EEL201 Term Paper: Serial Communication Over Parallel

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Serial Communication Over Parallel
Sachin Garg Indian Institute of Technology, Delhi-110016 E-mail: ee2110527@ee.iitd.ac.in
 Abstract—Communication is the essence of today’s technologically advanced world. The two most basic methods of digital communication are: Serial and Parallel. Parallel communication involves multiple wires running parallel to each other and can transmit data on all the wires simultaneously. However, Serial communication requires a single wire for transferring data bits one at a time. A natural question arises out of human curiosity: which one of the two methods is better. This term paper discusses these two methods, their various applications and the advantages of the Serial Communication method over the Parallel one. Index Terms—Parallel Communication, Serial Communication UART, RS-232 DTE, Synchronous, Asynchronous.

Parallel counterpart have been resolved. Although the Parallel Transmission Method is still used within internal buses and for devices such as printers, but the latest devices, ports and technologies are mostly based on Serial Techniques. For example - Universal Serial Bus and WAN, both make use of the Serial Type Data Transmission.

II. TERMS USED IN DATA COMMUNICATION A. Transmission Distance The transmission distance of a communication signal is the length of path the signal travels as it goes from transmitter to the receiver. B. Data Terminal Equipment (DTE) Data Terminal Equipment (DTE) is a device or equipment that acts either as a source or as destination during the transmission of data signals. A DTE controls the communication channel. However, DTEs don‟t directly communicate with each other. For this purpose, the DTEs need Data Communication Equipment (DCE). Some examples of DTEs are: computer, printer, dumb terminals, file and application servers etc [2]. C. Data Communication Equipment (DCE) Data Communication Equipment (DCE) is a term used for the devices and connections of a communication network which transmit or receive data signals, thereby, completing the communication network. DCEs also help in synchronization with a clock rate in serial communication. With the use of DCEs, DTE doesn‟t have to know how the data is sent or received. The details of the communication over the network are left with DCEs. A modem is a very good example of a DCE, it works as a translator of analog to digital signals and vice-versa. Other examples include satellites, ISDN adapters etc [2]. D. Baud Rate The Baud Rate of a data communication system is a measurement of transmission speed. It is the number of symbols transferred per second. It actually represents the number of bits that are actually being sent over the serial link including the overhead bits Start, Stop and Parity that are also transferred in asynchronous serial communication [3].

I. INTRODUCTION

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HE communication technology has gained very much importance over time and there are a lot of researches going on all over the world in this area of digital electronics. The big companies like- Apple and IBM are coming up with fastest of the computers ever made. Intel is producing highly efficient micro-processors using the latest Thunderbolt Technology which dramatically increases the data transfer performance giving up to 10Gbps bi-directional speed [1]. All of these tremendous innovations have possible because of the advancement in the data transmission methods. Basically, there are two different methods of data transmission: Serial and Parallel. Serial transmission involves a single stream of data being transmitted over a single channel whereas parallel transmission is able to transmit several streams of data along multiple channels. To achieve fastest transmission technology, it is very important to analyze the effectiveness of both the methods. At first site, it seems that the Parallel Transmission is faster because of the multiple bits transmission at a time. The earlier communication standards were more inclined towards the Parallel Transmission Architecture but recently, there is a shift towards the Serial Techniques as latest technology and research in the Serial Transmission domain has allowed it to become even faster than the Parallel Transmission. Also, many problems in the
Manuscript received November 10, 2012. Author Sachin Garg is a Sophomore Undergraduate in Indian Institute of Technology, Delhi-110016 and is pursuing a B.Tech. degree in Electrical Engineering (Phone: 09811604263; E-mail: ee2110527@ee.iitd.ac.in).

EEL201 Term Paper: Serial Communication Over Parallel E. Handshaking The process by which DTEs and DCEs initiate communication. Handshaking begins when one device sends a message to another device to establish a communication channel. After that, several messages are sent back and forth between the devices that enable them to agree on a certain communication protocol [4]. 1. DTE requests DCE to send data (RTS). 2. DCE indicates DTE when ready and clear to send data (CTS).

