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

Computer Technology Review, August, 2003 by Jim Stein

For more than fifteen years, Paralan Corporation has been assisting customers with SCSI bus subsystem performance problems. Many times, frustrated system integrators end up forfeiting all their profits making weekly trips to the customer's site to trouble-shoot system performance problems. Through this experience, it has become clear to those of us doing the troubleshooting that poorly designed and manufactured cabling is usually the culprit preventing a SCSI subsystem from running reliably with maximum data throughput. Fortunately, the problem is easy to correct.

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To prevent problems, designers must carefully define and build the cables to the specification and test them rigorously for reliability. With SCSI running at data rates of up to 320 Megabytes/sec, reliable SCSI cables are essential. The specification is clear on how to design and build reliable cables that will accommodate high speeds.

This article covers the indicators of poorly constructed SCSI cabling and defines how to specify cabling to assure correct assembly. Finally, it goes into detail on how to test the cables to assure that the specifications are met.

Symptoms of Cable Problems

Many symptoms indicate SCSI cable problems ranging from inconsistent operation to reduced data throughput to non-operation. Examples include:

* SCSI controller does not recognize the drives

* SCSI controller recognizes the drives but cannot exchange data

* System runs at reduced data throughput

* Occasional bus crashes or creates error reports

* Various problems occur when changing cable length or peripheral location

What Causes These Problems?

Software: If the right drivers for the peripherals are not loaded, the bus will not work correctly. If the latest version of the drivers is not loaded, the bus may run at reduced throughput.

Hardware: Although rare, there is a possibility that the SCSI controller or the peripheral failed. A failure in the controller will usually affect all the peripherals with the exception of a failure in the upper byte of a wide SCSI controller--which may not affect data transfer to narrow peripherals. Failure of a peripheral can be easily remedied by replacing the peripheral with one that is known to be good.

If one Single-Ended (SE) drive is placed on a multimode Low Voltage Differential (LVD) bus, the bus and all the peripherals will switch to the SE mode, which provides a maximum data throughput of Ultra SCSI (40 MB/sec for wide SCSI) Also, in the SE mode, the maximum allowable cable lengths are considerably shorter than for LVD.

Terminators: After cables, terminators and termination are the next major source of SCSI problems. For proper operation, each SCSI bus segment must be terminated at the two extreme ends of the segment. In a typical system, that means there must be termination at the SCSI controller and at the very end of the SCSI cabling. The termination must be appropriate for the type of SCSI used. That is, LVD or LVD/MSE terminators are used for LVD; HVD terminators for HVD; and SE active terminators for SE SCSI. Some SE drives have built-in terminations but most HVD drives and all LVD drives must be terminated externally.

Even the fastest SCSI hard drive with a large data buffer may burst data at up to 320MB/sec but will give a sustained data throughput of about 40MB/sec. Most tape drives do not exceed 30MB/sec. The best use of the available bandwidth is achieved when multiple high data throughput peripherals are used on the bus.

Cables: The first thing to check, when trouble shooting SCSI cabling, is the total length of each cable segment. Each implementation of SCSI has a different maximum recommended cable length. Exceeding the cable lengths specified for SE SCSI is ill advised. HVD and LVD are more tolerant of longer cable lengths, however, exceeding the maximum recommended lengths can result in problems that can be very difficult to trouble shoot. (Table 1 is a list of maximum cable lengths. The terminology conforms to the SCSI Trade Association [www.scsita.org] recommendation).

Stubs: A stub is an un-terminated length of transmission line. At the speed that SCSI operates today (now up to 320 MHz), a SCSI cable is a radio frequency transmission line. Stubs are typically created by a "Y" or a "T" in the cabling, or by having cable that extends beyond the terminator. Stubs are usually not a problem with round, shielded external cables but can be easily created with internal ribbon cables.

Incorrect Cable Type: Using cable that meets SCSI specifications for impedance, twisted pairs and shielding is critical. If the cable does not have the correct impedance, not all the signal is absorbed at the receiving device but is reflected back toward the transmitter. If these reflected signals arrive at a device that is expecting to receive signals, it will result in a SCSI resend or even a bus shut down. At a minimum, this results in a substantial reduction in data throughput.

Incorrect Cable Assembly: Even if the cable is made of quality components that meet SCSI specifications, if SCSI signal pairs are not matched to twisted pairs in the cable it increases crosstalk significantly. This can ,also result in SCSI resends or bus shut down. The faster the SCSI bus and the longer the cabling, the more pronounced the effect will be.

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

Computer Technology Review, August, 2003 by Jim Stein

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