Making SCSI Work

(Model SW1)

• EXTEND BUS LENGTH
• SHARE PERIPHERALS BETWEEN TWO HOSTS
• SWITCH BETWEEN TWO SCSI BUSES

Paralan is dedicated to SCSI, and to making your SCSI-interfaced components work to their full potential. We're know as the company that makes SCSI work, because our interface products reliably extend the capabilities of the SCSI bus.

The SW1 SCSI Bus Switch allows two hosts to share peripherals, or one host to access either of two sets of peripherals. Switching from one bus to another is accomplished with the push of a front panel switch. In mode One, Port A and Port 1 are connected. In mode Two, Port A and Port 2 are connected. Switching is electronic, and is executed only when transmission has been completed. Each port provides single-ended drivers and receivers, as well as active termination, and supports a full SCSI cable length. A front panel indicator displays the selected port. The SW1 is non-intrusive, does not require a SCSI device I.D., and is transparent to both Initiator and Target.

Quiet Cable

(SQ Series)

Paralan SCSI Quiet Cable is designed to give maximum performance to all single-ended and differential SCSI-1, -2 and -3 applications. All cables meet the ANSI X3T9.2 standards, and are FCC Class A tested.

Each Paralan SCSI Quiet Cable consists of a shielded cable with 25 twisted pairs of conductors and two connectors which may be SCSI-1, SCSI-2 or a combination of both. The shield is electrically connected to the metal shell of each connector and is isolated from all other signal wires and the connector pins.

The cabling used for Paralan SCSI Quiet Cable is a high grade UL and CSA-approved, shielded cable consisting of 25 twisted pairs of multistranded 28 gauge flexible copper wire. Signals are placed within the cable at specified locations to improve noise immunity and to decrease signal crosstalk. Figures on page 2 show the colors of the signal pairs and a cross section of the Belden 1401A cable used.

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 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 further information, please contact Paralan at:

Paralan
7875 Convoy Court
San Diego, CA 92111
Tel: (619) 560-7266
Fax: (619) 560-8929