2 automation, Internet and ISDN are a few of its major applications [6]. How it works: In this type of data transmission, the digital information is exchanged across the communication network using one wire only. Unlike parallel communication, only one data bit is sent across the link at a time. The electronics devices are able to communicate with the outside world using Serial method of Communication. The communication media in serial transmission is mostly wire based but can be optical or radio based also. The digital information flows as a single stream of data. However, the microprocessors inside the electronic gadgets process the data in bit-parallel mode, so a transmitter is required to convert parallel data to serial form at the transmitter end, and to convert from serial to parallel at the receiver end.

III. PARALLEL COMMUNICATION The parallel communication technology is used over very short distances, typically inside the computer itself and to printers. The parallel data transmission method, together with the connector, was first developed by Centronics and used by IBM in its first Personal Computer [5]. How it works: In this type of data transmission, the data information is exchanged across the communication network using multiple wires. Instead of squishing the bits together, more wires are used to send the bits simultaneously. So, there are several data streams instead of just one. Since multiple wires are used for transmission, this method is expensive. A constant clock signal is sent over a separate wire within the parallel cable for the purpose of handshaking. Also, “as the length of the cable gets longer, the synchronization timing between the multiple channels becomes more sensitive to the distance” [15].

Fig. 2. Simple Two-wire Line for Bit-Serial Data Transmission [6].

The D-shaped 9-pin connection-port is a very nice example of Serial Data Transmission port. It uses two loops of wire for the communication of data. Also, there are additional wires to control the flow of information. However, in any given direction, the data still flows over a single wire. Other examples include USB, FireWire, SATA and PCI Express. A UART (Universal Asynchronous Receiver/Transmitter) is used in the serial transmission technology. The UART: What is it and how it works A UART (Universal Asynchronous Receiver/Transmitter) is the microchip which is programmed such that it is able to control a computer‟s interface to the attached serial devices. Some internal modems also have their own UART. Looking down from a worm‟s eye view, UART actually provides the computer with the RS-232 Data Terminal Equipment (DTE) interface so that it can talk to and exchange data with the modems and other serial devices [7]. RS-232 DTE RS-232 is abbreviated for Recommended Standard-232. This is a standard interface that is approved by the Electronic Industries Association (EIA) for connecting serial devices. In other words, RS-232 is a long established standard that describes the physical interface and protocol for relatively low-speed serial data communication between computers and related devices [8].

Fig. 1. Parallel Data Transmission.

For example, in the case of a 25-pin parallel port, eight bits of data is transferred simultaneously through eight datacarrying wires.

IV. SERIAL COMMUNICATION The serial communication technology is increasingly being used for the transmission of digital data across the communication networks. A very large number of latest communication networks apply this method of data transmission. Computer networks for office communications, field-bus systems in process, building and manufacturing

EEL201 Term Paper: Serial Communication Over Parallel As a part of this interface, the UART also performs the following operations: 1) It converts the parallel-data bytes received from computer micro-processor chips into a single serial bit stream for transmitting the signal. 2) At the receiver‟s end, it performs just the reverse of above operation, i.e., converts serial bit stream back to the form compatible with micro-processor chips. 3) It also adds a parity bit (if selected for so) on outbound transmissions, and at the receiver‟s end, it checks the parity of incoming bytes and discards the parity bit. 4) It adds and transmits start and stop delineators during transmission and strips them from inbound transmissions. 5) It handles any interruptions caused from the keyboard and mouse (which are serial devices with special ports). 6) It can also handle other types of interruptions and can help in synchronizing computer‟s speed of operation with devices speed [7].

3 the receiver must know when to read the next bit coming from the sender‟s end. For handshaking, a clock is shared between the sender and the receiver. So, some extra wiring and control circuits are required to share the clock. Also, when there is no data-bit to be transmitted, a fill character is used, so that data is always being transmitted. Some of the examples where this is used are: phone calls, text chat, Skype etc [10].

