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Description  |
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BACKGROUND OF THE INVENTION
a. Field of the Invention
This invention broadly relates to digital communications systems. More
specifically, it concerns transmitting and receiving high speed digital
information signals in binary form, such as computer data, over any noisy
line medium, such as alternating current (AC) power lines.
b. Description of the Prior Art
Digital information, such as computer data, is known to be capable of
transmission over existing alternating current (AC) power lines. The
potential benefits of such data communication are well-recognized,
including extreme versatility for interconnecting electronic office
products, data terminals, remote printers, personal computers and the
like. Creation of data paths may be accomplished simply by plugging the
distributed terminals into any available AC outlet. Any type of equipment
that can be run by a central computer could be linked to that computer
through the same power cord already provided for such equipment. Such a
central computer could be used to control various process equipment,
including heating, lights, and air conditioning.
The presently common situation with the vast majority of users requiring
digital communication lines is the use of hardwiring to interconnect the
various components. This is expensive, inflexible, and generally provide
higher data rates than are necessary for the average user. Since AC power
wiring already exists in most, if not all, locations where data
transmission is needed, reliable high-speed digital communication through
this medium would provide significant cost savings and system flexibilty.
A widely available transmission medium, i.e., an AC power line in the
frequency range of approximately 100 kiloHertz (kHz) to 500 kHz, generally
exhibits unpredictable transmission characteristics such as extreme
attenuation at certain frequencies, phase changes along the route, notches
and discontinuities. Generally, three modes of noise most common: low
voltage Gaussian noise, low voltage impulsive interference, and very high
voltage spikes. Of these three, the low voltage impulsive interference
tends to be the predominant source of data transmission errors, i.e., data
transmission may be reliably accomplished even in the presence of Gaussian
noise. As for high voltage spikes, they are relative infrequent and
invariably cause data errors, with error detection/retransmission
(ACK/NACK) being commonly recognized as the best method of recovering the
lost information. Furthermore, these characteristics may vary
significantly as load conditions on the line vary, e.g., a variety of
other loads being added or removed from the current-carrying line. Such
loads include industrial machines, the various electrical motors of
numerous appliances, light dimmer circuits, heaters and battery chargers.
Past attempts to solve these problems have included a variety of single or
multi-channel, narrow band transmission techniques. Narrow bandwidth,
however, limits the data transmission capacity of the link. Furthermore,
the changing noise environment on the AC power line significantly impairs
the reliability of any technique which suffers when a transmission channel
(a predetermined bandwidth) is interrupted or lost. For these and other
reasons, AC power line communication has not in the past been regarded as
either fast or reliable.
While multi-channel digital coding techniques have modestly improved the
reliability and speed of power line communication systems, the cost of
improvement has been bulky, sophisticated and expensive signal processing
equipment. Thus the potential for power line data transmission has not yet
been achieved, nor realistically even approached. For example,
substantially error-free data transmission has been limited to data rates
under ten kilobits per second (kbps). Even with such improved systems,
reliability is highly suspect since any one or more of the predetermined
narrow bandwidth transmission channels may suddenly become unusable,
without warning, due to unpredictable variations in the power line
transmission characteristics.
In recent years, data transmission over power lines has become
significantly more difficult, due to changes in the nature of the
distortion encountered. Due to the widespread use of personal computers
and remote printers, the FCC issued regulations which place limits on
conducted or radiated digital emissions from computing devices onto power
lines. In order to satisfy these requirements, computer manufacturers
routinely added filters appearing, from the line side, as very low
impedances, such as very high capacitances having values on the order of
0.1 microfarad. This significantly affects distortion encountered by
wideband signals, and at the same time can cause severe attenuation of
certain narrow bandwidth signals.
Many common forms of carrier signal modulation have been attempted in
connection with power line communication systems. In each of these
schemes, the digital information is modulated onto a carrier and the
carrier is then added to the AC power line. A receiver picks off the
modulated carrier signal and then demodulates that signal to recover the
digital data information. Two of the more common types are amplitude-shift
keying (ASK) and frequency-shift keying (FSK). Both techniques have been
generally regarded as being susceptible to electromagnetic interference
(EMI) and radio frequency interference (RFI). A third principal modulation
technique, phase-shift keying (PSK), has also generally been considered
unsuitable because of increased susceptibility to noise interference and
consequently fluctuating carrier signal attenuation.
