AM79C974 datasheet(85/153 Pages) AMD

85 / 153 page

P R E L I M I N A R Y

AMD

Am79C974

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

message. The

ATN signal is asserted before

deasserting the

ACK signal.

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

(STIMREG),

Control

One

(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

(STIMREG),

Control

One

(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

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