Technical Section - SCSI Cables /Fiber Optic Cables

 Cabling FAQ

SCSI Cables

Cables built to SCSI III standards must meet impedance specifications of 90 ohms single-ended and 132 ohms differential. SCSI bus lengths are not to exceed 6 meters single-ended, 25 meters differential, and 3 meters for FAST SCSI. SCSI cables have the pairs located within three  layers inside the cable. Specific signals must be placed on specific layers within the cable to guarantee error-free operation.

Madison cable is used for all external cables. AMP connectors are used on all internal and external cable assemblies. The use of these materials insure consistent quality and performance.

SCSI Terminators

In many applications, passive terminators have been used successfully.  However, as clock rates and signal transition speeds increase and noise margins decrease, the passive terminator can no longer ensure a reliable signal. To create a reliable signal with longer cable runs of up to 6 meters, and higher data rates, you need active SCSI terminators. Through the use of a  voltage regulator, the terminator has immunity to TERM-PWR voltage variations.  Additionally, the lower impedance of the active terminator, typically 110 ohms, more closely matches that of the cable. If your cabling has different impedances or lengths exceeding 6 meters, a better choice may be a forced perfect terminator. The resistors and diodes of a forced perfect terminator work together to push impedance higher and lower the line impedance. This happens so quickly that it convinces the signal that it is seeing a perfect impedance match. If you're using differential drives and controllers, you will need a differential terminator. The differential terminator is a resistor network that provides a reliable signal for cable lengths up to 25 meters. It is important to note that differential terminators may not be used in conjunction with single-ended controllers or  drives. The same is true for single-ended terminators. These terminators are not compatible with differential drives or controllers.

Fiber Optic Cables

Unlike wire, data travels in one direction only in a fiber cable. It takes  two fiber cables to complete a duplex (both directions) data transmission.  Fiber optic assemblies may be built in either single or multi-mode styles.  Single-mode assemblies are yellow in color; multi-mode cables are orange.  Single-mode cable assemblies are used with lasers and laser diodes;  multi-mode cable assemblies are used with LEDS (light emitting diodes).  Single-mode fiber uses cable with a 9 micron glass core. Multi-mode may use either 50, 62.5 or 100 micron glass core; 62.5 micron is the most common.

Connectors used on fiber optic cable assemblies may be either physical  contact (PC), or non-physical contact. Physical contact connectors have,  for the most part, displaced non-physical contact connectors because of  their reliability and low loss characteristics. The highest reliability fiber optic connectors are the FC, SC and FDDI/MIC types.