SCSI Cable Interconnect Characteristics Simplified.
This article is intended to focus attention on significant cable characteristics and their interaction in the total cable assembly design. It will not address the multitude of connector styles or other options used for SCSI interconnect.
The biggest issue is understanding the different cable electrical parameters, to identify cable specific characteristics, and which cable geometry to use, while determining which of the characteristics are important. All of the parameters must be identified and weighed as to which are important and where they apply in the decision process. There are many parameters available for study such as impedance (loaded and unloaded), capacitance, inductance, attenuation, dielectric, propagation delay, insulation, skew, single-ended, differential, length, shielding, crosstalk near end, and crosstalk far end.
Additionally, there are a multitude of materials and construction techniques used in the cable construction that must be contended with in determining the best materials for a specific cabling need. All affect the electrical characteristics of the bus. An additional construction concern is which insulation is used--Polyvinyl Chloride (PVC), thermoplastic elastomer TPO/TPE, or Fluorinated Ethylene Propylene (FEP). All are available in multiple configurations such as Flat-Zip, Twisted pair, round, and in numerous styles, gauges, strand, and solid. Each provides different performance levels.
All of the electrical parameters and design techniques affect system performance. Working to specify the correct parameters and design techniques will result in the optimization of the SCSI system by achieving the best possible bus characteristics and ensuring data integrity. The SCSI bus signals on the cable media with termination must be treated in the context of signals on a transmission line. This means that the data signal round trip travel time to the end of the bus has to be understood and treated as a critical parameter, which should be longer than the rise time of the data signal.
The cable must be terminated with a value equal to its characteristic impedance; otherwise reflections will exist. The amount of reflection is a function of load impedance, RL, and the characteristic impedance, Zo, of the line. The ideal case is when RL of the load and Zo of the cable are equal. (There is no reflection). The important issue is that any value that is close to matching impedances will result in reduced reflections.
Skew is of the utmost importance in the Low Voltage Differential (LVD) signaling of Ultra2 and Ultra3 SCSI systems. Skew is the difference in time it takes for parallel signals to propagate down the cable. When the skew time becomes excessive, the SCSI controller timing will be violated and the system will not be reliable as it tries to synchronize multiple parallel data channels. There are two types of skew in LVD SCSI. The first skew type is within a pair and the second type is between all of the pairs. In order to minimize skew, it is imperative that line lengths and capacitive loading be matched within the signal pairs and then compared to skew between other pairs to minimize the signal pair to pair skew. Raw cable skew of .025ns to .035ns per foot is acceptable for LVD SCSI.
LVD SCSI is more sensitive to time delays than previous generations of SCSI. Cable propagation delay has to be thoroughly understood in order to design a reliable system. Propagation delay is defined as the time it takes a signal to travel from one end of the cable to the other. Cable propagation delay varies with the insulation material and its dielectric constant. Typical values of l.45ns to 1.59ns per foot are acceptable for use in Ultra2 LVD SCSI at cable lengths up to 12 meters. Loading of the cable with devices will add capacitance that will adversely affect the propagation delay. As a result, unbalanced line capacitances must be kept to a minimum. This means balanced line loads by the devices, as well as the cable and terminator. Increased line capacitance will show itself most seriously as propagation delay and in the signal skew mismatch. In turn, the mismatch will create poor signal quality that does not meet the SCSI signaling requirements. The result will be SCSI bus hangs or data errors. Data errors can also occur due to crosstalk.
Cross talk is the unwanted coupling of one signal to another and is the worst type of transmission line condition for LVD SCSI. LVD SCSI is a low voltage differential signal with fast rise times. The total SPI-2 and SPI-3 budget for differential crosstalk and noise, per the standards, is 55-60 mV. Cable crosstalk has two components--capacitive and inductive. Cross talk is the one characteristic that limits the use of Flat-Zip parallel ribbon cable in long cable lengths. Long ribbon cables have large capacitive and inductive cross talk components.
It is the inductive crosstalk component characteristic that makes LVD SCSI operate with Twisted Wire Pair (TWP) up to the maximum SCSI length, 12 meters with max configuration. The inductive component creates a protective shielding characteristic in twisted pairs known as common mode rejection and begins to shine at frequencies above 20mhz. Twisted pair is the best cable to use for system designs where long cable lengths are required.
Cross talk is measured as near end and far end. Near end data is the only data that shows the construction difference between Flat-Zip and TWP. The coupling varies with the insulation materials used in cable construction. Shielding one signal from another is the best way to control crosstalk such as in coax. This crosstalk between adjacent signals is at its worst when using flat-zip ribbon cable. The best method to control crosstalk is to use Differential twisted pair for interconnection such as a LVD-SCSI.
