Building Fast SCSI Subsystems





SCSI has become the interface of choice for high-performance

storage subsystems. The original SCSI specification envisioned

transfer rates of up to 5MHz. The new SCSI-2 specification allows

faster rates of up to 10MHz. However, by pushing these original

SCSI standards to their limits, system integrators have seen

reliability problems mount. The dilemma is that signal quality

problems, which have been present from the start, become more

apparent as busses become more heavily loaded and are operated at

faster data rates.



The SCSI electrical specification has two mutually exclusive

transceiver specifications:



 1. Differential RS-485 transceivers that allow for up to 10Mhz

    data transfer at a maximum cable length of 25 meters (82 feet).



 2. Single-ended TTL transceivers which allow:

  

    Slow synchronous data transfer up to 5MHz at a maximum cable

    length of 6 meters (19 feet).

  

    Fast synchronous data transfer up to 10MHz at a maximum cable

    length of 3 meters (9 feet).

  

    Asynchronous data transfer (no maximum transfer rate is given

    but typical rates are about 1.5MHz to 2MHz) at a maximum cable

    length of 6 meters (19 feet).



Each transfer may consist of one, two, or four bytes, depending on

the bus width option implemented. However, today most

implementations utilize only byte-wide data.



When 10MHz fast SCSI was first proposed, only differential SCSI

transceivers were envisioned. However, many drive manufacturers

have chosen to implement fast SCSI with single-ended drivers

because of savings in cost, size and power consumption. This

presents several problems to integrators, especially as systems

increase in speed and size. A very common symptom of an unreliable

single-ended interface is bus misoperation following the addition

of devices or cabling to the system. The failures increase as the

number of devices and length of the cable grow. The failures are

also unpredictable, generally catastrophic in nature and are not

necessarily the same from system to system.



Most data reliability problems stem from signal reflections and

noise which are read by SCSI receivers as incorrect data or false

SCSI bus phases. The SCSI cable is a transmission line which has a

characteristic impedance whose value depends upon the type of cable

used. Discontinuities in this impedance can cause signal

reflections to occur. These impedance variations can be the result

of extra capacitance internal to SCSI devices, connectors, improper

terminators, mixing of different cable types, cable stubs, etc. At

10MHz fast SCSI rates, these reflections are much more prevalent

than at the slower 5MHz SCSI rates. Additional noise picked up from

external devices, as well as from other signals on the SCSI cable,

can add to these false signals. Unfortunately, a 10MHz fast SCSI

bus is a more efficient transmitter of noise than a slower 5MHz

SCSI bus.



A carefully, well-configured single-ended SCSI bus can reliably

transfer data at 10MHz without a problem. However, good engineering

practices should be followed in order to guarantee success:



Use as short a cable length as possible. The ANSI XT3T9.3 SCSI-3

Parallel Interface working group is currently recommending that for

10MHz data transfers the total cable length should not exceed 3

meters (10 feet).



Avoid stub clustering. Do not space SCSI devices on the cable any

closer than 0.3 meters (12 in.) apart. When devices are clustered

closely together on the SCSI cable, their capacitances add together

to create an impedance discontinuity and thus reflections. SCSI

devices should be spaced as evenly as possible.



Cable stub length should not exceed 0.1 meters (4 in.). Some SCSI

devices may create stubs internal to the device which exceed this

value, resulting in excessive capacitive loading and signal

reflections. This parameter is under the control of the SCSI device

(i.e. tape drive or disk drive) manufacturer. The SCSI cabling

itself should include no stubs.



Watch out for capacitance. As devices are added to a SCSI bus,

capacitance is introduced to each signal from the connectors,

receivers, and PC board traces. The SCSI-2 specification limits

this capacitance to 25pF and this number will probably be lowered

to 20pF in SCSI-3. The reason for this limit is that the added

capacitance has the effect of lowering the impedance of the section

of cable to which these devices are added, as well as adding delay.

Both of these effects can be highly detrimental to a fast SCSI bus.

Look for input filters that may be attached to the SCSI front-end

of the printed circuit board. These filters add capacitance.



Avoid connector adapters. They are just another source of

capacitance and signal degradation.



Route cable with care. Avoid practices such as rolling the cable up

on itself, running the cable alongside of metal for long lengths or

routing the cable past noise generators (i.e. power supplies).

