am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 86

• Manufacturer Part Number: am79c974
• Description: Pcnettm-scsi Combination Ethernet And Scsi Controller For Pci Systems
• Manufacturer: Advanced Micro Devices

into the FIFO before issuing this command. This com-
mand will be terminated early in the following situations:
A Successful Operation/Service Request interrupt is
generated
successfully.
Select With ATN and Stop Steps Command
(Command Code 43h/C3h)
The Select with ATN and Stop Steps Command is used
by the Initiator to send messages with lengths other than
1 or 3 bytes. When this command is issued, the device
executes the Selection process, transfers the first mes-
sage byte, then STOPS the sequence. ATN is not de-
asserted at this time, allowing the Initiator to send
additional message bytes after the ID message. To
send these additional bytes, the Initiator must write the
transfer counter with the number of bytes which will fol-
low, then issue a Transfer Information Command.
(Note: the Target is still in the Message Out phase when
this command is issued). ATN will remain asserted until
the Current Transfer Count Register decrements
to zero.
The SCSI Timeout Register (STIMREG), Control Regis-
ter One (CNTLREG1), and the SCSI Destination ID
Register (SDIDREG) must be set to the proper values
before the Initiator issues this command. This command
will be terminated early if the STIMREG times out or if
the Target does not go to the Message Out Phase fol-
lowing the Selection Phase.
Enable Selection/Reselection Command
(Command Code 44H/C4H)
The Enable Selection/Reselection Command is used to
respond to a bus-initiated Selection or Reselection.
Upon disconnecting from the bus the Selection/
Reselection circuit is automatically disabled by the de-
vice. This circuit must be enabled for the Am79C974 to
respond to subsequent reselection attempts and the En-
able Selection/Reselection Command is issued to do
that. This command is normally issued within 250 ms
(select/reselect timeout) after the device disconnects
from the bus. If DMA is enabled, the device loads the
received data to the buffer memory. If the DMA is dis-
abled, the received data stays in the FIFO.
Disable Selection/Reselection Command
(Command Code 45H)
The Disable Selection/Reselection Command is used
by the Target to disable response to a bus-initiated
86
The SCSI Timeout Register times out
The Target does not go to the Message Out Phase
following the Selection Phase
The Target exits the Message Phase early
The Target does not go to the Command Phase
following the Message Out Phase
The Target exits the Command Phase early
AMD
when
this
command
is
completed
P R E L I M I N A R Y
Am79C974
Reselection. When this command is issued before a
bus-initiated Selection or Reselection is in progress, it
resets the internal state bits previously set by the Enable
Selection/Reselection Command. The device also gen-
erates a Successful Operation interrupt to the proces-
sor. If however, this command is issued after a
bus-initiated Selection/Reselection has begun, this
command and all incoming commands are ignored
since the Command Register (CMDREG) is held reset.
The Am79C974 also generates a Selected or
Reselected interrupt when the sequence is complete.
Select With ATN 3 Steps Command
(Command Code 46h/C6h)
The Select with ATN3 Steps Command is used by the
Initiator to select a Target. This command is similar to
the Select with ATN Steps Command, except that it
sends exactly three message bytes. When this com-
mand is issued the Am79C974 arbitrates for control of
the SCSI bus. When the device wins arbitration, it se-
lects the Target device with the ATN signal asserted and
transfers the Command Descriptor Block (CDB) and
three message bytes. Before issuing this command the
SCSI Timeout Register (STIMREG), Control Register
One (CNTLREG1), and the SCSI Destination ID Regis-
ter (SDIDREG) must be set to the proper values. If DMA
is enabled, the Start Transfer Count Register
(STCREG) must be set to the total length of the com-
mand. If DMA is not enabled, the data must be loaded
into the FIFO before issuing this command. This com-
mand will be terminated early in the following situations:
A Successful Operation/Service Request interrupt is
generated
successfully.
General Commands
No Operation Command
(Command Code 00h/80h)
The No Operation Command administers no operation,
therefore an interrupt is not generated upon completion.
This command is issued following the Reset Device
Command to clear the Command Register (CMDREG).
A No Operation Command in the DMA mode may be
used to verify the contents of the Start Transfer Count
Register (STCREG). After the STCREG is loaded with
the transfer count and a DMA No Operation Command
is issued, reading the Current Transfer Count Register
(CTCREG) returns the transfer count value.
The SCSI Timeout Register times out
The Target does not go to the Message Out Phase
following the Selection Phase
The Target removes Command Phase early
The Target does not go to the Command Phase
following the Message Out Phase
The Target exits the Command Out Phase early
when
this
command
is
executed