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Frequently Asked Questions on SCSI

Questions

  1. What does the term "SCSI" mean?
  2. What is SCSI?
  3. What can I do with SCSI?
  4. I seem to remember hearing the term SASI in the past. What is it?
  5. Does SCSI work in both directions?
  6. What are the differences between SCSI-1 and SCSI-2?
  7. What are the differences between SCSI-2 and SCSI-3?
  8. What is the difference between single-ended and differential SCSI?
  9. What is meant by "narrow" SCSI?
  10. What is meant by "wide" SCSI?
  11. What is HVD SCSI?
  12. What is Wide Ultra SCSI?
  13. What is LVD SCSI?
  14. What is "multimode LVD" or LVD/MSE?
  15. What are the benefits of LVD SCSI?
  16. Is LVD SCSI backward compatible?
  17. What is Ultra160 or U160 SCSI?
  18. I have heard of U160/m SCSI. What is it?
  19. Is Ultra160 SCSI backward compatible?
  20. Is Ultra 160 SCSI better than fibre channel?
  21. Is Ultra 160 SCSI better than EIDE?
  22. What is Fast-20 [or Fast-40 or Fast-80] SCSI?
  23. Can I connect an Ultra 2 Wide (LVD) disk to an Ultra Wide adapter?
  24. What is Double Transition clocking?
  25. What is "Domain Validation"?
  26. What is CRC?


Answers

(1) What does the term "SCSI" mean?

The term "SCSI" is an acronym for Small Computer System Interface. In the 1970s the name was appropriate. Today, SCSI is used for PCs, workstations, servers, mainframes, supercomputers.

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(2) What is SCSI?

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 to name a few. There are many implementations of SCSI starting with SCSI-1 to SCSI-2 to SCSI-3 including, Narrow, Wide, Fast, Ultra, Ultra-2 and Ultra160 SCSI. The SCSI specifications are approved and issued by ANSI and are developed by the X3T10 SCSI Committee.

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(3) What can I do with SCSI?

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.

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(4) I seem to remember hearing the term SASI in the past. What is it?

SASI is the acronym for Shugart Associates System Interface. It was developed in the 1970s by Shugart, at the time a dominant manufacturer of disk drives. It was meant to be an intelligent interface for disk drives only. Offering only 8-bit (Narrow), single-ended, asynchronous operation, by today's standards it was very slow (1.5 Mbytes per second). The standard connector for in-cabinet cabling is the non-shielded, 50-pin, female, low-density, connector having two rows of 25 pins each on 0.1 inch spacing. The standard connector for cabling outside the cabinet is the shielded, 50-pin, male, "centronics" type connector. In 1981 Shugart and NCR submitted SASI to the ANSI committee X3T9.2 as an open architecture I/O bus for disk drives. ANSI accepted the project, changed the name to Small Computer System Interface and added some major improvements to the specification. It was approved in 1986 by ANSI as document IEEE X3.131-1986. Today it is called SCSI-1. SASI is now long obsolete and, although many aspects of SCSI were backward compatible with SASI, it is very problematic.

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(5) Does SCSI work in both directions?

Yes. SCSI is a bi-directional bus and will not work at all if it does not work in both directions. That also means that SCSI expanders such as a single-ended (SE) to differential converter will work as a SE to differential or a differential to SE converter. In other words, it does not make any difference if the initiator is on the SE side or on the differential side of the expander.

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(6) What are the differences between SCSI-1 and SCSI-2?

The initial implementation of SCSI (now called SCSI-1) was designed primarily for Narrow (8-bit), single-ended, synchronous or asynchronous disk drives and was very limited relative to today's SCSI. It includes synchronous and asynchronous data transfers at speeds up to 5 Mbytes/sec. Only passive termination was defined. It did not include definitions of a device independent interface. The standard connectors are the familiar 50-pin, female, low-density (0.1 inch spacing), non-shielded connector (now termed the non-shielded Alternative 2, A-connector) for internal wiring and the equally familiar 50-pin, male, shielded "centronics" type connector for external wiring (now termed the shielded, Alternative 2, A-connector). This "centronics" type connector is frequently called the "SCSI-1 connector". 5 Mbyte/sec SCSI is termed "Slow" SCSI. SCSI cable lengths may be up to 6 meters (20 ft) for Slow SCSI. Even before X3.131-1986 was officially accepted by ANSI, the SCSI committee went to work on improving it.

