The Small Computer System Interface (SCSI) is now being used by faster and more complex devices. The SCSI protocol is very flexible and allows for faster transmission rates. A major limiting factor is the SCSI cabling. Parallel SCSI cables must be made correctly in order to handle the increased transmission rates of today's SCSI devices.
Most installers are using some form of shielded round cables when connecting external SCSI devices. Shielded round SCSI cables came into being because of the problems that unshielded flat ribbon cables have with electromagnetic interference (EMI). Unshielded flat ribbon cable can not pass FCC requirements, whereas properly shielded round cable can function within FCC specifications.
With many vendors not readily able to provide their specifications as to how they put their cable together, it usually is very hard to determine if the purchased round SCSI cable will work in the application. There are SCSI round cable suppliers providing cables with connectors that, though specified as SCSI 50-pin cables, may have as few as 25 lines wired from connector to connector. Differential applications will not operate under these conditions, and many single-ended SCSI applications will also have trouble maintaining uncorrupted signal transmissions with cable more than 6 feet long, as most of the ground lines are not connected.
Even if all of the wires are connected, and even if twisted pair cable made specifically for SCSI is used, depending upon how the cable is wired to the connector, there can also be problems. The twisted pairs on the cable must be matched with the correct pin numbers. Otherwise it is possible to have the plus side of two signal lines going through the same wire pair, inducing interference between the signals. This causes problems on fast SCSI-based systems, and systems with longer SCSI cable lengths.
When there is a problem with the cable, the symptoms vary greatly. The system may not operate at all, or there may be intermittent SCSI communication failures. In many cases the symptoms are initially thought to be due to the devices on the SCSI bus or the software drivers running the devices, resulting in excessive system installation delays and costs.
Over the years that Paralan Corporation has provided SCSI bus enhancements, more than 90% of the problems with a new installation of external SCSI-based devices (disk drives, tapes, scanners, optical drives, etc.) have been associated with the SCSI cables used in the installation. That's why Paralan has done something about the problem.
Paralan Corporation is now providing the SCSI Quiet Cable™ series of SCSI cables which are specified and tested to meet the requirements of the faster SCSI-based devices, wired to keep induced signal noise to a minimum, and priced to compete with other SCSI cables of unknown materials and construction.
For more information on SCSI cabling be sure to check out the SCSI White Paper "Cable Cautions" an excerpt from "Making SCSI Work" written by Paralan Staff.
Paralan also has SCSI cable testers that checks for twisted pair integrity, point-to-point continuity, opens, and shorts. The benchtop model ST123, and the model ST1000. - Marc Brooks -