Placing the cable near ground planes created by grounded metal

cabinetry reduces its impedance. For example, the free air

impedance of a unshielded 28 AWG, 0.050 in. center ribbon cable is

about 105 ohms, but direct contact with a metal ground plane cuts

that by 61 ohms. Such an impedance discontinuity will cause signal

reflections. The SCSI-3 working committee suggests that in order to

minimize discontinuities due to local impedance variation, a flat

cable should be spaced at least 1.27 mm (0.050 in.) from other

cables, any other conductor, or the cable itself when the cable is

folded.



Use high impedance cables wherever possible. This will allow for

closer termination impedance matching as well as provide more

cushion against the impedance reduction normally experienced during

cable routing.



Avoid mixing cable types. Select either flat or round, shielded or

non-shielded. Typically mixing cables mixes impedances. Cable

impedance mis-match is a common problem resulting in signal

reflections. If cable types must be mixed, use of 26 AWG wire in

0.050in. pitch flat cable will more closely match impedances of

many round shielded cables, resulting in fewer impedance

discontinuities and therefore improved signal quality. Internal

cables are typically flat ribbon cables, while external cables

should be shielded. Where they offer easier routing, size

advantages, and better air flow, round cables can be used

internally as well. This in fact, may be desirable if it allows for

better impedance matching to the external cable.



Ribbon cable shows fairly good crosstalk rejection characteristics

due to the GND-Signal-GND layout. However, more care needs to be

taken to insure adequate performance when round shielded cable is

employed. When round cable is used, select a cable that uses a wise

placement of key lines within the cable. The following is

suggested:  In the case of a standard 25 pair round construction,

pairs are arranged inside the cable in three layers. The closer the

pair is to the outside shield, the lower the impedance. Conversely,

pairs located closer to the center of the cable have higher

impedances. Using centrally located high impedance pairs for

speed-critical signals such as REQ and ACK is desired. By locating

data pairs in the outermost layer of the cable, crosstalk between

REQ, ACK, and the data lines is minimized. The middle layer might

contain status lines such as C/D, I/O, MSG, ATN, etc.  Another

thing to look for in a round shielded cable is to make sure that

the lowest impedance wire in the cable is used for TERMPOWER to

minimize transmission line effects on what is meant to be a voltage

supply line. Some SCSI cable vendors have put a low-impedance

conductor into the cable specifically for this purpose. Typically

a larger wire gauge along with a higher dielectric constant is used

on this conductor.





SCSI Cable Types:



SCSI systems can utilize cabling both inside and outside the

cabinet. Internal cables are typically flat unshielded ribbon

cables while external cables are generally round and shielded. The

most common internal cable is the 50 conductor flat-ribbon cable

which typically uses 28 AWG conductors on 0.050 in. centers.

Typical free air characteristic impedances for this type of cable

runs about 105 ohms. DPT has had good success with the 3365 Round

Conductor Flat Ribbon Cable manufactured by 3M Corp. It uses 28 AWG

stranded wire on 0.050 in. centers and has a nominal free air

characteristic impedance of 108 ohms.



External shielded 8-bit SCSI cables typically contain 25

twisted-pairs (50 conductor) with an overall foil/braid composite

shield. Typical free air characteristic impedances for this type of

cable have run about 65 to 80 ohms. Higher single-ended round

shielded cable impedances of 90 to 100 ohms are becoming available

and should be highly considered.



The SCSI-2 specification requires that systems which employ the

fast synchronous data transfer option shall use cables which

consist of 26 AWG or 28 AWG conductors. Characteristic impedance is

specified as between 90 and 132 ohms. In addition, signal

attenuation should be 0.095 db maximum per meter at 5MHz. The pair

to pair propagation delay delta should not exceed 0.2ns per meter.

Finally, the DC resistance is specified as 0.23 ohms maximum per

meter at 20 degrees C.





Passive Termination:



Passive termination (called Alternative-1 in the SCSI-2

specification) is the most common form of termination currently

utilized today. A typical single-ended SCSI system will employ

eighteen sets of 220 ohm pull-up and 330 ohm pull-down, thick film

resistors to equalize impedance and to absorb reflected signals.

The Thevenin equivalent impedance for this type of termination is

132 ohms.



In order to maintain the largest possible high-level noise margin

it is advisable to use resistors with a maximum tolerance of 2%

rather that 10%. In worst case conditions the difference could

easily add up to 140mV. Worst case occurs when the pull-up resistor

is high and the pull-down resistor is low.



