Summary:

Half Duplex Data Communication between Two computers using LASER is a senior project design for EE 411 course. After going through a hard time I represent this project which involves C/C++ programming in addition with two electronic circuits to send data between two computers using the parallel port. At the beginning of this report, I will present background about the project in chapter one. Then, in chapter two I will give brief information about each component that I used. Moreover, in Chapter three will be about the circuit Construction. After that, I will present information about C programming and the Codes that I used in chapter four. The next chapter will be about implementation and testing the circuit to show the results. The last chapter is conclusion.

Acknowledgments: First of all, I would like to thank my advisor Dr. Mohammad Ousman for his guidance in the project and helping me to understand the importance procedure of the project. He was great working with me on the project to get useful information as possible. His suggestions and ideas were very helpful to me to complete my project. Furthermore, I would to thank the lab engineer Mr. Azeddine Hamroouni for his support and guidance in this project. Finally, I would like to thank the University of Hail especially electrical engineering department. The electrical department is helped me in my life and build a good knowledge for me.

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List of Abbreviations

List of Tables

Chapter one: Introduction

1.1 Overview The project is based on the concept of free space optical communication. An optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal.

In this project, a communication between PC to PC using LASER beams is established. The communication over the short distance of 2-3 m is possible using photodiodes. The range could be increased up to hundred meters, using the laser diode.

The laser module used is easily available as laser pointer (having about 5 mW power output). Point the first laser beam to the second photodiode of the module connected to the other PC and vice-versa.

1.2 Free Space Optical Communication Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to transmit data between two points for telecommunications or computer networking. "Free space" means air, outer space, vacuum, or something similar. This contrasts with using solids such as optical fiber cable or an optical transmission line. The technology is useful where the physical connections are impractical due to high costs or other considerations.

Free space optical communications is a line-of-sight (LOS) technology that transmits a modulated beam of visible or infrared light through the atmosphere for broadband communications. Free space optics uses a light emitting diode (LED) or laser (light amplification by stimulated emission of radiation) point source for data transmission. However, in free space optics, an energy beam is collimated and transmitted through space rather than being guided through an optical cable.

* Free Space Optics is used to communicate between space-craft, since outside of the atmosphere there is little to distort the signal.

* The optical links usually use infrared laser light, although low-data-rate communication over short distances is possible using LEDs.

* Distances up to the order of 10 km are possible, but the distance and data rate of Connection is highly dependent on atmospheric conditions.
1.3 Transceiver A transceiver is a combination transmitter/receiver in a single housing. The term applies to wireless communications devices such as cellular telephones, handheld two-way radios, and mobile two-way radios. Occasionally the term is used in reference to transmitter/receiver devices in cable or optical fiber systems. In a radio transceiver, the receiver is silenced while transmitting. An electronic switch allows the transmitter and receiver to be connected to the same antenna, and prevents the transmitter output from damaging the receiver. With a transceiver of this kind, it is impossible to receive signals while transmitting. This mode is called half duplex. Transmission and reception often, but not always, are done on the same frequency. Some transceivers are designed to allow reception of signals during transmission periods. This mode is known as full duplex, and requires that the transmitter and receiver operate on substantially different frequencies so the transmitted signal does not interfere with reception. Cellular and cordless telephone sets use this mode. Satellite communications networks often employ full-duplex transceivers at the surface-based subscriber points.

1.4 Duplex A duplex communication system is a point-to-point system composed of two connected parties or devices that can communicate with one another in both directions simultaneously. An example of a duplex device is a telephone. The people at both ends of a telephone call can speak at the same time, the earphone can reproduce the speech of the other person as the microphone transmits the speech of the local person, because there is a two-way communication channel between them.

