SPI-3—also known as Ultra3 or Ultra160 SCSI—builds on the previous standard and doubles the speed again to Fast-80DT (double transition). This results in a maximum throughput of 160MBps. The main features added to SPI-3 (Ultra3) are

• DT (double transition) clocking

• Cyclic redundancy check (CRC)

• Domain validation

• Packetization

• Quick Arbitrate and Select (QAS)

Double transition clocking sends data on both the rising and falling edges of the REQ/ACK clock. This enables Ultra3 SCSI to transfer data at 160MBps, while still running at a bus clock rate of 40MHz. This mode is defined for 16-bit wide bus use only.

Cyclic redundancy checking (CRC) is a form of error checking incorporated into Ultra3 SCSI. Previous versions of SCSI used simple parity checking to detect transmission errors. CRC is a much more robust form of error-detection capability that is far superior for operation at higher speeds.

Domain validation allows better negotiation of SCSI transfer speeds and modes. With prior SCSI versions, when the bus is initialized, the host adapter sends an INQUIRY command at the lowest 5MHz speed to each device to determine which data-transfer rate the device can use.

The problem is that, even though both the host adapter and device might support a given speed, there is no guarantee that the interconnection between the devices will reliably work at that speed. If a problem occurs, the device becomes inaccessible.

With domain validation, after a maximum transfer speed is negotiated between the host and device, it is then tested at that rate. If errors are detected, the rate is stepped down until the connection tests error-free. This is similar to how modems negotiate transmission speeds before communicating and will go a long way toward improving the flexibility and perceived reliability of SCSI.

Packetization is a protocol that enables information to be transferred between SCSI devices in a much more efficient manner. Traditional parallel SCSI uses multiple bus phases to communicate different types of information between SCSI devices: one for command information, two for messages, one for status, and two for data.

In contrast, packetized SCSI communicates all this information by using only two phases: one for each direction. This dramatically reduces the command and protocol overhead, especially as higher and higher speeds are used.

Packetized SCSI is fully compatible with traditional parallel SCSI, which means packetized SCSI devices can reside on the same bus as traditional SCSI devices.

As long as the host adapter supports the packetization, it can communicate with one device using packets and another using traditional protocol. Not all Ultra3 or Ultra160 SCSI devices include packetization support, however. Ultra3 devices that support packetization typically are referred to as Ultra160+ SCSI.

Quick Arbitrate and Select (QAS) is a feature in Ultra3 SCSI that reduces arbitration time by eliminating bus free time. QAS enables a device to transfer control of the bus to another device without an intervening BUS FREE phase. SCSI devices that support QAS report that capability in the INQUIRY command.

Ultra160 and Ultra160+

Because the five main new features of Ultra3 SCSI are optional, drives could claim Ultra3 capability and not have a consistent level of functionality. To ensure truth in advertising and a minimum level of performance, a group of manufacturers got together and created a substandard within Ultra3 SCSI that requires a minimum set of features.

These are called Ultra160 and Ultra160+ because both indicate 160MBps throughput. These new substandards are not an official part of the SPI standard. Even so, they do guarantee that certain specifications will be met and certain performance levels will be attained.

Ultra160 is a specific implementation of Ultra3 (SPI-3) SCSI that includes the first three additional features of Ultra3 SCSI:

• Fast-80DT clocking for 160MBps operation

• CRC

• Domain validation

Ultra160 SCSI runs in LVD mode and is backward compatible with all Ultra2 SCSI (LVD) devices. The only caveat is that no SE devices must be on the bus. When Ultra2 and Ultra160 (Ultra3) devices are mixed, each device can operate at its full-rated speed independent of the other.

The bus will dynamically switch from single- to double-transition mode to support the differences in speeds. Ultra160+ adds the other two features, ensuring a full implementation of Ultra3:

• Packetization

• Quick Arbitrate and Select

With Ultra160 and Ultra160+, a known level of functionality ensures that a minimum level of performance will be met. Ultra160+ SCSI is the highest-performance PC-level storage interface and is best suited for high-traffic environments, such as high-end network servers or workstations. The adaptability and scalability of the interface enables high performance with high reliability.