Consider the situation where TERMPOWER is being driven across a 6

meter (19 feet) cable. Due to power supply tolerances and to the

fifteen or so SCSI bus signals drawing current, it is possible for

the remote end TERMPOWER to be sitting at 3.65 volts (see TERMPOWER

section for more details). If 2% resistors are used, the worst case

termination voltage divider will have a gain of 0.588 V/V and the

quiescent signal bias will be 2.15 V. If 10% resistors are used,

the worst case termination voltage divider will have a gain of

0.551 V/V and the quiescent signal bias will be 2.01 V. In this

worst case example, given the SCSI mandated minimum V(ih) of 2.0 V,

only 10mV of high-end noise margin will remain.





Active Termination:



The preferred termination for 10MHz fast SCSI busses is active

termination. This type of termination is known as Alternative-2 and

uses only one 110 ohm resistor per signal per bus end pulled up to

locally supplied, voltage regulated 2.85 V. Features of this

termination include:



Termination voltages, and therefore the currents flowing through

the 100 ohm termination resistors are at least partly immune to IR

voltage drops on the TERMPOWER line until TERMPOWER - 2.85 V equals

the dropout voltage of the voltage regulator, about 1.1 V.



Closer match to the characteristic impedance of the cable (110 vs.

132 for passive as compared to the typical 105 -108 ohms free air

impedance of the cable) minimizes reflections.



Increased high level noise margin of deasserted signals.



Higher pull-down currents avoid rising "staircase" waveforms seen

on weakly driven transmission lines.



Studies by Kurt Chan and Gordon Matheson, both of Hewlett Packard,

have shown that mixing termination types will yield better

performance than using passive termination alone. Wherever

possible, use SCSI devices that employ active termination.





Where To Terminate:



Termination should be installed only at the far ends of the cable.

If the DPT controller is at one end of the bus and a SCSI device is

at the other end leave all three controller termination resistor

packs in their sockets. If the DPT controller is supporting both

internal and external SCSI devices and thus is located in the

middle of the bus, the termination resistor packs must be removed

from their sockets. In both cases, disable the termination of any

SCSI devices that are not located at the cable ends. This can

usually be done by jumper configuration, removal of resistor packs,

or both.



Ideally, TERMPOWER should be located at the terminations, not in

the middle of the cable. Interface error rates are lower if the

termination voltage is maintained at the extreme ends of the cable.

From strictly a signal quality perspective, it is best if

terminators get power only from the device to which they attach,

and not over the bus. Unfortunately, cable-ended devices may be

powered-down and the bus would then be inoperative unless the

terminators are supplied from the other voltage sources along the

bus. This fact must be balanced against desired signal quality.



Most drives provide jumpers to select the manner in which TERMPOWER

is supplied to their on-board termination. DPT recommends that

drives be configured to supply their own isolated TERMPOWER unless

accidental power down concerns dictate otherwise.



The reason why TERMPOWER should be applied near terminations only,

is that TERMPOWER is a transmission line which shares many of the

same characteristics as the signal lines. Current surges entering

this line at the terminators will propagate and reflect exactly as

they would on any signal line, except where there is a

low-impedance voltage source. It follows then, that current surge

waveforms propagating down the bus from a point where many data

lines are changing simultaneously will couple into other signals

through the pull-up termination resistors if there is not a

TERMPOWER voltage source right at the terminator to absorb or

provide the current surge needed.



The worst real-life case is where data lines along with MSG, C/D,

and I/O all change at the same time, causing noise on signals of

opposite polarity (several signals going low causing a deasserted

signal to also go low, or signals going high causing an asserted

signal to also go high). This phenomenon has nothing to do with

crosstalk, or driver skew rate, but is instead a function of where

TERMPOWER is applied and where the drivers are.



Another reason to supply TERMPOWER locally is to prevent the loss

of receiver noise margin due to TERMPOWER DC voltage drop across

the cable. It is not uncommon to find TERMPOWER resistances of 2

ohms or more on maximally configured systems. When 15 to 18 signals

conduct, the TERMPOWER line will carry nearly 300mA to the far

terminators, which would cause a voltage drop across the cable of

about 0.6V during these periods.



