1 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE PDACE Glb Dept of E&CE 1. Introduction Power line communications (PLC) refers to the concept of transmitting information using the electrical power distribution network as a communication channel. This technology allows a flow of information through the same cabling that supplies electrical power. This novel idea of communication helps in bridging the gap existing between the electrical and communication network. It offers the prospect of being able to construct intelligent buildings, which contain many devices in a Local Area Network. During the last years the use of Internet has increased. If it would be possible to supply this kind of network communication over the power-line, the utilities could also become communication providers, a rapidly growing market. On the contrary to power related applications, network communications require very high bit rates and in some cases real-time responses are needed (such as video and TV). This complicates the design of a communication system but has been the focus of many researchers during the last years. Systems under trial exist today that claim a bit rate of 1 Mb/s, but most commercially available systems use low bit rates, about 10-100 kb/s, and provides low-demanding services such as meter reading. The power-line was initially designed to distribute power in an efficient way, hence it is not adapted for communication and advanced communication methods are needed.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
1. Introduction
Power line communications (PLC) refers to the concept of transmitting
information using the electrical power distribution network as a communication
channel. This technology allows a flow of information through the same cabling that
supplies electrical power. This novel idea of communication helps in bridging the gap
existing between the electrical and communication network. It offers the prospect of
being able to construct intelligent buildings, which contain many devices in a Local
Area Network.
During the last years the use of Internet has increased. If it would be possible to
supply this kind of network communication over the power-line, the utilities could
also become communication providers, a rapidly growing market. On the contrary to
power related applications, network communications require very high bit rates and in
some cases real-time responses are needed (such as video and TV). This complicates
the design of a communication system but has been the focus of many researchers
during the last years. Systems under trial exist today that claim a bit rate of 1 Mb/s,
but most commercially available systems use low bit rates, about 10-100 kb/s, and
provides low-demanding services such as meter reading.
The power-line was initially designed to distribute power in an efficient way,
hence it is not adapted for communication and advanced communication methods are
needed.
2 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
1.1 Benefits:
PLC integrates the transmission of communication signal and 50/60 Hz power signal
through the same electric power cable. The major benefit is the union of two
important applications on a single system. The data link appears 'transparent' to the
user. Although the devices are connected through the power line, consumers perceive
that there is a “separated” link available for data communications. Since the existing
power lines are used for signal transmission, the initial heavy cost and investment for
setting up a data communications system is avoided. Setting up such a
communications system then involves installation of transmitter and/or receivers at
appropriate points.
1.2 The Challenge:
Since the power line was devised for transmission of power at 50/60 Hz and at
most 400 Hz, the use of this medium for data transmission (especially at high
frequencies) presents some technically challenging problems. It is one of the most
electrically contaminated environments, which makes it very hostile for transmission
of data signals. The channel is characterized by high noise levels and uncertain (or
varying) levels of impedance and attenuation. In addition, the line offers limited
bandwidth in comparison to cable or fiber-optic links.
Power line networks are usually made of a variety of conductor types and cross
sections joined almost at random. Therefore a wide variety of characteristic
impedances are encountered in the network. This imposes interesting difficulties in
designing the filters for these communication networks.
3 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
1.3 Project aims:
The project aims to thoroughly explore the theoretical and practical aspects of
power line communications (PLC) techniques. We placed ourselves a number of goals
at the start of the project.
To gain a detailed knowledge of the challenges faced by PLC techniques
To explore the theoretical and practical aspects of PLC.
To research and design a working PLC and on/off control system.
To use the design and implement a power line communications system that
connects two personal computers and moreover one should be able to transmit
command over the power line to switch on/off an electrical device. The PC should be
able to transfer data using the power lines as their only link of communication.
1.4 Block Diagram Description
The block diagram shows the two personal computers(pc) used for
communication and device control connected through the powerline, which is the
communication channel. Since the communication is simplex, one pc is connected
through modulator and other is connected through demodulator to the powerline. The
scheme of modulation and demodulation used is FSK as it is inherently immune to noise
which is an important property as notoriously bad channel that has been developed
without regard for any communications considerations. The pc connected through
modulator to the powerline transmits data and pc connected with demodulator receives
data. The devices to be controlled by the transmitter pc are connected through the I/O
card of the receiver pc. The transmitter pc sends command to the receiver to on/off any
specific device.
