Electronic – Computer bus frequency and electricity frequency

buscomputersfrequency

  1. From http://en.kioskea.net/contents/pc/bus.php3

    A computer bus is characterised by the amount of information that can be
    transmitted at once. This amount, expressed in bits, corresponds to
    the width of the bus, i.e. the number of physical lines over which
    data is sent simultaneously. A 32-wire ribbon cable can transmit 32
    bits in parallel. The term "width" is used to refer to the number of
    bits that a bus can transmit at once.

    Additionally, the bus speed is also defined by its frequency
    (expressed in Hertz), the number of data packets sent or received per
    second. Each time that data is sent or received is called a cycle.

    This way, it is possible to find the maximum transfer speed of the
    bus, the amount of data which it can transport per unit of time, by
    multiplying its width by its frequency.

    A bus with a width of 16 bits and a frequency of 133 MHz, therefore,
    has a transfer speed equal to: 16 * 133*10^6 = 2128*10^6 bit/s.

  2. Also if I understand correctly, a signal/data transmitted in a
    computer bus is actually an electrical current with its electricity
    frequency. The power supply unit on the power chord for a computer
    is a AC->DC converter, so is the electricity frequency zero in
    computer buses?

I wonder if the bus frequency and the electricity current frequency are the same concept? If not, how are they related?

Also if I understand correctly, cables such as USB cables and Ethernet cables are also computer buses. So the above questions are asked for them too.

Thanks!

Best Answer

I think what might be missing here in your understanding is that when we send information on a computer bus, we are sending digital information. Bits are represented with two states, '1' or '0' - or simply "on" and "off". A string of bits represents a number in base 2. We represent these two states (1 and 0) with discrete voltage levels, for the sake of simplicity, something like +5V and 0V respectively (different devices may use different levels to represent their bits but the principle is the same).

So when we talk about a computer bus having a frequency, we're talking about how quickly it can toggle a bit - or a single wire in this case - from a logic '1' to a logic '0' or vice versa as this will determine the maximum bandwidth the bus is capable of (at the bit rate level). To further divide computer buses into categories, there are parallel buses and serial buses. A parallel bus breaks out the individual bits of a base 2 number (typically an address or data value) and gives each one a wire. Thus, the calculation that was quoted holds true for parallel buses. Then there are serial buses for something like USB like you mentioned. On a serial bus, all bits are sent on a single wire (two wires in USB's case, but they are a differential pair, so you can think of it as one wire). The bit rate calculation for that would simply be its maximum "toggling" frequency.

You should not try to relate the above with a sinusoidal AC signal frequency found in AC power. Yes you are correct in that a DC voltage should have a theoretical frequency of 0 - but in the context of digital communication we are not really interested in that.

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