AM53C96 datasheet(35/63 Pages) AMD

35 / 63 page

P R E L I M I N A R Y

AMD

Am53C94/Am53C96

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 it expected from the target.

When sending data to the SCSI bus, the FIFO is loaded

with null bytes and these bytes are sent out to the SCSI

bus. DMA has to be enabled when pad bytes are trans-

ferred to the SCSI bus. No actual DMA requests are

made but the device uses the Current Transfer Count

(CTCREG)

00H–01H

terminate

the

transfer.

When receiving data from the SCSI bus, the device will

receive the pad bytes and place them on the top of the

FIFO and unload them from the bottom of the FIFO.

The command terminates under the same condition as

the Information Transfer Command, only the device

does not keep the

ACK signal asserted during the last

byte of the Message In phase. If this command termi-

nates prematurely, due to a disconnect or a phase

change, before the CTCREG decrements to zero, the

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:

s If any select with

ATN command is issued and the

arbitration is won.

s An initiator needs the target’s attention to send a

message. The

ATN signal is asserted before deas-

serting 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.

Target Commands

Target commands are executed by the device when it is

in the target mode. If the device is not in the target mode

and a target command is received the device will ignore

the command, generate an illegal command interrupt

and clear the Command Register (CMDREG) 03H.

A SCSI bus reset during any target command will cause

the device to abort the command sequence , flag a SCSI

bus reset interrupt (if the interrupt is enabled) and dis-

connect from the SCSI bus.

Normal or successful completion of a target command

will cause a Successful Operation interrupt to be

flagged. If the

ATN signal is asserted during a target

command sequence the Service Request bit is asserted

in the Interrupt Status Register (INSTREG) 05H. If the

ATN signal is asserted when the device is in an idle state

a Service Request interrupt will be generated, the Suc-

cessful Operation bit in the INSTREG will be reset and

the CMDREG cleared.

Send Message Command

(Command Code 20H/A0H)

The Send Message Command is used by the target to

inform the initiator to receive a message. The SCSI bus

phase lines are set to the Message In Phase and mes-

sage bytes are transferred from the device FIFO to the

buffer memory.

Send Status Command

(Command Code 21H/A1H)

The Send Status Command is used by the target to in-

form the initiator to receive status information. The SCSI

bus phase lines are set to the Status Phase and status

bytes are transferred from the target device to the initia-

tor device.

Send Data Command (Command Code 22H/A2H)

The Send Data Command is used by the target to inform

the initiator to receive data bytes. The SCSI bus phase

lines are set to the Data-In Phase and data bytes are

transferred from the target device to the initiator device.

Disconnect Steps Command

(Command Code 23H/A3H)

The Disconnect Steps Command is used by the target to

disconnect from the SCSI bus. This command consists

of two steps. The first step consists of sending two bytes

of the Save Data Pointers commands by the target in the

Message In Phase. In the second step the target discon-

nects from the SCSI bus. Successful Operation and Dis-

connected bits are set in the Interrupt Status Register

(INSTREG) 05H upon command completion. If

ATN sig-

nal is asserted by the initiator then Successful Operation

and Service Request bits are set in the INSTREG, the

CMDREG is cleared and Disconnect Steps Command

terminates without disconnecting.

AM53C96 Datasheet(PDF) 35 Page - Advanced Micro Devices

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