4 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
Block Diagram
5 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
2. Focus on PLC
2.1 Noise:
The major sources of noise on power line are from electrical appliances, which
utilize the 50 Hz electric supplies and generate noise components, which extend well
into the high frequency spectrum. Apart from these induced radio frequency signals
from broadcast, commercial, military, citizen band and amateur stations severely
impair certain frequency bands on power line. The primary sources of noise in
residential environments are universal motors, light dimmers and televisions. This
noise can be classified as:
• 50 Hz periodic noise: Noise synchronous to the sinusoidal power line
carrier can be found on the line. The sources of this noise tend to be
silicon-controlled rectifiers (SCRs) that switch when the power crosses a
certain value, placing a voltage spike on the line. This category of noise
has line spectra at multiples of 50 Hz.
• Single-event impulse noise: This category includes spikes placed on the
line by single events, such as a lightning strike or a light switch turn on or
off. Capacitor banks switched in and out create impulse noise.
• Continuous Impulsive noise: This kind of noise is produced by a variety of
series wound AC motors. This type of motor is found in devices such as
found in vacuum cleaners, drillers, electric shavers and many common
kitchen appliances. Commutator arcing from these motors produces
6 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
impulses at repetition rates in the several kilohertz range. Continuous
impulsive noise is the most severe of all the noise sources.
• Non-synchronous periodic noise: This type of noise has line spectra
uncorrelated with 50 Hz sinusoidal carriers. Television sets generate noise
synchronous to their 15734 Hz horizontal scanning frequency. Multiples of
this frequency must be avoided when designing a communications
transceiver. It was found that noise levels in a closed residential
environment fluctuate greatly as measured from different locations in the
building. Noise levels tend to decrease in power level as the frequency
increases; in other words, spectrum density of power line noise tends to
concentrate at lower frequencies. This implies that a communications
carrier frequency would compete with less noise if its frequency were
higher.
• Background Noise: This is what every subscriber sees as already present
on the line, and not caused by subscriber’s appliances. Typically, this
originates from the Distribution Transformer, public lighting systems etc.
2.2 Attenuation:
Attenuation is the loss of signal strength as the signal travels over distance. For a
transmission line the input impedance depends on the type of line, its length and the
termination at the far end. The characteristic impedance of a transmission line (Zo) is
the impedance measured at the input of this line when its length is infinite. Under
these conditions the type of termination at the far end has no effect. A standard
7 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
distributed parameter model can obtain the characteristic impedance of an unloaded
power cable, and it is given by,
At the frequencies of interest for PLC communications (the high frequency range),
this approximates to,
where L and C are the line impedance and capacitance per length.
High frequency signals can be injected on to the power line by using an
appropriately designed high pass filter. Maximum signal power will be received when
the impedance of the transmitter, power line and the receiver are matched. Power line
networks are usually made of a variety of conductor types and cross sections joined
almost at random. Therefore a wide variety of characteristic impedances are
encountered in the network. Unfortunately, a uniform distributed line is not a suitable
model for PLC communications, since the power line has a number of loads
(appliances) of differing impedances connected to it for variable amounts of time.
Channel impedance is a strongly fluctuating variable that is difficult to predict. The
overall impedance of the low voltage network results from a parallel connection of all
the network’s loads, so the small impedances will play a dominant role in determining
8 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
overall impedance. Overall network impedances are not easy to predict either. The
most typical coaxial cable impedances used are 50 and 75-ohm coaxial cables. A
twisted pair of guage-22wire with reasonable insulation on the wires measures at
about 120 ohms. Clearly, channel impedance is low, it can even be as low as 0.1Ω. A
graph showing the variation of power loss with change in frequency for various load
impedance is shown below.
Fig I: Power loss versus carrier frequency for various load impedances at 1000-meter line.
In the above figure L [dB] is the power loss in decibels and ZL is the load impedance.
This presents significant challenges when designing a coupling network for PLC
communications. Maximum power transfer theory states that the transmitter and
channel impedance must be matched for maximum power transfer. With strongly
varying channel impedance, this is tough. We need to design the transmitter and
receiver with sufficiently low output/input impedance (respectively) to approximately
match channel impedance in the majority of expected situations.
9 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
2.3 Radiation of the Transmitted Signal:
When transmitting a signal on the power-line the signal is radiated in the air. One
can think of the power-line as a huge antenna, receiving signals and transmitting
signals. It is important that the signal radiated from the power-line does not interfere
with other communication systems. When using the frequency interval 1-20 MHz for
communication the radiation is extremely important because many other radio
applications are assigned in this frequency interval. It is not appropriate for a system
to interfere with, e.g., airplane navigation or broadcast systems. Recent research has
studied this problem and tries to set up a maximum power level of transmission. It is
important that this work is finished in the near future since it limits the use of this
bandwidth and the development of communication systems for the power-line
channel.
