SCSI FAQs


Q: What does SCSI stand for?
A: SCSI is an acronym for Small Computer System Interface.

Q: How do you pronounce SCSI?
A: "Skuzzy"

Q: What is SCSI?
A: SCSI is a type of data bus allowing for the transfer of data between a computer and peripheral devices. It was the first intelligent data bus relieving the CPU from the burden of processing I/Os between the computer and its peripherals.

Q: How do I choose between Fibre Channel and SCSI technology?
A: If shared storage is required then Fibre Channel is the technology of choice. If transfer speeds are more important than sharing storage, then SCSI is preferable.

Q: Is SCSI a proven technology?
A: Yes. In 1981 SCSI standardization began with the first products being introduced in 1983. Development cycles for SCSI products is short. Additionally, SCSI maintains a high performance standard making it the dominant interface for medium and large systems.

Q: Is it easy to connect devices to a system with SCSI?
A: SCSI makes it easy to add/connect peripherals to a system without changing an entire configuration and SCSI preserves current investments and also maintains a platform for future growth.

Q: What are some typical SCSI devices?
A: Types of SCSI devices include scanners, CD-ROMs, DVD, hard disk drives, tape drives, plotters, etc.

Q: What Environments Use SCSI?
A: Some typical environments include prepress, imaging, digital video and audio, data collection, databases, applications and file servers and other bus intensive applications.

Q: What is an initiator?
A: Device that starts or initiates communication.

Q: What is a target?
A: A device that receives communication from the initiator and responds.

Q: What does Plug and Play mean?
A: Devices conforming to Plug and Play SCSI specification - means that devices should work as soon as they are connected.

Q: What is a SCSI ID?
A: SCSI ID is a unique identification/addressing for each device on the SCSI bus. Each SCSI device needs to have a unique ID, including the host adapter. The factory default for ATTO host adapters is to use ID 7. ID 7 has the highest priority, followed by 6, 5..0, 15,14...8. It is recommended to keep the host adapter at ID 7, but this can be changed using the ExpressPro-Tools application program if necessary. Since a dual channel host adapter has two independent SCSI buses, there can be devices with the same IDs on different buses.

Q: What is a LUN?
A: A LUN (Logical Unit Number) is a 2nd level of device identification/addressing. It is a sub-set of SCSI ID's - there are 7 LUNs per SCSI ID.

Q: What is Double Transition Clocking?
A: Double Transition Clocking enables speeds up to 160 MB/sec. per channel while guaranteeing compatibility with older SCSI devices.

Q: What is Cyclical Redundancy Checking (CRC)?
A: CRC improves the reliability of SCSI data transmission through enhanced detection of communication errors.

Q: What is Domain Validation?
A: Domain Validation detects the configuration of the SCSI bus (domain) and automatically tests and adjusts SCSI transfer rate to optimize interoperability.

Q: What is packetization?
A: Information is sent in packets as opposed to multiple I/Os, reducing SCSI overhead and providing increased performance.

Q: What are the benefits of Quick Arbitration (QAS)?
A: QAS provides faster arbitration resulting in a reduction of disconnect/reconnect time on the SCSI bus.

Q: What types of SCSI host adapters are currently available from ATTO Technology?
A: ATTO offers a variety of high-performance SCSI host bus adapters (HBAs).

Q: How can I mix different SCSI devices on the same host adapter bus?
A: Since Ultra3, Ultra2, and Ultra SCSI are all backward compatible, simply attach the devices to the adapter and use the type of terminator that matches your slowest device.

Q: When is a dual channel adapter preferred over single channel?
A: There are a number of reasons to use a dual channel host adapter. Since, each channel is independent, you can put slower devices on channel one and the faster devices on channel two without performance degradation. Essentially, it is combining two single channel HBAs while utilizing only 1 PCI slot. You can put drives on both channels of the adapter, create a RAID Level 0 stripe group with ATTO's ExpressRAID software and stripe across channels for maximum performance. The same doubling of performance applies for Ultra2 (2 buses at 80 MB/sec. equals 160 MB/sec.) and Ultra3 SCSI host adapters Ultra2 (2 buses at 160 MB/sec. equals 320 MB/sec.)

Q: What is ATTO's Vpath Technology?
A: ATTO's exclusive Vpath Technology allows users of the ExpressPCI UL3S to own the most flexible single channel card available. This adaptability is achieved through the use of variable paths (hence Vpath) that can take full advantage of Ultra3 speeds and legacy devices simultaneously. The UL3S with Vpath Technology allows users to attach Ultra3 drives externally and slower legacy devices internally, or vice versa, without sacrificing performance of high-speed devices to run slower legacy equipment.
Top