Specifications

ATAPI For Streaming Tape QIC-157 Rev B

Implementor's Note: Implementors of ATAPI drivers which are intended to share a single cable

with a disk and disk driver should ensure that the device has completed any issued commands prior

to changing the DRV bit.

2.14.5. Default Timing

ATAPI Devices compatible with this specification shall support ATA mode 3 timing without

requiring the Host system to configure the ATAPI Device using any set features commands. ATAPI

Devices must therefore either be fast enough to always supply data at the maximum rate allowed by

Mode 3 or the ATAPI Device must be shipped with IORDY enabled.

ATAPI Devices shall revert to their default interface configuration on a Power On Reset or a

Hardware Reset.

[Implmentor’s Note: A low speed drive, Mode 0-2, may affect system performance when sharing the same cable with

hard disk drives capable of Mode 3 or faster data transfer timing.]

2.15. ATAPI Command Protocol and DSC Handling

2.15.1. ATAPI Media Access Commands and DSC

All ATAPI media access commands return a completion status with DSC set to 0 upon validation of

the packet command and transfer of any associated data. When the drive completes, it will set DSC

to 1 to indicate completion. This frees up the IDE bus so the Host can send commands to another

IDE device on the same cable. After DSC is set to 1, the Host can send a Request Sense to check for

any errors that might have occurred.

If another media access command is issued while DSC is 0, the device will go BSY until the

previous command is complete.

If an error condition exists upon receipt of a new command, the new command will be aborted and a

deferred error will be reported.

2.15.2. ATAPI Non-media Access Commands and DSC

ATAPI non-media commands and ATA commands (other than reset) are executed immediately, and

any error status (if needed) is set, before returning completion status. DSC is unaffected by these

commands.

2.15.3. ATAPI Read and Write Commands and DSC

ATAPI Read and Write commands use DSC to indicate buffer availability instead of command

completion. An initial Read or Write of 0 blocks is used to switch DSC handshake from completion

mode to buffer available mode. Any media access command other than read or write will switch

DSC back to completion mode.

Once DSC is in buffer available mode, DSC set to 1 indicates that a specified amount of data can be

transferred to/from a drive’s buffer without the overhead of tape I/O. The amount that can be

transferred is specified in the Continuous Transfer field in mode sense page 2Ah.