When the cables are below ground the radiation is small. Instead it is the radiation
from the households that makes the major contribution. Wires inside households are
not shielded and thus radiate heavily. A solution might be to use filters to block the
communication signal from entering the household.
2.4 Relevant Regulatory Standards:
Frequencies used by the devices communicating over the power line are restricted
by the limitations imposed by the regulatory agencies. These regulations are
developed to ensure harmonious coexistence of various electromagnetic devices in the
same environment. The frequency restrictions imposed by FCC and CENELEC are
shown in figures 1(a) and 1(b). Federal Communications Commission (FCC) and
European Committee for Electro technical Standardization (CENELEC) govern
regulatory rules in North America and Europe respectively.
10 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
Fig 1a: CENELEC frequency band allocation
Fig 1b: FCC frequency band allocation
In North America frequency band from 0 to 500 KHz can be used for power line
communications. However the regulatory rules in Europe are more stringent. Here,
the CENELEC standard only allows frequencies between 3 kHz and 148.5 kHz. This
puts a hard restriction on powerline communications and might not be enough to
support high bit rate applications, such as real-time video, depending on the
performance needed. According to this standard the spectrum is divided into five
bands based on the regulations. They are
3 – 9 KHz: The use of this frequency band is limited to energy provides;
11 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
9 – 95 KHz: The use of this frequency band is limited to the energy
providers and their concession-holders. This frequency band is often
referred as the "A-Band".
95 – 125 KHz: The use of this frequency band is limited to the energy
provider’s costumers; no access protocol is defined for this frequency
band. This frequency band is often referred as the "B-Band".
125 – 140 KHz: The use of this frequency band is limited to the energy
providers’ customers; in order to make simultaneous operation of several
systems within this frequency band possible, a carrier sense multiple
access protocol using center frequency of 132.5 KHz was defined. This
frequency band is often referred to as the "C-Band".
140 – 148.5 KHz: The use of this frequency band is limited to the energy
provider’s customers; no access protocol is defined for this frequency
band. This frequency band is often referred to as the "D-Band".
Thus in Europe power line communications is restricted to operate in the
frequency range from 95 – 148.5 KHz. Apart from band allocation, regulatory bodies
also impose limits on the radiations that may be emitted by these devices. These
reflect as restrictions on the transmitted power in each of these frequency bands.
Bandwidth is proportional to bit rate, in order to increase the bit rate, larger bandwidth
may be needed. Recent research has suggested the use of frequencies in the interval
between 1 and 20 MHz. If this range could be used, it would make an enormous
increase in bandwidth and would perhaps allow high bit rate applications on the
power-line. An important problem is that parts of this frequency band is assigned to
other communication system and must not be disturbed. Other communication
12 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
systems using these frequencies might also disturb the communication on the power-
line.
The power line was never intended for communications purposes. PLC device has
limited use because of strong interference, varying attenuation and impedance
problems. The PLCC system designer is limited in the bandwidth available for
communications not only by physical properties of the power line, but also by
regulatory standards imposed by governing bodies. But the applicability and the
benefits of this technology are so significant that, armed with many modern
communications techniques, and with careful thought, these challenges can be
definitely overcome.
13 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
3. PLC DEVICE DESIGN: PRACTIAL
ISSUES
For the design of any communications system, we have to address a number of
important design issues. Modulation techniques and transmission methods need to be
selected to give suitable performance in the communications environment of choice.
Our communications environment, that is the power line network possesses some
unique design issues of its own.
3.1 Modulation methods:
Transmission of data across a noisy communications channel requires some
manner of separating the valid data from the background noise. The most common
way to accomplish this is to modulate the data at the transmission end and to
demodulate the data on the reception endpoint, to make sure that that the data coming
from the receiver is the same as the data being presented to the transmitter. The
efficiency of the modulation/demodulation process determines the accuracy of the
data coming from the receiver. Therefore, careful consideration must be given to the
selection of an appropriate modulation-demodulation scheme.
The modulation band selected for power line communications must meet the
required data rate while maximizing resistance to noise and interference with the
signal because in any power line, there are several sources of noise and interference,
each with it own individual characteristics.
There are many different ways to modulate a signal, each with its own advantages
and disadvantages. The different types of digital modulation schemes are:
14 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
ASK is the simplest scheme but is very rarely used, because of its relatively poor
noise performance. The amplitude variations in an ASK signal becomes a source of
difficulty. Such signals when amplified by nonlinear amplifiers generate spurious out-
of-band spectral components, which are filtered out only with difficulty.. FSK is a
‘non return to zero’ modulation method. This means that the non-modulated condition
is between the “off” and “on” condition. In other words, the carrier should never be at
the center frequency when modulation is present. The benefit here is noise immunity.
