Air Media
A) MICROWAVE (MICROWAVE RADIO)
It is a transmission of high frequency (GHz) which is limited by the curvature of the earth. Uses parabolic dishes that are known as satellite dishes to transmit data.
Satellite dishes are mounted on top of buildings because there must be a line of sight between antennas.
B) RADIO VHF / UHF
It's a wave of high frequency electromagnetic transmission. The radio transmitter generates the signal and the antenna receives this information.
C) SATELLITE EGOCENTRIC
Geocentric satellites using a radio transmission of high frequency on the order of GHz between two locations. The satellite orbit is synchronous with the orbit of the earth. That is, the satellites orbit the earth using a geocentric orbit. However, the satellite maintains its position relative to a point on the earth. The satellites receive and transmit information using satellite dishes.
D) POWER LINE
With special equipment you can send and receive data over a power line from 120 VAC to 460 VAC.
COMMUNICATION PROTOCOLS
The protocol is a set of rules that define the meaning of a word of bits. It is established that a message be sent from an RTU to MTU to a through a series of digital bits. But that may represent the first, second, third bit, and what it can mean the bit twenty-four. The protocol takes care of us that means every bit.
So will a code to tell us what is the meaning of each bit in serial communication. This encodes the beginning of the transmission and decoded at the end of the transmission.
Needless to say that there are many protocols in the market and a supplier intends to use its technology as a standard protocol.
11 shows a diagram of a sent message based on a protocol of IEEE C37.1. Each bit is defined as a purpose. The total length of the transmission of the document is included as the top of all the fixed bits plus the number that is taken as a modifier. The string of the remote address, defines the station which is message is sent. Eight bits to identify up to 256 stations.
Figure 11
The function of the frame is set up to 256 types of messages. Defines the internal address register with the receiving station which routes the message. The switch modifies the internal management and defines how many words are included in the message. The data is variable length field that goes from 0 to 192 bits.
The CRC is a 16 bit redundant cycle based on a formula BoseChaudhuri - Ocquenghem to detect transmission errors.
Below is an example for calculating the CRC. The CRC calculation is a mathematical model that is implemented by digital circuits or microprocessors developed routines.
Figure 12
A) DIGITAL SIGNAL MODULATION
Modulation of digital signals is carried out by teams modems is known as contraction and demodulates Modulate. But what is meant by modulation? Generally this means vary or change the waveform according to a carrier wave.
When discrete signals are sent through a line, these signals are attenuated by the effect of physical resistance.
Figure 13 shows that a waveform can be separated into a series of mathematical series of sine waves. A French mathematician named Fourier described the method to analyze these waveforms. Showed that a rectangular waveform is the sum of sinusoidal components.
Fig.13
The original wave components are affected by the inductance and capacitance of the physical medium used. The main effect is the distortion of the phase angle of the signal. However, it was demonstrated that a base frequency is not affected by the distortion of the physical environment. In this sense, the idea is to use a carrier frequency for modulating the information.
Different modulation methods are:
1. Amplitude modulation AM.
2. FM Frequency Modulation.
3. PM phase modulation.
AM amplitude modulation varies the amplitude of the high frequency of the carrier wave multiplied by the width of the data. The result is a series of waves that vary in amplitude sinusolidales at the speed of the data.
Figure 14
FM frequency modulation varies the carrier wave frequency range according to the data. The amplitude of the output wave is constant. In these cases the atmospheric noise does not affect the carrier wave.
Figure 15
The phase angle modulation, the phase changes in response to the changing amplitude of the data. This modulation enables higher communication speeds respect to modulation AM.
A protocol governs the transmission format between two or more stations, including handshaking, error detection and error recovery. When selecting a protocol that best fits your application. The criteria to consider when selecting a protocol are:
Topology of the connection.
Mode of transmission.
Other application requirements, such as connections and existing equipment.
If your mode of transmission is one way to select the protocol is half duplex. If the mode is bidirectional transmission simultaneously select the protocol to be full duplex.
Depending on the provider of choice for master and slave station, the protocol will be suggested by the manufacturer of this equipment. For example, when working with teams from Allen Bradley, the provider will suggest the half-duplex DF1 protocol because it has the benefits:
Remote monitoring of data table of the remote station and ladder access to the program this season.
Messages between stations.
Lower cost because the protocol included with the equipment.
DF1 protocol is an asynchronous protocol. You need to work with another protocol if:
You are using communication links with satellites, radio-modems and software needed to work with handshaking.
Expanding an existing system that does not work with AB DF1 protocol.
However, many technology vendors provide solutions known as Gateway, which are devices or software that is responsible for converting a given protocol in another protocol.
Below is a table of different protocols available for master stations:
SCADA Protocols
RTU Protocols
SELECTION OF COMMUNICATION EQUIPMENT
A method for calculating the interval scan is based on several considerations mentioned in Unit 1. When you have critical applications, it is necessary to handle higher bandwidth with higher baud rates. In these cases the media to use consider the use of physical media such as fiber optics, radio waves, etc.. Implement a dedicated line is more sophisticated, but you need a phone company who can provide this service.
SELF-TEST QUESTIONS
1. Name three methods of modulating waveform.
a.Amplitud, phase and bandwidth.
b.Frecuencia, PWM and AM.
c.Carrier power, FM and AM.
d.Amplitud, frequency and phase.
e.Ninguna of the above.
2. Is it possible to transmit more than 1 200 bps modem 1 200 bps?
a.Sí.
b.No.
c.Si modulation technique allows.
d.Sólo c are valid.
e.Ninguna of the above.
ANSWERS TO QUESTIONS OF SELF
1) d
2) c