VI. ASYNCHRONOUS SERIAL TRANSMISSION In this method, no clock signal is used by the sender. But the receiver must be having a way to distinguish between the valid data bits and noise. So, special bits are added by the sender to each word for the purpose of handshaking. These special bits include- a start bit, parity bit (may or may not be) and a stop bit. Thus, sometimes also called Start-Stop Communication. 1. Start bit - indicates the beginning of the data word. 2. Stop bit - indicates the end of the data word. 3. Parity bit - added for error detection (optional) [11]. PS2 ports on the computer use this type of communication to connect with keyboard and mouse.

VII. SERIAL COMMUNICATION VS PARALLEL The communication channels or links across which the computers and other related gadgets talk to one another may be either parallel or serial. A serial link transmits a single stream of data whereas a parallel one is able to transmit several streams of data along multiple cables or channels. Looking down from a bird‟s eye view, it seems that a parallel link must be superior to a serial one, because it can transmit more amounts of data on each clock tick. However, in actual reality, it is the serial links that achieve a higher Baud Rate as compared to the parallel ones because serial links can be clocked faster than the later ones due to a number of factors. Some of them are: A. Clock Skew Superficially, the speed of parallel link is equal to the number of channels used multiplied by the bit rate of each channel; so eight channels imply, eight times faster. But, in practice, clock skew reduces the speed of every channel to the slowest of all the channels. Clock Skew is the phenomenon due to which there is a time difference in the clock arrival time at different components in the circuit. “For two sequentiallyadjacent registers Ri and Rj with clock arrival times at register clock pins as TCi and TCj respectively, then the clock skew can be defined as: TSkew i, j = TCi - TCj The clock skew can be positive or negative” [12]. B. Isolation The cables used in a serial connection are fewer (e.g. wires/fiber) and hence occupies less space. This extra space is

Fig. 3. DE9 connector (DB9 connector), commonly used for serial ports (RS-232) [9].

During the serial data communication, the receiver must know whether the received signals are valid or not. For this purpose, the transmitter and the receiver must be synchronized. Based on how this is done, the serial communication can be further sub-divided into two categories: 1. Synchronous Transmission 2. Asynchronous Transmission

V. SYNCHRONOUS SERIAL TRANSMISSION In this method, each end of the communication network response before initializing a new conversation. The receiver‟s response is either Success or Resend [8]. Success means the receiver is ready to receive next piece of information. Whereas Resend implies some problem occurred and asks the transmitter to send the previous data packet again. Similarly,

EEL201 Term Paper: Serial Communication Over Parallel used for providing better isolation of the channel from the surroundings [13]. C. Crosstalk Crosstalk creates interference between the parallel lines, and this effect worsens as the length of the communication channel increases. Crosstalk is any phenomenon due to which a signal transmitted on one channel or circuit creates an undesired effect in the neighboring channel or circuit [14]. Advantages of Serial Data Transfer over Parallel: 1) Transmission Distance: The length of the serial cables can be much larger than the parallel cables. This is because a parallel port transmits a „0‟ as 0 Volts and a „1‟ as 5 Volts whereas a serial port transmits a „0‟ as -3 to -25 Volts and a „1‟ as +3 to +25 Volts [7]. So, a maximum swing of 50 Volts is allowed in the serial transmission compared to just 5 Volts in the parallel transmission. Since the maximum swing is very less in parallel, so cable loss causes a lot of problems. But, in serial communication, maximum swing allowed is much higher providing more flexibility. 2) Economical: The number of cables used in serial transmission is very less as compared to the parallel one. For example, in a Null Modem Configuration, the application of serial communication (3 core cables required) is going to be a lot cheaper than the later one (where 19 or 25 core cables are required) [7]. 3) Space Efficient: Since less cables are used in serial data transmission, therefore, it is much more space efficient than the parallel method of data transmission. 4) Easier to Decipher: Infra-red devices are based on serial data transmission method. It is very difficult to decipher eight bits (as in parallel transmission). Therefore, former method is used in which only one-bit data is transmitted at a time. The first infra-red, IrDA-1 was capable of 115.2k baud and it was interfaced into a Universal Asynchronous Receiver/Transmitter (UART) [7]. 5) Reduces Pin Count: Micro-controllers are in quite popularity nowadays. Most of these are based on the Serial Communication Interfaces (SCIs) which are used to communicate with the outside world. Serial Communication reduces the pin-count of these micro-controllers as in most of them, just two pins are commonly used, Transmit Data (TXD) and Receive Data (RXD) instead of minimum eight pins if eight-bit parallel method is used [7]. VIII. CONCLUSION