Since FSK relies on frequency change, and not amplitude change, to indicate data
states, an FSK receiver is inherently immune to amplitude noise. This increased noise
immunity suggests a potential for higher data rates. In fact, FSK systems can achieve
significantly higher data rates than the ASK counterparts, albeit at the sacrifice of cost
and power consumption.
Considering now the phase shift keying techniques, BPSK and QPSK generate
discontinuities in the carrier phase, which are a further source of difficulty. When it is
necessary to avoid such amplitude and phase discontinuities, frequency modulation is
the feasible solution. The FSK waveform has a constant amplitude and no matter how
discontinuous the modulating waveform maybe, its phase is continuous. Phase delay
in the PLC channel is expected and is also unpredictable. The reliable performance of
FSK with any reasonable amount of phase delay makes it the modulation scheme of
choice for PLC techniques.
15 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
3.1.1 Insight of FSK
Frequency shift keying (FSK) is the most common form of digital modulation in
the high-frequency radio spectrum. Binary FSK (usually referred to simply as FSK) is a
modulation scheme typically used to send digital information between digital equipment
such as tele-printers and computers. Data is transmitted by shifting the frequency of a
continuous carrier in a binary manner to one or the other of two discrete frequencies. One
frequency is designated as the “mark” frequency and the other as the “space” frequency.
The mark and space correspond to binary ‘1’ and ‘0’, respectively. By convention, mark
corresponds to the higher frequency but the reverse can also be done.
Frequency measurements of the FSK signal are usually stated in terms of “shift”
and center frequency. The shift is the frequency difference between the mark and space
frequencies. The deviation is equal to the absolute value of the difference between the
center frequency and the mark or space frequencies.
v(t)= A * cos[ ω* t+ d( t) *Ω* t]
Where A = amplitude
ω = center frequency
d(t) = +1 for logic level ‘1’
d(t) = -1 for logic level ‘0’
Ω = frequency deviation
Thus the transmitted signal is either
v(t)= A * cos[ ω* t+Ωt]
v(t)= A * cos[ ω* t-Ωt]
16 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
And hence two analog waves of different frequencies are obtained.
Fig 2a: FSK input digital data
Fig 2b: FSK modulated signal
The concept of FSK can be very well explained by the figures above. Fig 2a is the
digital input for the FSK modulation. As examples consider that higher frequency is used
for space (i.e. logic “0”) and a lower frequency is used for mark (i.e. logic “1”). The Fig
2b shows the FSK modulated signal. One can clearly observe the variation in the
frequency of the modulated signal as per the digital input.
FSK is one of the candidate modulation techniques for PLC due to inherent
system physical conditions. The system consists of many noise sources as well as thermal
noise such as man-made noises, effects of electrical machines, and variable loads. Phase
modulations give worst performance with respect to FSK. Bit error probability Pe for
coherent FSK is given by,
17 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
where erfc(.) is the complementary error function. Eb/N0 is the energy per bit to noise
density ratio and is given as,
where C is the carrier power, N is the noise power, Bw is the receiver noise bandwidth,
and fb is bit rate. Carrier-to-noise ratio is given as,
where (C/N) t is the carrier-to-noise ratio due to thermal noise on the communication
channel, (C/I) k is the carrier to kth
interference ratio.
FSK-PLC channel performances are shown in the figure below. The figures below
illustrate the relationships between bit error probability and frequencies, load and
distance. These results give an idea for PLC channel.
Fig 3a: Bit error probability with frequency and load impedance for 1000 meters
line.
18 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
Fig 3b: Effects of line length (meters) on the bit error probability.
The above figures illustrate the relationships between bit error probability,
frequencies, loads and distance.
3.2 The Coupling Circuit
Once the data signal has been generated, it needs to be placed on the power line
by some kind of coupling circuit so as to avoid the circuit to be damaged by the power
line high voltage. There are three possible combinations of lines on which to couple the
signal:
Live to Ground,
Neutral to Live, and
Neutral to Ground.
Among all the three mentioned above the best method to avoid the 230 V AC line
was to use the last option i.e. Neutral to Ground coupling. This was opted as there
19 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
suppose to be no voltage existing between the neutral and ground terminals and hence
there should be minimum noise present when compared to the other two ways of coupling
the signal to power line where we use the live wire which actually carries the 230V AC!!
Now as there exists no voltage between the neutral and ground the design of the coupling
circuit gets reduced to nothing, just for safety a RC filter circuit with center frequency
same as that of FSK modulator should be used.