Figure: Half and Full duplex Communication A half-duplex (HDX) system provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying (antennas are of trans-receiver type in these devices, so as to transmit and receive the signal as well). An example of a half-duplex system is a two-party system such as a walkie-talkie, wherein one must use "Over" or another previously designated command to indicate the end of transmission, and ensure that only one party transmits at a time, because both parties transmit and receive on the same frequency. A good analogy for a half-duplex system would be a one-lane road with traffic controllers at each end. Traffic can flow in both directions, but only one direction at a time, regulated by the traffic controllers. In automatically run communications systems, such as two-way data-links, the time allocations for communications in a half-duplex system can be firmly controlled by the hardware. Thus, there is no waste of the channel for switching. For example, station A on one end of the data link could be allowed to transmit for exactly one second, and then station B on the other end could be allowed to transmit for exactly one second. And then this cycle repeats over and over again.

Chapter Two: Components

2.1 Laser:
'Laser' stands for (light amplification by stimulated emission of radiation). Lasers are devices that amplify light and produce coherent light beams, ranging from infrared to ultraviolet. A light beam is coherent when its waves, or photons, propagate in step with one another. Laser light, therefore, can be made extremely intense, highly directional, and very pure in color (frequency). Laser devices now extend into the X-ray frequency range.
Why we Used Laser for Communication?
This project is designed for the entry level laser experimenter. The circuit allows any two computers with parallel port (DB25) communication capability to communicate using a laser beam. A low cost transmitter only circuit is also presented here for use in two way communication.
The laser source for this project is an inexpensive laser pointer. As well as being readily available, the circuit is designed in such a way so that the laser pointer is not damaged, and can be used for other experiments. Because this is an entry level circuit, costs have been kept to a minimum cost for the transceiver and the transmitter.
A laser as a communications medium has some unique properties compared to other forms of media. A line-of-sight laser beam is useful where wires cannot be physically connected to a remote location. A laser beam, unlike wires, also does not require special shielding over longer distances. Lasers offer at least an order of magnitude longer distances compared to infrared LEDs. Since the laser medium is line-of-sight and the beam being only several millimeters in diameter it is very difficult for the data stream to be tapped. This offers secure communication since any attempts to intercept the laser beam would be detected at the receiver as a loss in data.

Stimulated Emission: Before I talk about Stimulated Emission, I would like define the photon which is the initial particle responsible for electromagnetic phenomena and the basic unit of light and all forms of electromagnetic radiation. If an electron is already in an excited state (an upper energy level, in contrast to its lowest possible level or "ground state"), then an incoming photon for which the quantum energy is equal to the energy difference between its present level and a lower level can "stimulate" a transition to that lower level, producing a second photon of the same energy.

Figure ##: Stimulated Emission
When a sizable population of electrons resides in upper levels, this condition is called a "population inversion", and it sets the stage for stimulated emission of multiple photons. This is the precondition for the light amplification which occurs in a laser, and since the emitted photons have a definite time and phase relation to each other, the light has a high degree of coherence.

Figure%%: Coherent or Population inversion
Characteristic of Laser:
Laser light has three unique characteristics, which make it different than ordinary light. The characteristic are:
1- Coherent:

Coherent means that all the individual waves of light are moving precisely together through time and space. They are in phase.

2- Monochromatic:

Monochromatic means that it consists of one single color or wavelength, having its origin in stimulated emission from one set of atomic energy levels. So the laser light has a single spectral color and is almost the purest monochromatic light available.
3. Collimated

Collimated (parallel) means that the beam is directional and travels over long distances with very little spread.

2.2 Bipolar Junction Transistor (BJT): Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor's ability to change between these two states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics). The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labeled as the Emitter (E), the Base (B) and the Collector (C) respectively. Bipolar Transistors are current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The principle of operation of the two transistor types PNP and NPN, is exactly the same the only difference being in their biasing and the polarity of the power supply for each type.

