SCSI Cables: more than a couple of connectors and a few pieces of wire - Connectivity

Computer Technology Review, August, 2003 by Jim Stein

It should be noted that the SCSI specifications demand that the Request (REQ) and Acknowledge (ACK) signal pairs be located in the core of the SCSI cable. The control pairs should be in the second layer and the data pairs should be in the outside layer next to the shield (see Figure 1). The REQ and ACK signals control the timing of the SCSI bus and are most susceptible to noise. Placing the REQ and ACK pairs in the core of the cable provides the greatest shielding from external noise sources. SCSI cable assemblies that ignore pair placement are suspect and should not be used.

It is not safe to assume that a cable designated as a SCSI cable is manufactured with the parameters cited above. It is best to specify how cables are to be assembled and then test them to be sure every instruction was followed.

[FIGURE 1 OMITTED]

How to Specify Good SCSI Cables

Specifications for correct SCSI cables can be found in the SCSI Parallel Interface (SPI) document in the SCSI-3 specifications. The latest version, which includes Ultra320 SCSI, is SPI-4. (See http://www.tl0.org for the T10 SCSI developers website.)

Quality parts: Components used to make SCSI cables are as important as the construction. Be sure to specify a quality manufacturer of connectors, back shells and cable.

Cable Length: It is not sufficient to specify that cable assemblies be manufactured with cable containing 50 or 68 wires. Adequate cabling must meet the SCSI specifications including cable impedance, stranded or solid wire, wire gauge, 25 or 34 twisted pairs and shielding. Note, too, that Very High Density Cable Interconnect (VHDCI) SCSI connectors typically do not accept the 28-gauge wire of most SCSI cables. The SPI-4 document of the SCSI-3 specification acknowledges that fact and allows the use of 30-and 32-gauge wire, but the higher resistance of smaller wires results in a reduction of the maximum allowable SCSI cable lengths.

Proper Assembly: As important as using the correct cable is making sure that the SCSI signal pairs are assigned to twisted pairs in the cable and that correct pair location within the cable is observed. Correct pair selection will be found in the SPI documents of the SCSI specifications as well as on websites such as http://www.paralan.com/lvdmsepinout.html.> How to Test SCSI Cables

The first test is to visually inspect all the pins in the connectors on both ends of the cable (the SCSI specifications define that all external, shielded SCSI cables have male connectors) to be sure that none are bent. Centronics and VHDCI connectors do not have pins, however the 50-and 68-pin high-density connectors have pins that are quite susceptible to damage.

In order to test the electrical function of the cable, the cable could be torn apart; however, that is not practical and is certainly not practical in the field. There are a number of cable testers available that can test SCSI cable parameters. On the high end are expensive ($2K to $20K) testers that are designed to test many kinds of cables and require set up by a trained technician. Many of these testers do little more than test pin-to-pin continuity.