3.3 Interface with the PC
In our project we were aiming to transfer data and command (on/off control) over
the power line in between two PC’s. To do this the hardware has to be interfaced with the
PC. The best way and no doubt the simplest way to do this, was to use the serial port
(COM port) of the PC. The standards used by the COM port or the serial port of the PC is
the RS-232 standards. The specifications of the serial port are in Appendix A.
The driver for the COM port is, in case of Windows Xp operating system,
c:\windows\system32\drivers\serial.sys.
3.4 Interface with the devices for on/off control
Initially while thinking of a on/off control circuit we had thought of a separate
DTMF transmitter and receiver. The transmitter will be connected to the computer where
as the receiver will work independently and the devices will be connected to the DTMF
receiver through relays. We intended to use 4-bit dual-tone multi-frequency (DTMF) data
to be sent through the mains line to switch on/off the desired appliances via eight relays.
Eight 4-bit data words (0000 to 0111) are used to switch off eight appliances. Another
eight 4-bit words (1000 to 1111) are used to switch on the appliances. If the MSB bit is
high it is ‘on’ signal and if it is low it is ‘off ’ signal.
20 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
The above-mentioned concept is no doubt correct but it seemed to be much more
complicated when compared to device controlling using the I/O Card. I/O Card seemed to
be a far better option, as it was extendable i.e. in the DTMF control we could not connect
more than eight devices where as using the I/O Card the number of devices can go even
up to 48! For the specifications of the I/O Card please refer to the Appendix B.
21 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
4. PRACTICAL IMPLEMENTATION
In here we will give the details about the practical implementations of a working
power line carrier communications system. The hardware and the software both will be
covered. The devices that are chosen and circuits used are covered. Results of testing the
individual sections of the system, the problems encountered and their solutions are
discussed.
4.1 HARDWARE
In the previous sections of this report we suggested that the most suitable type of
modulation for transmitting data over the power line was FSK. So the first and foremost
part to be designed was FSK modulator and demodulator. As we had done a similar
experiment in one of our previous semester (using IC XR 2206 or 555 timer as the FSK
modulator and a PLL (IC LM565) as the demodulator circuit) we thought of using the
same circuit. But back in the previous semester and now also we encountered problems
especially with the PLL so to enhance the hardware we decided to use a dedicated
demodulator IC, which would act as a counter part to the transmitter, which is the IC XR
2211. The design equations are provided in the Appendix C. The details of each part of
the hardware are as follows.
4.1.1 Implementation of FSK modulation scheme:
In our final working circuit FSK modulation is performed by the application of a
Voltage Controlled Oscillator (VCO). A voltage-controlled oscillator produces an AC
waveform, output frequency directly proportional to the DC input voltage. By using the
22 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
23 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
binary waveform as an input to the VCO, we can vary the frequency of the output
sinusoid, giving FSK modulation. The integrated circuit employed for this purpose is the
EXAR device XR-2206. The XR-2206 is a monolithic function generator IC capable of
producing high quality sinusoid of high-stability and accuracy. Frequency of operation
can be selected externally over a range of 0.01Hz to more than 1MHz. The circuit is
ideally suited for communications, and function generator applications requiring
sinusoidal tone, AM, FM, or FSK generation. The system is designed to operate at a mark
’1’ frequency of 1270 Hz and a space ‘0’ frequency of 1070 Hz. Testing of this circuit
was done using a square wave input stream (representing a constantly varying 0-1-0
binary waveform). The mark and space frequencies were confirmed along with general
speed and stability. The circuit diagram of FSK modulator is on next page.
XR-2206
The XR-2206 is comprised of four functional blocks; a voltage-controlled
oscillator (VCO), an analog multiplier and sine-shaper; a unity gain buffer amplifier; and
a set of current switches. The VCO produces an output frequency proportional to an input
current, which is set by a resistor from the timing terminals to ground. With two timing
pins, two discrete output frequencies can be independently produced for FSK generation
applications by using the FSK input control pin. This input controls the current switches,
which select one of the timing resistor currents, and routes it to the VCO. The block
diagram and the pin description of IC XR-2206 is as shown in the figure on the next page.
The XR-2206 can be operated with two separate timing resistors, R1 and R2,
connected to the timing Pin 7 and 8, respectively, as shown in Figure 13. Depending on
the polarity of the logic signal at Pin 9, either one or the other of these timing resistors is
activated. If Pin 9 is open-circuited or connected to a bias voltage ≥ 2V, only R1 is
24 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
activated. Similarly, if the voltage level at Pin 9 is ≤1V, only R2 is activated. Thus, the
output frequency can be keyed between two levels. f1 and f2, as:
f1 = 1/R1C and f2 = 1/R2C
Fig 4: block diagram of XR 2206
Table 1: pin description of XR 2206
25 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
4.1.2 Implementation of FSK demodulation scheme:
The integrated circuit employed for this purpose is the EXAR device XR-2211.