BJT Regions:
Then bipolar transistors have the ability to operate within three different regions: * 1. Active Region - the transistor operates as an amplifier and Ic = β.Ib * 2. Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation) * 3. Cut-off - the transistor is "fully-OFF" operating as a switch and Ic = 0

Applied voltages | B-E Junction | B-C Junction | The mode | Application | E < B < C | Forward | Reverse | active | amplifier | E < B > C | Forward | Forward | Saturation | switch | E > B < C | Reverse | Reverse | Cut-off | switch |
Table $$: BJT Regions BJT Construction:

2.3 Photodiode: A photodiode is a type of photodetector capable of converting optical signal (light) into electrical signal (current/voltage), depending upon the mode of operation. The common, traditional solar cell used to generate electric solar power is a large area photodiode.

Figure : Photodiode Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode use a PIN junction rather than a p-n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias.
Photodetector Requirements: * High sensitivity at the desired wavelength. * Low noise. * Fast response time. * Insensitive to temperature variations * physical dimensions * Long operating life

Figure: Photodiode symbol
Principle of operation: A photodiode is a p-n junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron (and a positively charged electron hole). This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimized to enhance the sensitivity of the device.

2.4 Parallel Port: Parallel port is a simple and inexpensive tool for building computer controlled devices and projects. The simplicity and ease of programming makes parallel port popular in electronics hobbyist world. The primary use of parallel port is to connect printers to computer and is specifically designed for this purpose. Thus it is often called as printer Port or Centronics port (this name came from a popular printer manufacturing company 'Centronics' who devised some standards for parallel port). You can see the parallel port connector in the rear panel of your PC. It is a 25 pin female (DB25) connector (to which printer is connected). Unfortunately, as of today parallel port is a completely deprecated interfacing standard. It is impossible finding a new laptop with parallel port these days. Even almost all branded PCs already shed Parallel Ports from their back panels. Other alternative available in market such as PCI parallel port expansion cards and USB to Parallel Port converters aren't good for anything other than connecting your old printer because of their architectural difference. As of now the only possible alternative is to use serial ports along with serial to parallel converters (Unfortunately serial ports aren't available on laptops these days) or use USB which is the most popular interface standard in the market as of today. But inherent complexity of implementing USB protocol and hardware requirements keep the hobbyists away for experimenting with USB.

Figure$$: Parallel port
A register is a place where you can write values and retrieve them. There are different types of registers called Data Register, Control Register and Status Register.
Data Register
This is the register that allows the user to write values into the port. In simple words, these pins can be used to output a specific value in a data register. You can also change voltages in specific pins. These are called output pins. There are altogether 8 output pins available, ranging from D0 to D7.
Status Register (Pins)
These are called input pins or status registers and can hold a value that the outside world gives to the Parallel Port. So, this port acts like a reader and it has 5 pins for inputs. The pin range is S4 to S7.
Control Register (Pins)
This register can be used in both ways: it enables a user to write values to the outside world, as well as read values from the outside world. However, you need to remember that most of the control pins work in an inverted manner. You can see them with a dash sign on the top of the pin. Pin range is C0 to C3. Ground pins are used as neutral; these pins are used as (-) in batteries. If you're connecting a device to a Parallel Port in order to read or write, you have to use one or more ground pins and a read/write pin to work. For example, if you're trying to light up an LED, then you have to connect the (-) of the LED to a ground pin and the (+) of the LED to an output pin. For reading purposes, use the same mechanism.

