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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 ]

Negotiation and Domain Validation

SCSI hardware supports many different speeds, and newer, faster hardware is generally backwards-compatible with older, slower devices. You can use a host adapter capable of 160 MB/s throughput with drives that can only support 20 MB/s transfers, or vice-versa. This leaves an obvious question: how does each device determine what speeds the others on the bus are capable of? Without knowing this, senders can't figure out how fast receivers can handle data being sent.

Since this is so important, the SCSI protocols build in support for a method by which the host adapter can interrogate all devices on the bus to find out what speeds they support. This process is called negotiation, and is one of the first tasks performed by the SCSI host adapter when the system power is applied. Under conventional SCSI rules, this negotiation is done with each device; the host adapter records the maximum transfer speed that each device claims to support, and then uses that information when the device is accessed.

This works great in theory, but there's a problem with it: theory doesn't always translate into practice, especially when the technology "pushes the envelope" with high-speed signaling. For example, even if the host adapter can support Ultra160 transfers and the device says it can as well, this doesn't mean that 160 MB/s signaling is actually possible on the bus. Perhaps the cabling being used is inferior or too long, or there's a problem with a terminator, or the system is in a particularly electrically noisy environment. Regular negotiation just "trusts" that everything will work at the speed the hardware decides is possible, but it may not actually work. If there are difficulties, they may manifest themselves in the form of data errors or reliability problems.

To improve negotiation, the SPI-3 standard introduced a new feature called domain validation, sometimes abbreviated DV. This feature basically adds a verification step to the normal negotiation procedure (note that "domain" is another word for a SCSI channel or bus). After a device tells the host adapter that it is capable of transfers at a particular speed, the host adapter tests the device by sending write requests to the device's internal buffer at that speed. The data just written is then read back and compared. If the data is different, or if parity or CRC errors occur during either the read or the write, the host adapter knows that communication at that speed is not reliable. It will then retry at the next lower speed, and continue until reliable operation is established. (If this sounds similar to the way that two regular analog modems determine a communications speed, that's because it is!)

Domain validation is one of the five "optional" features of Ultra3 SCSI, and is a required feature for hardware meeting the Ultra160 or Ultra160+ specifications. This feature may be expanded in the future to include more frequent validation during the operation of the system, since over time errors may occur on a channel that worked fine when the system was first powered up.

Next: Quick Arbitration and Selection (QAS)


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