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Kyle Brost

Kyle Brost. SCSI – Small Computer System Interface. What it is.

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Kyle Brost

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  1. Kyle Brost SCSI – Small Computer System Interface

  2. What it is • SCSI provides a high-speed, intelligent interface that allows an easy connection for up to 16 devices (8 devices for Narrow SCSI) on a single bus. These devices may be hard disks, floppy disks, CDs, tape drives, printers and scanners to name a few. Peripherals may be mounted in the computer or in an external enclosure. Total SCSI cable length is dependent on the type of SCSI.

  3. The Small Computer System Interface is a high-speed, intelligent peripheral I/O bus with a device independent protocol. It allows different peripheral devices and hosts to be interconnected on the same bus. Depending on the type of SCSI, you may have up to 8 or 16 devices connected to the SCSI bus. The number of devices can be dramatically expanded by the use of LUNs (Logic Unit Numbers). There must be at least one initiator (usually a host) and one target (a peripheral device) on a bus. There is a large variety of peripheral devices available for SCSI, including hard disk drives, floppy drives, CDs, optical storage devices, tape drives, printers and scanners

  4. SCSI is used to connect peripherals to an computer. It allows you to connect hard disks, tape devices, CD-ROMs, CD-R units, DVD, scanners, printers and many other devices. SCSI is in opposite to IDE/ATA very flexible. Today SCSI is most often used servers and other computers which require very good performance. IDE/ATA is more popular due to the fact that IDE/ATA devices tend to be cheaper.

  5. How it works, why it works • All the SCSI types work according to a mechanism – using controllers and cables to connect with the devices. The SCSI controllers are what coordinate the computer and SCSI buses. These SCSI controllers or host adapters can be in the form of a card or built-in form in the motherboard. The software needed to access and controlled vices is placed on the controller and is termed as SCSI BIOS. In order for the SCSI devices to work properly and in an orderly manner, each of the devices is assigned a unique identifier (UI). Now while the external devices attach themselves to the controller using a thick round cable, internal devices connect to the SCSI controller using a ribbon cable.

  6. Various SCSI variations use different connectors with varying pin numbers such as 50, 68 or 80 pins. Once all the devices are installed on the bus and allotted their own UI, the end of the bus should be closed; else the signal in the bus might reflect back interfering with the communication between other devices and controller.

  7. Basic Info • Small Computer System Interface -SCSI-is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives, although not all controllers can handle all devices.

  8. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements. SCSI is an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner.

  9. Up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host. SCSI uses handshake signals between devices, SCSI-1, SCSI-2 have the option of parity error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data are error checked by a CRC32 checksum. The SCSI protocol defines communication from host to host, host to a peripheral device, peripheral device to a peripheral device.

  10. However most peripheral devices are exclusively SCSI targets, incapable of acting as SCSI initiators—unable to initiate SCSI transactions themselves. Therefore peripheral-to-peripheral communications are uncommon, but possible in most SCSI applications. The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as a SCSI target.

  11. SCSI has several benefits. It's fairly fast, up to 320 megabytes per second. It's been around for more than 20 years and it's been thoroughly tested, so it has a reputation for being reliable. Like Serial ATA and FireWire, it lets you put multiple items on one bus. SCSI also works with most computer systems. • However, SCSI also has some potential problems. It has limited system BIOS support, and it has to be configured for each computer. There's also no common SCSI software interface. Finally, all the different SCSI types have different speeds, bus widths and connectors

  12. History • SCSI was derived from "SASI", the "Shugart Associates System Interface", developed in 1978 and publicly disclosed in 1981. Since its standardization in 1986, SCSI has been commonly used in the Amiga, Apple Macintosh and Sun Microsystems computer lines and PC server systems. Apple started using Parallel ATA (also known as IDE) for its low-end machines with the Macintosh Quadra 630 in 1994, and added it to its high-end desktops starting with the Power Macintosh G3 in 1997. Apple dropped on-board SCSI completely (in favor of IDE and FireWire) with the (Blue & White) Power Mac G3 in 1999. Sun has switched its lower end range to Serial ATA (SATA).

  13. Commodore included a SCSI interface on the Amiga 3000/3000T systems and it was an add-on to previous Amiga 500/2000 models. Starting with the Amiga 600/1200/4000 systems Commodore switched to the IDE interface. SCSI has never been popular in the low-priced IBM PC world, owing to the lower cost and adequate performance of ATA hard disk standard. However, SCSI drives and even SCSI RAIDs became common in PC workstations for video or audio production.

  14. SCSI Types • SCSI has three basic specifications SCSI-1: The original specification developed in 1986, SCSI-1 is now obsolete. It featured a bus width of 8 bits and clock speed of 5 MHz. • SCSI-2: Adopted in 1994, this specification included the Common Command Set (CCS) -- 18 commands considered an absolute necessity for support of any SCSI device. It also had the option to double the clock speed to 10 MHz (Fast), double the bus width from to 16 bits and increase the number of devices to 15 (Wide), or do both (Fast/Wide). SCSI-2 also added command queuing, allowing devices to store and prioritize commands from the host computer. • SCSI-3: This specification debuted in 1995 and included a series of smaller standards within its overall scope. A set of standards involving the SCSI Parallel Interface (SPI), which is the way that SCSI devices communicate with each other, has continued to evolve within SCSI-3. Most SCSI-3 specifications begin with the term Ultra, such as Ultra for SPI variations, Ultra2 for SPI-2 variations and Ultra3 for SPI-3 variations. The Fast and Wide designations work just like their SCSI-2 counterparts. SCSI-3 is the standard currently in use.