DPT controllers drive TERMPOWER onto the cable through a thermistor

and a Schottky diode. Taking into account power supply tolerances

it is not inconceivable that under maximum loading conditions

TERMPOWER at the controller connector may be lowered to 4.25V.

Subtract 0.6V due to TERMPOWER DC resistance and far-end TERMPOWER

ends up at 3.65V. This would bias a quiescent signal to 2.19V

((330/220+330) * 3.65)). Comparing this to the SCSI specified

minimum V(ih) of 2.0V for single-ended inputs leaves a high end

noise margin of only 190mV. This quick and dirty worst case

analysis does not even include termination resistor tolerances

which could exacerbate the problem. Its a good thing that TTL

receivers typically switch near 1.4 to 1.5V (the middle of the

V(ih) range) rather than at 2.0V.



For all the reasons discussed above it is advised that TERMPOWER be

maintained at as close to nominal voltage as possible. In the case

of DPT controllers this means that the voltage level to the

controller be maintained at nominal and not allowed to droop.





TERMPOWER Bypassing:



The ANSI XT3T9.3 SCSI-3 Parallel Interface working group recommends

that all TERMPOWER lines be decoupled at each terminator to

minimize TERMPOWER glitch coupling.



The minimum recommended values is a 2.2uF solid tantalum capacitor

along with .01uF ceramic capacitor in parallel to help with high

frequency, low voltage noise. These capacitors, when utilized, will

supply the high frequency, low impedance path to ground necessary

to filter out glitches. Without the capacitors, TERMPOWER acts

simply as a high-impedance node and couples noise from signal to

signal. With the capacitors, an "AC ground" exists which filters

this noise.



For cables of significant length and configurations without

TERMPOWER at each terminator there is a high probability of signal

corruption without adequate decoupling. Thus, DPT recommends that

the system integrator inspect the system to insure that all SCSI

devices used are properly bypassed.



DPT controllers provide power to the on-board termination resistors

directly from a highly decoupled power plane thus insuring minimal

TERMPOWER glitch coupling. However, it is important to keep in mind

that decoupling in the middle of the bus is not sufficient. If the

DPT controller is supporting both the internal and external SCSI

busses simultaneously then the SCSI devices at the ends of the

cable need to be bypassed at their terminations. This requirement

applies to both passive and active termination.





Fast Differential SCSI:



When the total cable length of a fast, synchronous SCSI bus cable

must exceed 3 meters (9.84 ft.) DPT recommends the use of a

differential-ended SCSI interface.



An important concern is cable selection. When twisted-pair cable is

used, differential-ended SCSI busses provide greater signal

integrity over longer distances than does single-ended, because

noise coupled into a twisted-pair generally appears equally on both

wires. Because differential receivers respond to differences

between the conductors of the twisted-pair, the coupled common-mode

noise is rejected.



On the other hand, the signal positioning of a differential SCSI on

a flat non-twisted ribbon cable causes two problems. First, noise

introduced into parallel conductors tends not to be common mode.

Second, while the single-ended conductor arrangement naturally

interleaves ground wires between signal wires, there are not enough

conductors to interleave grounds between each differential signal

pair. These factors lead to increased crosstalk between adjacent

conductors on a ribbon cable.



DPT strongly recommends the use of twisted-pair cable (either

twisted-flat or discrete wire twisted-pairs) for differential-ended

SCSI interfaces.



The maximum cumulative cable length permitted is 25 meters (82

feet) with devices not to be spaced any closer then 0.3 meters

apart (12 in.) and stub lengths not to exceed 0.2 meters (8 in.).

As in single-ended, SCSI bus terminators should be installed only

at each end of the cable.





Acknowledgements:



Information for this paper is based upon testing conducted in the

DPT labs as well as the work of many others. Much valuable research

has been done under the auspices of the SCSI working committees and

we are very appreciative for all their excellent publications. The

major portion of the wording in this report has been copied

directly from the SCSI specifications themselves or from papers and

articles published by the committee membership, as well as others.

In particular, we would like to thank Kurt Chan of Hewlett-Packard,

Gordon Matheson of Hewlett Packard, Paul Boulay of Laser Magnetic

Storage International, James E. Schuessler of National

Semiconductor Corp, Skip Jones of Emulex Corp, Peter Blackford of

Cooper Industries, and John Lohmeyer of NCR Corp.





          For reprints, ask for Technology Focus Paper:

                "Building Fast SCSI Subsystems"

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