25 | Direction In/out | Register | Hardware Inverted | 1 | In/Out | Control | Yes | 2 | Out | Data | | 3 | Out | Data | | 4 | Out | Data | | 5 | Out | Data | | 6 | Out | Data | | 7 | Out | Data | | 8 | Out | Data | | 9 | Out | Data | | 10 | In | Status | | 11 | In | Status | Yes | 12 | In | Status | | 13 | In | Status | | 14 | In/Out | Control | Yes | 15 | In | Status | | 16 | In/Out | Control | | 17 | In/Out | Control | Yes | 18 - 25 | Gnd | | |
Table: Pinouts for parallel port connectors
Pin1 = Data acknowledgement when the signal is low.
Pin 2 - 9 = Data transfer pins.
Pin 10 = Acknowledge that the data has finished processing and when the signal is high indicates ready for more.
Pin 11 = When the signal goes high indicate that the printer has accepted the data and is processing it. Once this signal goes low and Pin 10 goes high will accept additional data.
Pin 12 = Printer paper jam when signal is high or no signal if printer jam.
Pin 13 = When high signal printer is indicating that it is on-line and ready to print.
Pin 14 = When low signal PC has indicated that the printer inset a line feed after each line.
Pin 15 = Printer sends data to the computer telling it that an error has occurred.
Pin 16 = When low signal PC has requested that the printer initiate an internal reset.
Pin 17 = When low signal the PC has selected the printer and should in return prepare for data being sent.
Pin 18 - 25 = Ground.

Chapter three: Construction

3.1 The circuit Construction:
The Construction of the two electronic circuits is the same and both of them are transceiver which is transmitting and receiving at the same time.
The components for the two boards that I used are: No. | Component | Quantity | 1 | Resistor ($$ Ω) | 2 | 2 | Resistor ($$Ω) | 2 | 3 | BJT NPN 2222 | 4 | 4 | Photodiode | 2 | 5 | LASER | 2 | 6 | Parallel Port (DB25) | 2 | 7 | Power source with 5 Volt DC | |
Table $$: components for the two boards
3.2 Darlington Pair: Sometimes the high current gain of a single-transistor, common-collector configuration isn't enough for a particular application. If this is the case, multiple transistors may be staged together in a popular configuration known as a Darlington pair. The Darlington pair (Darlington transistor) is a compound structure consisting of two bipolar transistors (either integrated or separated devices) connected in such a way that the current amplified by the first transistor is amplified further by the second one. Furthermore, A Darlington pair is used to amplify weak signals so that they can be clearly detected by another circuit. The Darlington Pair is a useful circuit configuration for many applications within electronic circuits. The Darlington transistor configuration provides a number of advantages that other forms of transistor circuit are not able to offer and as a result it is used in many areas of electronics design. The Darlington Pair also occasionally referred to as a super beta pair is renowned as a method for obtaining a very high level of current gain, using just two transistors. It is able to provide levels of gain that are not possible using single transistors on their own, but it may not be used in all circumstances because it does have a number of limitations. It has been in use for very many years. It was invented in 1953 by Sidney Darlington who was working at Bell Laboratories. Darlington pair of transistor is also called as super-beta transistor. It has the capacity to amplify the current output many a times a normal transistor configuration. A general relation between the compound current gain and the individual gains is given by:

Darlington pair current gain, hFE = hFE1 × hFE2

Darlington Pair Advantages: * Very high current gain. * Very high input impedance for overall circuit. * Darlington pairs are widely available in a single package or they can be made. from two separate transistors. * Convenient and easy circuit configuration to use.

Darlington pair applications:

Darlington pairs find applications in many areas. Offering a very high level of current gain they can be used to good effect in many areas. However when using the Darlington pair, its limitations must also be considered. As a result, its use is limited to a number of areas where its limitations are not a major problem. Some typical areas where the Darlington is used include:

Chapter Four: C programming and Coding

4.1 C-Programming: C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system.
Although C was designed for implementing system software, it is also widely used for developing portable application software. C is one of the most widely used programming languages of all time and there are very few computer architectures for which a C compiler does not exist. C has greatly influenced many other popular programming languages, most notably C++, which began as an extension to C.
Importance/Advantages of programming in C:

1) Easy to understand.
2) Freedom of using different type of data.
3) Short listed words could be use.
4) Efficient and fast programming.
5) It can be used as mid-level language.
6) Any type of software and operating system be developed with the help of C language.
4.2 Coding

Chapter Five: Testing and Implementation
Chapter Six: Conclusion
References