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Many advances have been developed in the Serial Communication Techniques over the last couple of years and researches are still going on in this domain. Many latest technologies providing high speed data transmissions are based on Serial Techniques. WANs also use Serial Transmission over long distance communication. Super speed USB 3.0 providing transfer speed as high as 5 Gbps is another example. Also, the latest Thunderbolt Technology developed by Intel with bi-directional 10Gbps speed, which can transfer an entire HD movie within seconds, uses single cable transmission. “Enhancing performance with innovative design, Thunderbolt technology combines PCI Express and DisplayPort I/O protocols onto a single highly-efficient metaprotocol” [1]. So with the advancements, Serial Transmission outruns the Parallel counterpart. ACKNOWLEDGMENT I take this opportunity to thank Prof. Shouri Chatterjee and Prof. Anuj Dhawan for exemplary guidance, monitoring and constant encouragement throughout the course. The blessing, help and guidance given by them time to time shall carry me a long way in the journey of life on which I am about to embark. I also thank my friends and family for their help, encouragement and constant support. REFERENCES
[1] "Thunderbolt™ Technology for Developers." Internet: www.intel.com/content/www/us/en/io/thunderbolt/thunderbolttechnology-developer.html [Nov. 6, 2012]. "Principles of Data Communications: Basic Concepts and Terminology." Internet: www.sqa.org.uk/elearning/NetTechDC01BCD/index.htm [Nov. 4, 2012]. A. Awadhi. "What is Serial communication?" Internet: www.efarabi.com/en/resarchnarticles/rs232%20serial%20communicatio n.pdf [Nov. 5, 2012]. "Handshaking." Internet: www.webopedia.com/TERM/H/handshaking.html%20[Nov. 9, 2012]. "Parallel Communications Explained." Internet: hardwarehell.com/articles/parallel.htm [Nov. 4, 2012]. "Serial Data Transmission." Internet: www.samson.de/pdf_en/l153en.pdf [Nov. 6, 2012]. B. R. Mundari, P. Priyadarshan. "Serial Communication By Using UART." B.Tech. thesis, National Institute of Technology, Rourkela, India. Available: ethesis.nitrkl.ac.in/65/1/10407023.pdf [Nov. 6, 2012]. "Introduction to Serial Communication." Internet: sena.com/download/tutorial/tech_Serial_v1r0c0.pdf [Nov. 2, 2012]. "D-sub 9 Pinout." Internet: www.db9-pinout.com/ [Nov. 4, 2012]. "Synchronous Communication." Internet: learningwithcomputers.pbworks.com/w/page/15921339/Synchronous% 20Communication [Nov. 5, 2012]. "Serials Communications." Internet: www.silabs.com/Support%20Documents/Software/Serial_Communicati ons.pdf [Nov. 2, 2012]. "Clock Skew." Internet: en.wikipedia.org/wiki/Clock_skew, Sep. 16, 2012 [Nov. 8, 2012]. "Serial Communication." Internet: en.wikipedia.org/wiki/Serial_communication [Nov. 7, 2012]. "Crosstalk (electronics)." Internet: en.wikipedia.org/wiki/Crosstalk, Oct. 16, 2012 [Nov. 2, 2012]. “Serial and Parallel Transmission.” Internet: www.bookrags.com/research/serial-and-parallel-transmission-csci-02/ [Nov. 2, 2012].

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