  15. Wide SCSI is the term that is used for 16-bit SCSI. It can usually be identified by 68-pin connectors. From SCSI-2 until the SPI-3 document in SCSI-3, this term also applied to 32-bit SCSI. SPI-3 obsoletedthe 32-bit SCSI bus. Ultra SCSI, defined in the SPI-2 document of SCSI-3 offers a maximum data throughput of 20 Mbytes/sec for Narrow (8-bit) SCSI. Ultra Wide SCSI is the 16-bit version that offers 40 Mbytes/sec data transfers. Ultra Wide single-ended SCSI has a maximum cable length of 1.5 m (5 ft) with more than 4 active IDs and 3 m (10 ft) with 4 or fewer active IDs. Ultra Wide differential SCSI has a maximum cable length of 25 m (82 ft).

  16. Ultra 160 is defined in SPI-3. It offers data throughput of 80 Megatransfers/sec or 160 Mbytes/sec for Wide (16-bit) SCSI which is the only defined bus width. For this speed, clocking on both the rising and falling edges of the REQ and ACK clock is required. This is called Double Transition (DT) clocking. Also called Fast-80 or Ultra 3 SCSI. Ultra 160, also called Ultra 3 is backward compatible through the single-ended interface, if it is multimode Ultra 160. It is doubtful that anyone will build Ultra 160 devices that are not multimode. Remember that if a single-ended device is placed directly on a multimode Ultra 160 bus the entire bus will switch to the single-ended mode with its limitations on data throughput and cable length.

  17. Working • All of these SCSI types are parallel -- bits of data move through the bus simultaneously rather than one at a time. The newest type of SCSI, called Serial Attached SCSI (SAS), uses SCSI commands but transmits data serially. SAS uses a point-to-point serial connection to move data at 3.0 gigabits per second, and each SAS port can support up to 128 devices or expanders. • A SCSI controller coordinates between all of the other devices on the SCSI bus and the computer. Also called a host adapter, the controller can be a card that you plug into an available slot or it can be built into the motherboard. The SCSI BIOS is also on the controller. This is a small ROM or Flash memory chip that contains the software needed to access and control the devices on the bus.

  18. Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example, if the bus can support sixteen devices, their IDs, specified through a hardware or software setting, range from zero to 15. The SCSI controller itself must use one of the IDs, typically the highest one, leaving room for 15 other devices on the bus. • Internal devices connect to a SCSI controller with a ribbon cable. External SCSI devices attach to the controller in a daisy chain using a thick, round cable. (Serial Attached SCSI devices use SATA cables.) In a daisy chain, each device connects to the next one in line. For this reason, external SCSI devices typically have two SCSI connectors -- one to connect to the previous device in the chain, and the other to connect to the next device.

  19. The cable itself typically consists of three layers Inner layer: The most protected layer, this contains the actual data being sent. Media layer: Contains the wires that send control commands to the device. Outer layer: Includes wires that carry parity information, which ensures that the data is correct. Different SCSI variations use different connectors, which are often incompatible with one another. These connectors usually use 50, 68 or 80 pins. SAS uses smaller, SATA-compatible connectors. Once all of the devices on the bus are installed and have their own IDs, each end of the bus must be closed. If the SCSI bus were left open, electrical signals sent down the bus could reflect back and interfere with communication between devices and the SCSI controller. The solution is to terminate the bus, closing each end with a resistorcircuit. If the bus supports both internal and external devices, then the last device on each series must be terminated.

  20. Termination • Types of SCSI termination can be grouped into two main categories: passive and active. Passivetermination is typically used for SCSI systems that run at the standard clock speed and have a distance of less than 3 feet from the devices to the controller. Activetermination is used for Fast SCSI systems or systems with devices that are more than 3 feet from the SCSI controller. • SCSI also employs three distinct types of bussignaling, which also affect termination. Signaling is the way that the electrical impulses are sent across the wires. • Single-ended (SE): The controller generates the signal and pushes it out to all devices on the bus over a single data line. Each device acts as a ground. Consequently, the signal quickly begins to degrade, which limits SE SCSI to a maximum of about 10 ft. SE signaling is common in PCs.

  21. High-voltagedifferential (HVD): Often used for servers, HVD uses a tandem approach to signaling, with a data high line and a data low line. Each device on the SCSI bus has a signal transceiver. When the controller communicates with the device, devices along the bus receive the signal and retransmit it until it reaches the target device. This allows for much greater distances between the controller and the device, up to 80 ft. • Low-voltagedifferential (LVD): LVD is a variation on HVD and works in much the same way. The big difference is that the transceivers are smaller and built into the SCSI adapter of each device. This makes LVD SCSI devices more affordable and allows LVD to use less electricity to communicate. The downside is that the maximum distance is half of HVD -- 40 ft.

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