SCSI Long Cables, High Voltage Differential Signals and Data Skew?

Discussion in 'A+ Certification' started by Will Hay, Mar 4, 2004.

  1. Will Hay

    Will Hay Guest

    Can someone explain why using differential signals on a SCSI cable allow for
    such long cable lengths ~20m while single ended cable is typically 6m. From
    reading the reference below I understand that with differential cable 1 bit
    is carried by two wires each having opposite signals with the difference
    being taken to represent the data (1). What I'm not sure about is how does
    this eleviate the problem of data skew i.e. data on wires in a parallel bus
    arriving at different times.

    Thanks


    http://www.pcguide.com/ref/hdd/if/scsi/protDiff-c.html
     
    Will Hay, Mar 4, 2004
    #1
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  2. Will Hay

    Will Hay Guest

    Posted too soon - found an explanation using Google
    http://groups.google.co.uk/groups?q...=88rufs$v91$&rnum=2

    (Mark S. Bilk)
    Pasted below

    Two basic principles: Electrical signals are always sent
    using two conductors, because voltage is only meaningful as
    the voltage difference between two points. When current
    flows through a conductor, it causes a voltage difference
    to appear between the two ends; this can be minimized by
    using a large (wide and thick cross-section) conductor.

    Most electrical connections on a circuit board or in a cable
    use a single conductor -- wire, coax, or printed foil -- to
    carry each signal, and a ground conductor, or series of them,
    to establish a zero-voltage reference between the circuits
    sending and receiving the signals. Since the receiver re-
    sponds to the voltage difference between the signal and ground
    conductors, and the ground is common to all the various signals
    being sent, the ground conductor has to be large and very well
    bonded at both ends in order to prevent reference voltage
    fluctuations between source and destination, because the
    ground conductor also carries signal currents -- from all the
    signals. Fluctuations between the ground voltage of sender
    and receiver (caused by signal currents flowing in the ground
    conductor) would cause errors in the received signal.

    Differential transmission uses a separate pair of conductors
    to carry each signal; if the voltage of one is changed by the
    transmitter circuit in a positive direction, the voltage of
    the other is changed equally in a negative direction. The
    receiving circut senses the difference in voltage between the
    two signal conductors, rather than between one signal conduc-
    tor and a common ground connection. So each signal is sent
    and received independently of all the others, instead of
    sharing the reference (ground) conductor.

    Although a ground conductor is still used, it carries much
    less signal current than it does in the single-ended (non-
    differential) method, and so can more easily maintain a
    consistent ground voltage between the two ends. However,
    since the ground is not used as a signal reference, this con-
    sistency is much less important in the differential system.

    Interactions between signals (which could cause reception
    errors) are much smaller with differential transmission for
    three reasons -- first, the electric and magnetic fields of
    the two equal and opposite signals in each pair tend to cancel
    each other out in their effect on neighboring signal pairs;
    second, any remaining effects on neighboring conductors tend
    to be in the same direction in both wires of the affected
    pair, and so get ignored by its receiver, which only responds
    to changes in opposite directions between the two wires;
    finally, there is no interaction between signals in the ground
    conductor, because it doesn't carry signals, and is not used
    as a reference by the receivers.

    Since signal interactions and consequent reception errors
    increase with signal frequency, and the interactions are much
    smaller with the differential system, it can carry signals at
    a higher frequency, and thus higher data rate, without errors.
    It's disadvantage is higher cost due to more complex sender
    and receiver circuits, and sometimes more cable conductors and
    connector pins.
     
    Will Hay, Mar 4, 2004
    #2
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