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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Protocols and Interface Features ]

Bus Parity and Cyclic Redundancy Checking (CRC)

As mentioned in a few places in this discussion of SCSI, parallel buses can have signal integrity problems, especially if used on long cables or at high signaling speeds. To help ensure that the data sent from one device arrives intact at its destination, various SCSI buses use two different data protection methods.

The first technique is SCSI bus parity. The parity method uses an extra bit for each eight bits of data, which is computed by the sending device so that the sum of all the "ones" in the nine bits taken together is either odd or even--one is chosen as the standard for the interface, and for SCSI odd parity is used. At the receiving device, the data is checked to see if the total is still odd; if an even number of "ones" is seen, this means there was a data corruption problem (because one bit is the wrong value somewhere) and the sender is signaled to retransmit. This simple data protection method is not unique to SCSI; it has also been used for years for serial communications and in memory circuits. For more details on parity, including examples showing how it works, see this comprehensive discussion in the memory section.

Note: SCSI parity is almost universally supported, but some very early host adapters may not work with parity checking enabled; to allow for this, many drives include a jumper to disable parity operation.

SCSI parity is useful, but is limited in its effectiveness, especially for very high transfer rates. It cannot detect if two bits in a given byte of data flip, for example. To further safeguard data, the SPI-3 standard introduced cyclic redundancy checking or CRC to the SCSI world when double transition clocking was introduced, allowing 160 MB/s data throughput on the bus. CRC is another technique that is not new, just "new to SCSI". It has been used in a variety of places in the computing world for decades, for example, in modems.

CRC is a bit difficult to describe in brief terms, and a full explanation is beyond the scope of this site. In a nutshell, it is a more robust way of checking for data corruption that can occur anywhere in a transmitted data message. A special algorithm is used that calculates a binary code as a result of arithmetic operations on the data; this is called a cyclic redundancy code (also abbreviated CRC). This code is sent along with the data over the bus. The recipient runs the same computation on the data and checks to see if it gets the same value that the sender computed; if there is any difference then an error occurred. In fact, modern CRC implementations don't actually run the computations using formulas, but rather use pre-created tables to speed up the process. This is sort of like using "multiplication tables" as you might have memorized as a child, instead of doing the computation from scratch each time.

At any rate, the bottom line is that CRC does a much better job of protecting data transmitted on the bus, especially at high signaling speeds. It can in theory be used in conjunction with parity, as they are independent. However, once CRC is being used, parity is somewhat unnecessary, except perhaps for compatibility with older hardware. CRC is one of the "optional" features of Ultra3 SCSI, and is a required feature for hardware meeting the Ultra160 or Ultra160+ specifications.

Tip: If you really want the "full scoop" on CRC, try this site. Be warned that it is definitely not for the technically faint of heart. :^)

Next: Command Queuing and Reordering

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