am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 85

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

switch to the Message In phase or if the Target discon-
nects from the SCSI bus. This command does not utilize
the Internal State Register ((B)+18h).
Message Accepted Command
(Command Code 12h)
The Message Accepted Command is used to release
the ACK signal. This command is normally used to com-
plete a Message In handshake. Upon execution of this
command the device generates a Service Request in-
terrupt after REQ is asserted by the Target.
After the device has received the last byte of message, it
keeps the ACK signal asserted. This allows the device
to either accept or reject the message. To accept the
message, Message Accepted Command is issued. To
reject the message the ATN signal must be asserted
(with the help of the Set ATN Command) before issuing
the Message Accepted Command. In either case, the
Message Accepted Command has to be issued to re-
lease the ACK signal.
Transfer Pad Bytes Command
(Command Code 18h/98h)
The Transfer Pad Bytes Command is used to recover
from an error condition. This command is similar to the
Information Transfer Command, only the information
bytes consists of null data. It is used when the Target ex-
pects more data bytes than the Initiator has to send. It is
also used when the Initiator receives more information
than expected from the Target.
When sending data to the SCSI bus, the SCSI FIFO is
loaded with null bytes which are sent out to the SCSI
bus. Although an actual DMA request is not made, DMA
interface must be enabled when pad bytes are transmit-
ted since the Am79C974 uses the Current Transfer
Count Register to terminate transmission.
This command terminates under the same conditions as
the Information Transfer Command, but the device does
not keep the ACK signal asserted during the last byte of
the Message In phase. Should this command terminate
prematurely due to a Disconnect or a phase change be-
fore the Current Transfer Count Register decrements to
zero, the SCSI FIFO may contain residual Pad bytes.
Set ATN Command
(Command Code 1Ah)
The Set ATN Command is used to drive the ATN signal
active on the SCSI bus. An interrupt is not generated at
the end of this command. The ATN signal is deasserted
before asserting the ACK signal during the last byte of
the Message Out phase.
Note: The ATN signal is asserted by the device without
this command in the following cases:
If any select with ATN command is issued and the
arbitration is won.
An Initiator needs the Target’s attention to send a
P R E L I M I N A R Y
Am79C974
Reset ATN Command
(Command Code 1Bh)
The Reset ATN Command is used to deassert the ATN
signal on the SCSI bus. An interrupt is not generated at
the end of this command. This command is used only
when interfacing with devices that do not support the
Common Command Set (CCS). These older devices do
not deassert their ATN signal automatically on the last
byte of the Message Out phase. This device does deas-
sert its ATN signal automatically on the last byte of the
Message Out phase.
Idle State Commands
The Idle State Commands can be issued to the device
only when the device is disconnected from the SCSI
bus. If these commands are issued to the device when it
is logically connected to the SCSI bus, the commands
are ignored, an Invalid Command interrupt is generated,
and the Command Register (CMDREG) is cleared.
Select Without ATN Steps Command
(Command Code 41h/C1h)
The Select without ATN Steps Command is used by the
Initiator to select a Target. When this command is is-
sued, the device arbitrates for the control of the SCSI
bus. When the device wins arbitration, it selects the Tar-
get device and transfers the Command Descriptor Block
(CDB). Before issuing this command the SCSI Timeout
Register
(CNTLREG1), and the SCSI Destination ID Register
(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 command. If DMA is
not enabled, the data must be loaded into the FIFO be-
fore issuing this command. This command will be termi-
nated early if the SCSI Timeout Register times out, if the
Target does not go to the Command Phase following the
Selection Phase, or if the Target exits the Command
Phase prematurely. A Successful Operation interrupt
will be generated following normal command execution.
Select With ATN Steps Command
(Command Code 42h/C2h)
The Select with ATN Steps Command is used by the In-
itiator to select a Target. When this command is issued,
the device arbitrates for the control of the SCSI bus.
When the device wins arbitration, it selects the Target
device with the ATN signal asserted and transfers the
Command Descriptor Block (CDB) and a one byte mes-
sage. Before issuing this command the SCSI Timeout
Register
(CNTLREG1) and the SCSI Destination ID Register
(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 command and mes-
sage. If DMA is not enabled, the data must be loaded
message. The ATN signal is asserted before
deasserting the ACK signal.
(STIMREG),
(STIMREG),
Control
Control
Register
Register
AMD
One
One
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