In light of the aforementioned difficulties, power line communications has
not been regarded as a potential local area network (LAN) medium, despite
what should be a natural extension of LAN systems to an already existing
data transfer medium reaching into virtually every office in a building,
every home in a neighborhood, or anywhere else AC lines or other
two-conductor media can reach. Instead, LANs are generally expensive
hardwired installations delivering data transfer capacity far in excess of
that required by most users (nodes) on the network.
OBJECTS OF THE INVENTION
A broad object of the invention is to provide a power line communication
system capable of substantially error-free data transmission at both low
and high data rates, utilizing any available existing lines, such as
ubiquitous alternating current (AC) power lines, for data transmission.
Another broad object is to provide an inexpensive, highly reliable AC power
line communication system capable of data transmission at faster speeds
than those of presently known systems.
An object of the invention is to eliminate the need for expensive and
inflexible data communication line hardwiring for otherwise portable data
processing equipment.
Another object is to provide a highly flexible power line communication
link requiring minimal installation, having small volume for ease of
portability and reconfigurability.
An additional object of the present invention is to provide a digital data
transmission system having enhanced error detection/error correction
capabilities in order to increase data transmission rates.
Yet another object is to provide a power line communication system which is
substantially immune to constantly changing power line data transmission
characteristics, and especially robust even under impulsive noise
conditions.
Still another object of the invention is to provide a power line
communication system having data transfer reliability and an internal
protocol sufficiently robust to allow networking between a number of
devices participating in a local area network (LAN), even in token passing
configurations.
SUMMARY OF THE INVENTION
According to the invention, a novel circuit is provided for data
communication over noisy lines. The circuit includes a transmitter section
and a receiver section, enabling two-way communication with similar
circuits located elsewhere on the line. Intelligent control of the
transmitter/receiver characteristics is provided for giving improved
reception and transmission under constantly changing, adverse noise
conditions, distortion and attenuation.
A data modulated carrier is provided using a wideband technique, creating a
waveform having, in each period, substantially uniform power over the
available bandwidth. Thus, sensitivity to noise, distortion and
attenuation is reduced because the signal frequency spectrum is
substantially wider than any impulsive noise signal, and is wide enough so
that frequency dependent attenuation phenomena are reduced in effect.
The receiver includes means for receiving the noise-containing, distorted,
modulated carrier signal impressed upon a line signal, such as a 60 Hz AC
line voltage. The receiver also works over dead lines. The line signal, if
present, is removed by appropriate filter means, leaving only the
noise-containing, distorted, modulated carrier signal. Selectively
controlled low and high pass filters act on the modulated carrier signal
in response to control signals which may be generated by microcomputer
means in a predetermined fashion. In particular, the filters may be
controlled so as to seek a filtering arrangement in which distortion due
to transmission over the AC lines is equalized as much as possible.
There is provided means for converting or demodulating the filtered carrier
signal to a digital signal or pulse train representing discrete
information bits. The resulting digital signal contains the desired
"intelligence" to be recovered, and also serves as the driving signal for
controlling various adaptive circuit control means, including the filters
mentioned above. A logic circuit searches the incoming digital signal for
recognizable data patterns in order to establish a receiver
synchronization substantially in concert with the highest
energy-containing portion of each new information bit. The logic circuit
combines novel adaptive filter control, signal correlation means, and may
also employ special error detection/error correction means in order to
search, track, verify and lock onto a valid data transmission preamble for
further processing by the receiver/transmitter, or by the host device,
such as a personal computer, to which the inventive device is connected.
The transmitter of the invention is arranged to generate a properly
modulated carrier signal encoded with information to be transmitted. The
transmitted signal is a wideband signal having energy components spread
substantially across the usable frequency spectrum of the transmission
medium. In accordance with an aspect of the invention, the wideband
transmitter is uniquely feedback-controlled so as to adaptively control
the transmitted signal strength in response to changing impedances on the
transmission line.
