannot
be disabled. (Note: /RST is not latched in bit 7 of the
Current SCSI Bus Status Register and is not active when
this port is read. For this case, the Bus Reset interrupt is
determined by default.)
Figure 17. Bus and Status Register
The proper values for the Bus and Status Register and the
Current SCSI Bus Status Register are displayed in Figures
19 and 20, respectively.
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PS97SCC0200