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What is the difference between a hard drive labeled U2W and U2W SCA?
The SCA acronym stands for Single Connector Attachment which has an 80-pin connection vice the typical SCSI connection of 68-pins. In addition, there are pinouts for assigning the SCSI ID, powering the drive, and some control pins. In order to use a U2W SCA SCSI hard drive, you will need a compatible SCA adapter that will give you an increased stub length. However, if you use more than one or two on a given SCSI bus segment, you may have problems. Unless you are using a SCSI backplane, it would be a good practice to avoid the drives with the SCA connector.
How can I get data fast enough to the print controller?
There are high-speed printers that use the SCSI bus such as the Ultra160 LVD SCSI that will transfer data at a rate of 160MB/second. For point to point connections, the distance can be as far as 25 meters. Two controllers can be used to ship data to the printer faster. Ultra320 SCSI devices are now coming to market that will double the transfer rate to 320MB/second.
Is there a maximum stub length on getting power to SCSI devices?
It is important to first of all realize that power Y cables are not desirable because they increase the stub length. The maximum stub length including the PCB paths is 0.1 meter for single-ended SCSI or LVD SCSI and 0.2 meters for HVD SCSI. You can find all the latest specifications on SCSI from the SCSI Website.
I have an UltraSCSI controller (20MB/sec), with UltraSCSI disks (20MB/sec) and Fast SCSI disk (10MB/sec). Which is the real hard disk throughput: 10MB/sec or 20MB/sec, or both, depending on the hard drive that has been accessed?
When the SCSI bus initializes, the controller "asks" each peripheral what kind of SCSI it is---data throughput, wide or narrow, synchronous or asynchronous, etc. All the devices on the bus then have that information and when any two devices exchange data they know how to "talk" to each other. So when your UltraSCSI controller exchanges data with your UltraSCSI disk, it will do so at UltraSCSI throughput. When it exchanges data with your FastSCSI disk, it will do so at the FastSCSI throughput.

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Author: Patrick G. Koehler
Date of Last Revision: 21 December 2001
Extracted from Computer Technology Review dated March 2001.