The XR-2211 is a monolithic phase-locked loop (PLL) system especially designed for
data communications applications. It is particularly suited for FSK modem applications. It
operates over a wide frequency range of 0.01Hz to 300kHz. The circuit for this design is
shown in Figure on next page and the calculations of the biasing components are included
later in this report in Appendix C. Testing the FSK demodulation section involved
passing an FSK input into the demodulator, and tuning the phase locked loop parameters
until a stable binary output was obtained. Specifically, a square wave was passed into the
FSK modulator section, producing an FSK output. This output was passed directly to the
demodulator section, and the demodulator output compared to the overall input. The
circuit diagram of FSK modulator is on the next page.
XR-2211
The main PLL within the XR-2211 is constructed from an input preamplifier,
analog multiplier used as a phase detector and a precision voltage controlled oscillator
(VCO). The preamplifier is used as a limiter such that input signals above typically
10mVrms are amplified to a constant high-level signal. The multiplying-type phase
detector acts as a digital exclusive or gate. Its output (unfiltered) produces sum and
difference frequencies of the input and the VCO output. The VCO is actually a current
controlled oscillator with its normal input current (fO) set by a resistor (R0) to ground and
its driving current with a resistor (R1) from the phase detector. The output of the phase
detector produces sum and difference of the input and the VCO frequencies (internally
connected). When in lock, these frequencies are fIN+ fVCO (2 times fIN when in lock) and
fIN - fVCO (0Hz when lock). By adding a capacitor to the phase detector output, the 2 times
fIN component is reduced, leaving a DC voltage that represents the phase difference
26 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
between the two frequencies. This closes the loop and allows the VCO to track the input
frequency. The FSK comparator is used to determine if the VCO is driven above or below
the center frequency (FSK comparator). This will produce both active high and active low
outputs to indicate when the main PLL is in lock (quadrature phase detector and lock
detector comparator). The block diagram and pin description of XR 2211 is given below.
Fig 5: block diagram of XR-2211
Table 2: pin description of XR-2211
27 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
28 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
4.1.3 The coupling circuit:
As mentioned previously the coupling circuit need not be complicated, as there
exists no voltage across the neutral and ground terminals. A simple capacitor coupling is
more than enough for this type of connection to couple the transmitter to the neutral and
ground lines. Where as at the receiver end we added a narrow band pass filter to eliminate
the noise form the signal.
4.1.4 Relay switching circuit:
The relay switching is a simple single transistor driven set up as shown in the
figure on the next page. As we were describing the applications of PLC we did controlled
eight devices connected to the port A. The pins of port A are connected to the relay
driving circuit which in turn switches the devices. The pin connections of the FRC are
29 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
also shown in the next page. A BC 547 transistor is used for switching to whose collector
the coil of the relay is connected. A resistor is provided at the base of the transistor so as
to limit the base current. A diode 1N4007 is used as a protection for the transistor mainly
by the reverse voltage generated by the relay coil, also known as the fly back voltage.
4.1.5 Power supply:
Power supply is an essential part of every electronic equipment. Since the
healthy functioning of all stages in the equipment requires a well-designed power supply.
A great many things like voltages required, current ratings, power drawn and the
percentage of regulation required influence the design of a particular power supply.
Generally the kind of power supply used in an instrument used is of fixed voltage type.
Since the various voltages required at various points are already known. However in some
rare cases, a facility to vary the power supply voltage may be provided. In general, the
power supply section provides higher loads as well as line regulation along with main
isolation.
The power supply section mainly consists of two parts, the Transformer and the
Rectifier.
4.1.5.1 Transformer:
A transformer along with reducing the main voltage to required small voltage
provides isolation from mains to avoid any electrical shock to the operator. We used a
step down transformer (12-0-12) which steps downs the 220v AC main voltage. The
current rating of the transformer is 1 amp, this was chosen as we intended to connect the
device to a PC whose current are about 250-mamp and more over at the receiver we
wanted to connect the relays. It is always better to go for a higher value than the exact so
we went for 1 amp rather than 500-mamp transformers.