According to other especially advantageous aspects of the invention, the
microcomputer means is arranged to selectively provide a variety of
network access modes, including "master/slave," "token bus/token passing,"
and other data transfer control arrangements. The invention may be
selectively operated in synchronous or standard asynchronous communication
protocols over an RS-232-C interface with its host device. These features,
as well as others more specifically described below, provide a PLC device
useful for LAN applications, the invention achieving previously
unachievable speed, reliability, versatility and ease of operation, at low
cost.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention mentioned in the
above brief explanation will be more clearly understood when taken
together with the following detailed description of an embodiment which
will be understood as being illustrative only, and the accompanying
drawings reflecting aspects of that example, in which:
FIG. 1 illustrates, in block form, part of a circuit for receiving a
wideband data signal;
FIG. 2 illustrates schematically an adaptive filtering circuit wherein the
filtering is controlled in response to varying line conditions;
FIG. 3 illustrates a code representation of a filter control signal;
FIG. 4 illustrates schematically a circuit for transmitting a modulated,
wideband carrier signal having digital data encoded therein, the power of
the signal being adaptively controlled in response to changing impedance
of the transmission medium;
FIG. 5 illustrates schematically the transmit voltage control block of FIG.
4;
FIG. 6 illustrates a voltage supply function provided by the transmit
voltage control circuit of FIG. 5;
FIG. 7 illustrates schematically a transmitter power amplifier block of
FIG. 4;
FIG. 8 illustrates a data bit period according to the invention, detailing
the particular wave characteristics used in a synchronization scheme;
FIG. 9A illustrates a useful waveform for wideband power line communication
signals;
FIG. 9B illustrates a power spectrum for the waveform of FIG. 9A; and
FIG. 10 is a state diagram depicting a method of searching for and
verifying reception of a data signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows part of a power line communication (PLC) circuit 10 for
receiving data signals directly over a power line 20 supplying AC power to
a peripheral unit 46, such as a remote printer, personal computer, or the
like. Separate data wires are eliminated by the inventive circuit. The
circuit 10 may be selectively coupled to the line ("hot") and neutral
conductors, or to the ground and neutral conductors (not shown). In one
embodiment, the AC line conductor pair to be connected to is automatically
selected depending upon which of the two available pairs possesses better
signal transmission characteristics at any given moment, i.e., less
distortion, and better signal-to-noise ratio for the data-carrying signal.
A line surge protector 22, such as a gas tube surge absorber, may be
applied across the signal line. The selected conductors, in this example
hot and neutral, may be connected to the primary winding of a coupling
transformer 24, the transformer removing the 60 Hertz AC voltage provided
by the AC power line 20. Line signals VH and VL are taken from the
secondary winding terminals of coupling transformer 24, the importance of
these signals to be explained below in connection with feedback control of
the inventive transmitter section.
Transient voltage suppressors 26 may be applied between each terminal of
the transformer secondary and ground. First order low pass filtering 28
and first order high pass filtering 30 is applied to the received signals,
using well-known filters. According to the invention, the modulated
carrier used to convey data is a wideband phase shift-keyed (PSK) signal,
having substantial energy across the entire usable frequency range, i.e.,
between approximately 100 kiloHertz (kHz) to 500 kHz. Below 100 kHz, high
power noise spikes may provide interference to such an extent that any
reliable transmission may be difficult. Transmission at frequencies over
500 kHz may radiate energy into the broadcast AM frequency band, in
violation of applicable FCC regulations. Accordingly, the low and high
pass filters 28,30 substantially confine the signal frequency to the
desired bandwidth. It will be understood that in signal environments other
than AC power lines, the defined usable frequency spectrum may differ, and
in that case, the wideband data signal will contain energy over that
particular bandwidth.
The filtered signals are applied to the inputs of a difference amplifier
32, for example, including a JFET input op amp, to provide the modulated
carrier signal 33, less the common mode noise. This signal 33 will be
highly distorted and will contain substantial noise components, despite
the signal conditioning already applied. Second order low and high pass
filters 34,38 may thus be used for equalization and additional filtering
of the modulated carrier signal 33, enabling further signal processing,
including reliable demodulation. In a highly advantageous manner,
selectively controllable equal-component value, second order, Sallen-Key
filters are used. Digital switching control 36 is provided to adaptively
adjust both the cutoff frequency and damping of the low pass section 34.
Similarly, digital control 40 is applied to the high pass section 38 to
adjust its cutoff frequency and damping, as will be explained in greater
detail below.
After adaptive equalization, the signal is converted from an analog,
modulated carrier to a digital pulse signal 43 by any suitable means 42.
In one simple and inexpensive embodiment, the conversion is accomplished
by two-stage clipped amplification of the carrier signal 41, the output of
the second amplifier then being applied to one input of a high speed
comparator in a circuit using hysteresis. The digital pulse signal 43 is
provided at the output of the comparator. In another embodiment, an A/D
converter having longer bit length may be used to enhan | | |