Twisted Wire Pair (TWP) inductive coupling can be described as a separate transformer type coupling action within each signal pair. The currents in the TWP, which travel in opposite directions within the TWP pair, produce the coupling. This is the characteristic of the LVD SCSI bus that makes it so attractive. This coupling is a separate additive coupling action between each pair only. The coupling from and to the adjacent pairs is minimized with this inductive characteristic created by TWP and LVD design.
Due to a false sense of security, the Flat-Zip PVC ribbon is the cable of choice and most used for short connections due to its ease of manufacture, low cost, and ability to operate over the SCSI protocol for short distances. Flat-Zip ribbon has poor crosstalk electrical characteristics and is the cause of many undiagnosed system failures. Using TPE insulation can improve the Near End crosstalk by 5db over PVC in Zip configurations. System reliability dictates that Flat-Zip cable should be replaced with Twisted pair cable for its better crosstalk and impedance characteristics. It must be replaced with twisted pair to achieve reliable SCSI performance at max cable length with maximum devices.
The unaware continue to take the risk of using this for internal cable in their system. Why do they get away with the Flat-Zip configurations? They can because it works in small configurations at short cable lengths, specifically less than 3 feet. The problem arises with the expansion to longer cable lengths and the addition of more devices; trouble shooting begins as the system fails to operate reliably.
Impedance mismatch between the signal line characteristic impedance, Zo, and the driver's output or the receiver's input impedance can cause reflections. These reflections are more commonly known as ringing and can cause undershoot or overshoot when the reflections return and combine with the original signal. The characteristics of twisted pair cable are more defined than those of the parallel lines in the Flat-Zip ribbon. The twisted pair can be more accurately terminated, in turn reducing reflections. Typical bus termination is equal to the nominal bus characteristic impedance, Zo.
The overall SCSI bus impedance is not consistent and will have discontinuities depending on configuration. Each additional capacitive load will reduce the bus impedance and will cause a reflection on the incident wave. The unloaded SCSI bus impedance should be 110[less than]135 ohm for Low Voltage Differential (LVD) and 80[less than]100 ohm for Single-Ended. The loaded configuration impedance will probably be 85[less than]110 ohm for LVD and 50[less than]80 for Single-Ended.
The curves of Fig 1 and 2 compare the Near and Far End crosstalk characteristics of several cable configurations over frequency. The least resistant to crosstalk is the Flat-Zip ribbon with PVC, FEP, and TPE insulation. Per Fig 2, there is a 5dB improvement of FEP and TPE insulation over PVC. These three flat ribbon cables should only be considered for use in configurations where the length is short (1.5m) and there are two or three devices maximum. The data indicates similar Near end characteristics for TPE and FEP Zip cables.
The Twist-N-Flat PVC, in 28Ga, 30Ga solid, and 30Ga stranded are 20% more resistant to crosstalk than the Flat-Zip. They will work in maximum Ultra2 LVD SCSI bus configurations at maximum cable lengths. This resistance to crosstalk is the characteristic that makes these three the cables of choice for use in the Ultra2 SCSI bus. The best cable with the best crosstalk resistance characteristics is the 30 Gauge stranded.
External SCSI cables use TWP and signal placement within the cable bundle to minimize and control crosstalk. REQ and ACK are placed in the center of the cable bundle with data located on the outer loop near the cable shield.
Terminating the Single-Ended SCSI bus has historically been done with passive terminators at both ends of the bus, equal to the nominal cable characteristic impedance. As SCSI bus speed increased, this passive termination was not adequate. More versatile active single-ended terminators were developed to satisfy the termination requirements through the Ultra SCSI generation. Active termination provided better impedance matching, pull-up current, and bias stability for performance SCSI architectures. A variation of the Single-Ended termination included active negation to enhance and better match the cable and driver termination. With the shift to LVD SCSI signaling, the need for a new and improved terminator existed. The Single Ended Active negation termination used on Ultra SCSI would not work on the New LVD SCSI signaling.
Unitrode conceived a new generation of low voltage differential termination that combines the Single Ended active negation features into a new concept that includes features of series and parallel termination. The new features are commonly referred to as Y - Termination. Using the best of series and parallel termination in a new analog technology, series termination is best for reflections from impedance discontinuities and parallel termination is best for loads distributed along the cable. This revolutionary analog Y - termination scheme provides an LVD SCSI active termination that properly terminates the SCSI bus signals up to 500mhz and beyond, while compensating for discontinuities and load placement. This revolutionary analog termination is excellent for the LVD SCSI bus with its varying impedance characteristics depending on devices, drivers, receivers, cable, length, and host adapter, etc.
Don Getty is the senior applications engineer at Unitrode (San Jose, CA).