30 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
Circuit Diagram for Power Supply
31 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
4.1.5.2 Rectifier:
We designed a full wave rectifier to convert the reduced voltage AC signal to DC,
but this only was not sufficient as still the signal lacked stability in it. So the best way to
over come this instability was to use the IC voltage regulators available in the market. In
the transmitter we required two different voltage levels +12 volts and +5 volts for the XR-
2206 and MAX-232 IC’s respectively. So we used regulator IC’s 7812 and 7805
respectively. Where as in the receiver we required three different voltage levels +12 volts,
+5 volts and -12 volts for XR-2211, MAX-232 and the LM324 (amplifier) so we had to
use regulator IC’s 7812, 7805 and 7912 respectively. The circuit diagram for combined
power supply design is as shown on the previous page.
4.2 SOFTWARE
The whole software for this project was written in C programming language. We
developed a graphical user interface, rather than making the user to opt from the given
options in a DOS like environment, to make the project easy to use. We made the user
interface to look very much similar to the Windows, as almost every one is familiar to the
user interface of windows. In the case of device control we even provided the privilege to
the user that he/she can turn off the software but still when it is turned on the program
remembers the previous state of devices (even if the power is switched off to the PC this
will happen, but the devices will be switched off the instant the PC is turned off) and to
do so we have maintain a file. Initially even before first time loading the software we
have to initialize such that it will display as no devices as being turned on. To do so we
did write a small program. The source code written in C programming language is
attached in Appendix E.
32 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
4.3 Problems Encountered
We encountered several problems, as mentioned earlier we faced problems with
designing a FSK modulation and demodulation circuit using 555 timer and LM 565
(PLL) and hence we had to opt for the dedicated IC’s for them (XR-2206 and XR-2211).
Later on we faced a major problem with our channel. As we had decided to use the
neutral and ground connections and typically speaking there should exist no voltage
between them. But on the contrary we found that there existed considerable voltage
between neutral and ground. We first checked in all our houses but there was at least
3vrms and then we checked in our college. In the college the voltage varied from 3-17vrms
but only in the communication lab that to at a few terminals the voltage was about 1vpp
(all these voltage levels were a 50Hz signals). So we had no other option other than
working only in the communication lab. We tried to send over signal over the neutral and
ground but we could not retrieve the signal. Upon observing the received signal we came
to know that our FSK modulated signal was getting super imposed over the existing
voltage between ground and neutral. Hence we amplified our transmitting FSK signal so
that it would over come the existing voltage (the design has been included in Appendix F)
and we also designed an active first order band pass filter and tested our project on the
power line. We expected this setup to at least work and yes it did but only for about
fifteen minutes. Soon after the mentioned time as if every thing collapsed nothing was
wrong but we confirmed the whole setup is working or not, by removing the power line
and connecting the transmitter and receiver directly every thing was working! Later on
we did not achieve any thing so we decided to power amplify our signal using LM 386
and designed a fourth order active band pass filter at the receiver (rather than first order),
but still we were unable to receive the signal. We once again observed the received signal
33 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
(before the filter) on CRO and to our surprise our signal was no were to be seen! The
signal was not getting super imposed on the existing voltage (between neutral and
ground) we couldn’t see any trace of our signal. We could only marvel the variations in
the characteristics of the power line and yes why not that could only be the reason that
our setup works for some time and it doesn’t work for rest of the time. We decided not to
change the setup and tried again, starting with checking for super imposition. We could
observe the super imposing of FSK over the neutral line so we applied our set up (which
was not changed). Once again the set up worked only for a short period of time. We could
just conclude that the due to improper grounding and instability of the power line we are
not able to achieve the desired results.
34 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
5. Advantages and disadvantages
Advantages:
1. It makes use of an existing infrastructure, hence cost effective
compared to other systems.
2. There near light speed propagation makes them very powerful for fast
delivery of data and control of devices.
3. If extended it could replace the LAN connecting wires used now a
days for inter connecting computers.
4. It utilizes readily available hardware components for its
implementation.
Disadvantages:
1. Noise is generated from all loads, also broadcast radio interference
with the communication.
2. Attenuation is a parameter of physical length of the channel and
impedance mismatches.
3. Channel is time variant, complicates the design of a communication
system.
4. Communication is simplex. .
35 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
6. Applications
This project is based on power-line communication i.e. communication over the
existing power-lines. The main advantage of this kind of communication system is the
existing infrastructure, which simplifies the implementation. This project definitely brings
to surface the tremendous potential in using the power line as a data communication link.
In this section we would also like to discuss some major applications driving the Power
Line Communication (PLC) technology. They are:
Automatic Meter Reading (AMR) – For the readings of Electricity,
Water, Gas or any other meters in the customer premises to be transmitted
to a central base station for further processing, billing etc. With tens of
millions of meters to be read periodically and regularly, this alone
represents an enormous market.
