AM79C974 datasheet(83/153 Pages) AMD

83 / 153 page

P R E L I M I N A R Y

AMD

Am79C974

continue until the Current Transfer Count Register dec-

rements to zero.

Non-DMA commands do not modify the Current Trans-

fer Count Register and are unaffected by the value in the

Current Transfer Count Register. For non-DMA com-

mands, the number of bytes transmitted depends solely

on the operation in progress.

When non-DMA commands are used, the host com-

puter must use programmed I/O to transfer the data be-

tween the SCSI FIFO and the host memory.

Table 7. Summary of Commands

Initiator Commands

Information Transfer

Initiator Command Complete Steps

Message Accepted

–

Transfer Pad Bytes

–

Set

ATN*1A

–

Reset

ATN*1B

–

Idle State Commands

Select without

ATN Steps

Select with

ATN Steps

Select with

ATN and Stop Steps

Enable Selection/Reselection*

Disable Selection/Reselection

–

Select with

ATN3 Steps

General Commands

No Operation*

Clear FIFO*

–

Reset Device*

–

Reset SCSI Bus**

–

Non-

DMA

Mode

DMA

Mode

Code

(Hex.)

Command

Notes:

* These commands do not generate interrupt.

** An interrupt is generated when SCSI bus reset interrupt

reporting is not disabled (see Control Register1/DISR

bit6).

Command Stacking

The microprocessor may stack commands in the Com-

mand Register ((B)+0Ch) since it functions as a two-

byte deep FIFO. Non-DMA commands may not be

stacked, and commands which transfer data in oppos-

ing directions should not be stacked together; other-

wise, the results are unpredictable.

If DMA commands are queued together, the Start

Transfer Count must be written before the associated

command is loaded into the Command Register. Since

multiple interrupts can occur when commands are

stacked, it is recommended that the ENF bit in Control

phase bits in the SCSI Status Register ((B)+10h) at the

completion of a command. This allows the host to deter-

mine the phase of the interrupting command without

having to consider phase changes that occurred after

the stacked command began execution.

Note: Command stacking should only be used during

SCSI Data In or Data Out transfers.

Invalid Commands

When an illegal command is written to the Am79C974,

the Invalid Command Bit (Bit 6, Register (B)+14h) will be

set to ‘1’, and an interrupt will be generated to the host.

When this happens, the interrupt must be serviced be-

fore another command may be written to the Command

An Invalid command is defined as a command written to

the Am79C974 that is either not supported, not allowed

in the specified mode, or a command that has an unsup-

ported command mode.

The following conditions will also cause an Invalid Com-

mand interrupt to occur:

An Initiator Information Transfer, Transfer Pad, or

Command Complete is issued when

ACK is still

asserted.

A Selection command is issued with the DMA bit

enabled, if the Selection command was previously

issued with the DMA enabled.

Command Window

The window at the point where the Disable Selection/

Reselection command (45h/C5h) has been loaded into

the Command Register ((B)+0Ch), and before bus-initi-

ated Selection begins, has been eliminated. This pre-

vents a false Successful Operation Interrupt from being

generated when the Selection sequence continues to

completion after the Disable command has been

loaded.

Initiator Commands

Initiator commands are executed by the device when it

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

mode and an Initiator command is received the device

will ignore the command, generate an Invalid Command

interrupt and clear the Command Register.

Should the Target disconnect from the SCSI bus by

deasserting the

BSY signal line while the Am79C974

(Initiator) is waiting for the Target to assert

REQ, a Dis-

connected Interrupt will be issued 1.5 to 3.5 clock cycles

following

BSY going false.

Upon receipt of the last byte during Message In phase,

ACK will remain asserted to prevent the Target from is-

suing any additional bytes, while the Initiator decides to

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