Released by the ANSI Committee as specification IEEE X3.131-1994, SCSI-2 is also a complete, stand-alone document. Arguably the most significant addition of SCSI-2 is the expanded definition of the common command set (CCS) providing a common software interface for all disk drives and many peripherals other than disk drives. SCSI-2 defines the differential interface and the 16-bit and 32-bit "Wide" data bus; doubles data throughput to 10 Megatransfers per second (called "Fast" SCSI), which translates to 10 Mbytes/sec for Narrow (8-bit) SCSI and 20 Mbytes/sec for Wide (16-bit) SCSI; adds the smaller 50-pin, high density, micro-D connector (termed Alternative 1, A-connector); and terms all 50-pin cables "A" cables. This 50-pin high-density connector is commonly called the "SCSI-2 connector". SCSI-2 recommends active terminators in place of passive terminators for the single-ended bus. Backward compatible to SCSI-1. Note that in SCSI-2 the 16-bit bus requires two cables (one "A" cable and one "B" cable) to make a connection. This seriously limited growth of the Wide bus. SCSI-2 maximum recommended single-ended SCSI cable length is up to 3 m (10 ft) for Fast SCSI. Differential cable length is 25 m (82 ft) for Fast or Slow SCSI.

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(7) What are the differences between SCSI-2 and SCSI-3?

SCSI-3 changes the complete SCSI document structure and is no longer one document but a collection of documents, each with its own revision number. Some of these documents are the SCSI Primary Command (SPC) set layer, SCSI Block Commands (SBC) for hard disk interface, SCSI Stream Commands (SSC) for tape drives, SCSI Controller Commands (SCC) for RAID arrays, Multimedia Commands (MMC) , Media Changer Commands (MCC) and the SCSI Enclosure Services (SES) commands. For a complete overview see the SCSI Architecture Model (SAM) on the T10 Committee Website.

Let's take a look at some other important SCSI-3 documents:
  • SPI
    The SCSI Parallel Interface (SPI) defines the electrical signals and connections for parallel SCSI. A very quickly adapted new feature defined in SCSI-3 is the 68-pin, high density, micro-D connector for 16-bit Wide SCSI (termed the Alternative 3, P-connector). The SCSI specification terms cables with this connector the "P" cable. This connector eliminates the necessity of using two cables for 16-bit SCSI and gave a tremendous boost to the growth of Wide SCSI. It is commonly referred to as the "SCSI-3" connector.

    There are several revisions of the SPI document. SPI includes Fast SCSI data transfer speeds up to 10 Megatransfers (20 Mbytes/sec for 16-bit). The Ultra SCSI (Fast-20) modification of SPI includes doubling the data throughput to 20 Megatransfers/sec (40 Mbytes/sec for 16-bit). Ultra SCSI speeds reduce the maximum single-ended cable length to 1.5 m (5 ft) with 5 or more devices and 3 m (10 ft) for systems having up to 4 devices. The maximum recommended differential cable length remains at 25 m (82 ft).

  • SPI-2
    SPI-2 doubles bus speed again to the Ultra 2 (Fast-40) SCSI data throughput of 40 Megatransfers/s (80 Mbytes/s for 16-bit). To attain this speed, a new electrical interface is defined. This interface uses 3 V logic instead of TTL voltage levels and is known as Low Voltage Differential (LVD) SCSI. The older TTL based differential SCSI is now called High Voltage Differential (HVD) and it is not compatible with LVD signals. Most LVD device interfaces are designed as LVD/SE.

    Multimode operates at the LVD voltage levels and bus speed as long as all devices connected are LVD. Connecting a single-ended device to a multimode LVD bus causes all LVD/SE devices to switch to the single-ended interface. It will then operate at a maximum of 20 Megatransfers/sec (40 Mbytes/sec for 16-bit) with single-ended cable length limitations. Connecting an HVD device to an LVD bus will cause the bus to shut down. LVD cable length is specified as 12 m (40 ft). For a single initiator-single target application this length may be increased to as much as 25 m (82 ft). Note that single-ended signals cannot be used for bus speeds greater than Ultra SCSI (Fast-20).

    The low power requirements of the LVD interface allow the differential drivers to be included on the interface ASIC. Not having to place external driver chips on the PCB reduces the amount of PCB real estate required and reduces the cost of the board design.