Home Bus- For making the buildings "Intelligent", where all appliances
are to be monitored or controlled continuously and automatically for
convenience comfort, safety and energy - saving. This makes use of the
intra-building wiring.
Distribution Automation, and Supervisory Control and Distribution
Automation (DA and SCADA) – This is for the utility companies
themselves to monitor and control the Power Distribution Process.
Rural Communication Applications - Where user densities are low and
distances are large which makes installation of fresh infrastructure
expensive and also non-profitable.
Also during the last years the use of Internet has increased. If it would be possible
to supply such a kind of network communication over the power-line, it would bring this
technology out of the embedded systems area right to the personal computer industry.
36 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
Systems under trial exist today that claim a bit rate of 1 Mb/s, but most commercially
available systems use low bit rates, about 10-100 kb/s, and provides low-demanding
services such as meter reading. With the availability of power line communications
speeds, similar to those of Ethernet, the technology will soon become available in
products for personal computer networking within the residence. As electric utilities
begin to explore this avenue for enhanced services, a far greater value will be found in the
power line than simply delivering electrical power.
37 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
7. Conclusion
The power line communications channel is a notoriously bad channel that has
been developed without regard for any communications considerations. However, it is so
widely distributed that considerable cost savings can be achieved, if use is made of its
cable infrastructure. This project definitely brings to surface the tremendous potential in
using the power line as a data communication link
Trends in both the electric and telecommunications industry have lead to a
climate where PLC should be a big player. These trends are driven by the customer’s
demand for affordable and high speed Internet access. PLC technology is an exciting
alternative to connecting to the Internet via phone and modem. Though this technology is
not commercially available yet, it should be available before other broadband
technologies due to the relatively low cost of its local loop. So perhaps it will not be long
before the power socket on your wall doubles as a broadband communications gateway.
The future will see power-line technology in business data communication applications
and particularly in home automation applications.
38 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
8. References
1. Electronics Devices and Circuit Theory. By: Robert L. Boylestad,
Louis Nashelsky.( 6th
edition)
2. Digital communications By: Simon Haykin
3. IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 19, NO. 1,
JANUARY 2004
4. Op-Amps and Linear Integrated Circuits. By: Ramakant A.
Gayakwad (Third Edition)
5. Let Us C By: Yashavant Kanetkar (3rd
Edition)
6. Let Us C Graphics By: Yashavant Kanetkar
7. www.klm-tech.com
8. www.powerlineworld.com
9. www.enersearch.se/knowledgebase/
publications/thesises/PowerlineCom.pdf
10. http://pcmag.dit.net
39 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
APPENDIX – A
Serial Port Details
The RS-232 port is most commonly used in the PC. But, when we look through
Web pages and catalogs at different devices designed to be connected to the PC often use
the other inter faces available. The basic reason is pretty logical and obvious once we start
working with RS-232.RS-232 can be used in an awful lot of different ways to
interconnect devices with many different options and quirks that one has to understand
before one can successfully use the PC’s serial ports to interface with other devices.
One can wire two RS-232 devices together 16 different basic ways (and when all
of the small variances are taken into account, there is probably twice that number of
different ways again). First the serial port is cheaper method of interfacing devices to the
PC than the parallel port. Telegraphy was the first form of modern long distance
electronics asynchronous serial communication which can said to be the origin of
standards for RS-232. At this point, we are up to the early days of computing (the 1950s).
Although data could be transmitted at high speed, it couldn’t be processed and read new
incoming data back continuously. So, a set of handshaking lines and protocols were
developed for what became known as RS-232 serial communications.
The PC’s serial ports consist of basically the same hardware and BIOS interface
that was first introduced with the first PC in 1981. Since that time, a 9-pin connector has
been specified for the port. For the most pare, the serial port has changed the least of any
40 ON/OFF CONTROL AND DATA COMMUNICATION THROUGH POWER LINE
PDACE Glb Dept of E&CE
component in the PC for the last 20+ years. Usually, a PC (initially but not now) has four
serial ports, called COM1, COM2, COM3 and COM4. COM1 is usually for connecting a
serial mouse while COM2 is available to the user where as the rest of the two COM ports
3 and 4 are internally used by the computer. Now a days the COM ports are vanishing
from the PC’s and their place is being taken by high-speed serial interface very well
known as USB (Universal Serial Bus). But still we can find COM1 and COM2 ports (or
at least COM1) are fitted at the backside of the PC. They can be 25-pin D-type or 9-pin
D-type male connectors. The newer computers come with a 9-pin male connector. Fig 1
shows pin configurations of both the 25-pin D-type male connectors and the 9-pin D-type