Adaptec was quick to market with the AHA-2940U2W, the first of its Ultra2 SCSI products and the first Ultra2 device to grace APC Lab’s benches. So what advantages does Ultra2 SCSI have over Ultra Wide? It all comes down to more speed and longer cable.
The maximum rate that an Ultra Wide SCSI controller could transfer data was 40M per second. The AHA-294OU2W and other Ultra2 devices can transfer at double that — 80M per second. To achieve the speed increase, Adaptec increased the dock of the controlling chip used on the AHA-2940U2W (the AIC-7890) to 40MHz.
The other major improvement over Ultra Wide SCSI, the increased cable length, is the result of the incorporation of a new signalling technique into Ultra2 SCSI — low voltage differentia) (LVD), which measures the difference between two voltages rather than a voltage and ground to read logical Is and Os. This signalling method increases the cable’s protection from noise and electromagnetic impulses (EMI). If you look at an Ultra Wide SCSI adapter, it can have up to 15 devices on it, but the cable can be no longer than 3m. This is fine for desktop systems, but try to implement an external storage cabinet for a server and the cable distance becomes a limiting factor. The Ultra2 SCSI specification supports cables of up to 12m in length.
One of the single biggest reasons why consumers are slow to take up new SCSI technology is the price of peripherals that use the new technology. Hard disks, scanners, or removable storage devices that used Ultra Wide SCSI technology were very expensive when they were first released. Ultra2 SCSI reduces the cost of devices by removing the need to have a terminator on any of the peripherals. The controller and the SCSI cable handle termination of Ultra2 devices. Removing the need for termination from the peripherals lowers their manufacturing cost significantly.
The AHA-2940U2W that APC reviewed comes with four different cables to support all legacy SCSI devices (including the Fast, Ultra and Wide variants). Ultra SCSI2 devices need to be connected via an Ultra2 SCSI compatible cable. Trying to run Ultra2 devices over an Ultra Wide SCSI cable degrades performance.
Within the original Ultra2 SCSI specification, an Ultra2 controller can only run as fast as its slowest peripheral. If you have a Fast Narrow SCSI 11 hard disk, all of your Ultra Wide and Ultra2 devices would be slowed down to 10M per second. To get around this all of the SCSI controller manufacturers have had to come up with proprietary solutions to this problem. Adaptec has developed a technology it calls Speed-Flex. The SpeedFlex technology allows all devices connected to the controller to run at their intended maximum speed.
As you can see from our benchmark results, the AHA2940U2W used with a Quantum Ultra2 SCSI drive transfers data 2M per second faster than the Ultra Wide solution. Even using an Ultra Wide SCSI hard disk with the new Ultra2 SCSI controller proved faster than using a Ultra Wide controller.
At the time of writing, Adaptec was demonstrating a twin channel controller — the AIC 7897 — which can support up to 30 SCSI devices. Adaptec has also stated that it is working on having RAID products based on the Ultra2 specification ready by the middle of the year.
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