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SCSI Bus Speed

SCSI buses run at a variety of different speeds. Generally, newer buses run faster than older ones, reflecting the increased performance of newer hardware. In order to understand SCSI bus speeds, we must first tackle some terminology issues (yet again, sorry)   There are several different ways that SCSI bus speeds are specified, which can lead to a tremendous amount of confusion. The situation is particularly bad because base SCSI speeds vary by powers of two, and the differences between the various ways of specifying SCSI bus speeds also differ by powers of two!

These are the three ways that SCSI bus speeds are commonly quoted:

• Clock Speed: This refers strictly to the frequency of the clock (strobe) used to control synchronous transfers of data on the SCSI bus. With current technology this can be 5, 10, 20, 40 or 80 MHz. For more on clocks and how clock speeds work, see this fundamentals page.
• Transfer Rate: This refers to the number of times per second that data is transferred across the interface. This is only the same as the clock speed of the bus if single transition (conventional) clocking is used. Faster SCSI implementations now use double transition clocking, and this means the transfer rate (in millions of transfers per second) will be double the clock speed in MHz.
• Throughput: This number represents the theoretical maximum amount of data that can be moved across the SCSI bus, and is measured in millions of bytes per second (MB/s). On a narrow bus, throughput and transfer rate are the same, because each transfer is of 8 bits (one byte). But for a wide bus, throughput is double transfer rate, because each transfer is of 16 bits--two bytes.

Now that we understand all of that--we do understand it, right? :^)--we can look at the various bus speeds used in the SCSI world and understand what they mean. The table below shows all of the bus speeds used for parallel SCSI. (You may also find that looking at the table makes more clear the relationship between clock speed, transfer rate and throughput):

Note: The "(DT)" in "Fast-80(DT)" and "Fast-160(DT)" represents the fact that this suffix is sometimes attached to represent the use of double transition clocking for those interfaces.

As you can see, the use of double transition clocking and wide buses means that the numbers in the latest transfer modes do not refer to the actual speed of the bus at all. The "160" in "Ultra160" represents the maximum throughput of such devices, but the clock speed is "only" 40 MHz.

Finally, I must include my standard disclaimer: we are discussing interface transfer rates here. These represent only the maximums that data can be transmitted across the interface under theoretical conditions. The big numbers that are popularly discussed ignore command overhead and other inefficiencies, so you will not actually get a full 160 MB/s on an Ultra160 interface. Also, remember that true performance will be limited by the speed of the devices on the interface. Simply increasing the speed of the interface is not enough to really improve performance unless the interface was already the limiting factor (such as if multiple drives were saturating it). See here for more on this issue. Also remember that the maximum throughput of any SCSI device will be limited by the throughput of the host adapter's system bus interface.

Next: Bus Parity and Cyclic Redundancy Checking (CRC)

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