Another new feature of SPI-2 is the SCSI Interlock Protocol (SIP) which defines the parallel command set. Also, SPI-2 adds two new SCSI connectors:

  1. The 80-pin Single Connector Attachment (SCA-2) connector (termed the non-shielded Alternative 4, P-connector) that includes the 16-bit SCSI signals as well as power for the peripheral. This connector is designed for hot swapping of peripherals in SCSI backplanes.
  2. The Very High Density Cable Interconnect (VHDCI) connector (termed the shielded Alternative 4, P-connector) is a small connector that allows as many as four separate 68-pin Wide SCSI connectors to be placed on one standard width PC backplate. Some of the newer LVD host adapters include this connector.

SPI-2 is a complete stand-alone document for all parallel interfaces up to Ultra 2 (Fast-40) SCSI and does not refer to older documents. To do this, it has incorporated the 50-conductor "A" cables defined in SCSI-2 and the 68-conductor "P" cables defined in the original SPI document.
  • SPI-3
    SPI-3 again doubles the SCSI bus speed to Ultra 3 (also known as Ultra160 and Fast-80) providing SCSI bus speeds up to 80 Megatransfers/sec (160 Mbytes/sec for 16-bit). 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 and is defined for the 16-bit bus only.

    SPI-3 also includes a 32-bit CRC (Cyclic Redundancy Check) for better data security and Domain Validation. Domain Validation is new for peripheral buses. Basically, SCSI Domain Validation will not accept a negotiated data throughput speed until a validation test is performed. To perform this test, the initiator sends out a Write Buffer command to the target at the full data throughput. The initiator will then read the data back to see that it is correct. If it is not, the initiator will switch to the next lower speed and perform the test again. When the test passes, that speed is compatible with both the initiator and the target and is used for data transfers between the two devices.

    SPI-3 is also a complete document defining parallel SCSI interfaces up to 80 Megatransfers/sec and does not refer to previous SCSI documents. SPI-3 obsoletes HVD and 32-bit data bus designs. For specifications of the HVD and 32-bit bus, refer to SPI-2. The maximum cable length for Ultra 3 SCSI is 12 m (40 ft) or 25 meters (82 ft) for point-to-point applications.

    Ultra 160 (U160/m) is a sub-set of Fast-80 that includes Double Transition clocking, CRC and parts of Domain Validation. It is not yet a recognized form of SCSI.

  • EPI
    For Paralan a very significant development released in the Enhanced Parallel Interface (EPI) is the documentation of SCSI Expanders, Bridging Expanders, Switches and some connectors not otherwise documented. This finally incorporates into the SCSI specification the types of products that Paralan has been designing, marketing and selling for years. EPI also describes the design of SCSI systems, defining the electrical specifications for cable lengths and loads. Also included is a description of how to work with both Wide (16-bit) and Narrow (8-bit) devices on the same SCSI bus.

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(8) What is the difference between single-ended and differential SCSI?

Single-ended and differential are two methods of placing SCSI signals on the cabling. Single-ended uses one wire driven against ground and the signal is the voltage difference between that wire and ground. The differential interface drives two wires. The signal is the voltage difference between the two wires. Single-ended and differential are not directly compatible. (It should be noted that HVD and LVD are also not directly compatible). They can be interconnected by the use of a SCSI expander called a Single-ended to Differential Converter. Single-ended cable lengths are 6 to 1.5 meters (20 to 5 ft), decreasing with increasing data throughput, while differential (HVD and LVD) offers cable lengths to 25 meters (82 ft), regardless of the speed of the bus.

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(9) What is meant by "Narrow" SCSI?

Narrow SCSI is the term that is used for 8-bit SCSI. It can usually be identified by 50-pin connectors.

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(10) What is meant by "Wide" SCSI?

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 obsoleted the 32-bit SCSI bus.

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(11) What is HVD SCSI?

This is the "old" differential SCSI using TTL voltage levels that was originally defined in SCSI-2, offering 25 meter (82 ft) cable length. It was functionally replaced by LVD (Low Voltage Differential) SCSI in the SPI-2 document of SCSI-3 and obsoleted in the SPI-3 document of SCSI-3. HVD and LVD SCSI are not directly compatible but can be interconnected by the use of a SCSI expander called an LVD to HVD Converter.

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(12) What is Wide Ultra SCSI?

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).

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(13) What is LVD SCSI?

LVD, which stands for Low Voltage Differential, was introduced in the SPI-2 document of SCSI-3. It is also called Ultra 2 or Fast-40 SCSI. It uses 3 volt instead of 5 volt logic level and is not directly compatible with the "old" differential (HVD) SCSI. LVD again doubles SCSI data throughput to 40 Megatransfers/sec. Cable lengths are 12 m (40 ft). Single initiator-single target applications may use up to 25 m (82 ft) of cable. The "multimode" implementation of LVD is backward compatible with single-ended SCSI. However, connecting one single-ended peripheral to a multimode LVD bus will cause the entire bus to switch to the single-ended mode with the single-ended limitations on data throughput and cable length. LVD can be interconnected with HVD by the use of a SCSI expander called an LVD to HVD Converter.

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(14) What is "multimode LVD" or LVD/MSE SCSI?

Multimode LVD and LVD/MSE (Multimode Single-Ended) are terms for the same interface. It is an implementation of SCSI that automatically switches between the LVD and the single-ended mode. When a single-ended device is connected to a multimode LVD/MSE bus, the entire bus switches to the single-ended mode. Otherwise LVD/MSE devices operate in the LVD mode.

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(15) What are the benefits of LVD SCSI?

In addition to the obvious benefits of longer maximum cable length than single-ended and a doubling of data throughput, there are a number of other benefits. LVD/MSE and single-ended offer some compatibility. The lower operating voltage of the LVD bus means lower power dissipation, so the differential drivers can be included on the LVD ASIC rather than having to mount them external to the chip. This results in smaller boards, less heat dissipation, higher reliability and lower cost. Also, manufacturers will no longer have to design and build devices with both single-ended and differential interfaces. This results in lower costs.

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(16) Is LVD SCSI backward compatible?

LVD is backward compatible through the single-ended interface if it is multimode LVD. It is doubtful that anyone will build LVD devices that are not multimode. Remember that connecting a single-ended device to a LVD/MSE bus will cause the entire bus to switch to the single-ended mode with its data throughput and cable length limitations. To add a single-ended peripheral to an LVD bus and preserve the data throughput and cable length of LVD, you can use a SCSI expander called an LVD to SE or LVD/MSE to LVD/MSE converter. This converter divides the SCSI domain into two bus segments - one segment will operate at the LVD data throughput and cable length and the other bus segment will operate at the single-ended data throughput and cable length.

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(17) What is Ultra160 or U160 SCSI?

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.

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(18) I have heard of U160/m SCSI. What is it?

The SPI-3 document defines 5 new features for SCSI: Double Transition Clocking, CRC, Domain Validation, Quick Arbitration and Select (QAS), and Information Units (Packetization). In order to be compliant with the SPI-3 U160 specification, at least one of these features must be implemented. A group of industry leaders agreed to incorporate three of these features in order to speed up introduction of U160 products. These three features are Double Transition Clocking, CRC and Domain Validation. U160 devices with these three features are called U160/m.

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(19) Is Ultra160 SCSI backward compatible?

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.

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(20) Is Ultra 160 SCSI better than fibre channel?

This is a discussion that will go on for some time and there is no simple answer. It depends on the application. At least until now, fibre channel (FC) implementations of SCSI have been FC-AL or FC-Arbitrated Loop, so I will limit my comments to FC-AL. At the time of writing this FAQ, SCSI is beginning to ship devices that are capable of 160 Mbytes/sec data transfer rate while fibre channel (FC) is stalled at a maximum of 100 Mbytes/sec and the only peripherals with true FC interface are disk drives -- and only one manufacturer makes them. Mark up the data throughput advantage to SCSI.

  • FC proponents say that connectivity is more important than data throughput and that FC can have up to 126 nodes. If that were true why would every increase in SCSI data throughput be immediately adopted? Data throughput rules in nearly every serious application we have encountered. Anyway, on a practical basis, FC is limited to only a couple dozen nodes which is very similar to what SCSI can handle. Call the connectivity issue a draw.
  • Well then, FC can have up to 10 kilometers of fiber. Well, there are a number of manufacturers of SCSI Extenders that offer the same or longer fiber cable lengths. Call this one a draw.
  • Again, at the time of this writing, an issue that FC proponents do not raise is interconnectivity. There are great problems in getting FC devices from different manufacturers to work together. Sometimes this is true with products from the same manufacturer. SCSI went through these problems many years ago. There are now very few problems with SCSI interconnectivity. Of course, these FC interconnectivity problems will eventually be solved, but for now, score this one for SCSI.
  • Cost is always a factor. FC is more expensive. As FC gets wider application its cost will go down, but for now there is no contest. Score this one for SCSI.
  • Overall, at this time, FC has a place in large scale storage and backup systems but there is almost no reason for the home user and very little reason for the small business to consider FC.

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(21) Is Ultra 160 SCSI better than EIDE?

Again, the answer depends on the application. SCSI is an intelligent interface that can perform data transfers with no intervention from the host CPU. SCSI is multi-tasking. An initiator can issue a command to a target. The target can then disconnect from the bus to perform the task and free the bus up for another task. Ultra 160 SCSI can have up to 16 devices connected to the bus and they can be any of a large variety of peripherals, including hard drives, floppy drives, tapes, CDs, scanners, printers, etc. The number of devices can be substantially increased through the use of LUNs. EIDE can have two internal drives connected. Your PC probably has two EIDE buses, so it may have up to four peripherals. Ultra 160 SCSI allows up to 12 m (40 ft) of cabling, which may be internal or external to the computer. For point to point applications you may have up to 25 m (82 ft) of cable. EIDE is for internal cabling only and the maximum cable length is only 18 inches. And, don't forget that 160 Mbytes/sec is much faster than any EIDE bus.

For a home user with a single hard drive, EIDE is probably better as it is less expensive and almost as fast as Ultra SCSI. In a compute-intensive or storage-intensive application, however, SCSI is the clear choice.

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(22) What is Fast-20 [or Fast-40 or Fast-80] SCSI? Answer: The term "Fast-xx" refers to the maximum data throughput that a particular version of SCSI is capable of, expressed in Megatransfers/sec. For example, Fast-20 is 20 Megatransfers/sec which is 20 Mbytes/sec for 8-bit (Narrow) SCSI and 40 Mbytes/sec for 16-bit (Wide) SCSI.
  • Fast-10 is the same as Fast SCSI
  • Fast-20 is the same as Ultra SCSI
  • Fast-40 is the same as Ultra 2 (uses LVD transmissions)
  • Fast-80 is the same as Ultra 3 or Ultra 160 SCSI (uses LVD transmissions)

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(23) Can I connect an Ultra 2 Wide (LVD) disk to an Ultra Wide adapter?

The answer is definitely yes. However, exactly how you do it depends on the type of Ultra Wide host adapter you have. If it is single-ended SCSI and the LVD disk is multimode LVD, you can connect the disk directly to the host adapter. The multimode LVD interface on the disk will switch to the single-ended mode.

If the Ultra Wide adapter is differential (HVD), they can still be connected, however, you will have to use a SCSI expander called an HVD to SE or an HVD to LVD Converter.

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(24) What is Double Transition clocking?

Double Transition (DT) clocking is used to double the data transfer rate from Ultra 2 (Fast-40) to Ultra 3 (Ultra 160 or Fast-80) SCSI without having to increase the clock speed. That means that both edges of the REQ and ACK signals are used to clock data. The REQ and ACK signals run at 40 MHz on Ultra SCSI, so double clocking increases the rate at which data is clocked to 80 MHz. This provides data throughput of 160 Mbytes/sec for Wide SCSI.

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(25) What is "Domain Validation"?

Domain Validation is a method used in Ultra 160 (Fast-80 or Ultra 3) to test for the optimum rate for data exchange. Once the host adapter (initiator) has located a peripheral and negotiated a data transfer rate, the initiator sends a Write Buffer command to the target at that negotiated data transfer rate and then reads it back to determine if what it reads is what it wrote. If not, it will resend the Write Buffer command at the next lower data transfer rate. This will continue until a speed is reached where the test is successful. This is all accomplished automatically.

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(26) What is CRC?

Cyclic Redundancy Check is a means of detecting errors that is much more effective than the simple parity check that SCSI has used for years. CRC detects all single bit errors, all two bit errors, all errors with an odd number of bit errors, and all burst errors up to 32-bits long. CRC uses a 32-bit polynomial checksum to test data integrity. A similar process is used in Fibre Channel, Ethernet and other buses.

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