Jntuhubupdates.com Jntuhubupdates UNIT-V COMMUNICATION INTERFACE SERIAL DATA TRANSFER INTRODUCTION Data transmission, digital transmission or digital communications is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media. The data is represented as an electro-magnetic signal, such as an electrical voltage, radiowave, microwave or infra-redsignal. While analog communications is the transfer of continuously varying information signal, digital communications is the transfer of discrete messages. The messages are either represented by a sequence of pulses by means of a line code ( baseband transmission), or by a limited set of continuously varying wave forms ( passband transmission), using a digital modulation method. The passband modulation and corresponding demodulation (also known as detection) is carried out by modem equipment. According to the most common definition of digital signal, both baseband and passband signals representing bit-streams are considered as digital transmission, while an alternative definition only considers the baseband signal as digital, and passband transmission of digital data as a form of digital-to-analog conversion. Data transmitted may be digital messages originating from a data source, for example a computer or a keyboard. It may also be an analogsignal such as a phone call or a video signal, digitized into a bit-stream for example using pulse-code modulation (PCM) or more advancedsource coding (analog-to-digital conversion and data compression) schemes. This source coding and decoding is carried out by codecequipment. Base band and Pass band signals The physically transmitted signal may be one of the following: 1. A baseband signal ("digital-over-digital" transmission): A sequence of electrical pulses or light pulses produced by means of a line coding scheme such as Manchester coding. This is typically used in serial cables, wired local area networks such as Ethernet, and in optical fiber communication. It results in a pulse amplitude modulated signal, also known as a pulse train. 2. A passband signal ("digital-over-analog" transmission): A modulated sine wave signal representing a digital bit-stream. The signal is produced by means of adigital modulation method such as PSK, QAM or FSK. The modulation and demodulation is carried out by modem equipment. This is used in wireless communication, and over telephone network local-loop and cable-TV networks. Serial and Parallel transmission In telecommunications, serial transmission is the sequential transmission of signal elements of a group representing a character or other entity of data. Digital serial transmissions are bits sent over a single wire, frequency or optical path sequentially. Because it requires less signal Jntuhubupdates.com Jntuhubupdates.com Jntuhubupdates processing and less chances for error than parallel transmission, the transfer rate of each individual path may be faster. This can be used over longer distances as a check digit or parity bit can be sent along it easily. In telecommunications, parallel transmission is the simultaneous transmission of the signal elements of a character or other entity of data. Indigital communications, parallel transmission is the simultaneous transmission of related signal elements over two or more separate paths. Multiple electrical wires are used which can transmit multiple bits simultaneously, which allows for higher data transfer rates than can be achieved with serial transmission. This method is used internally within the computer, for example the internal buses, and sometimes externally for such things as printers, The major issue with this is "skewing" because the wires in parallel data transmission have slightly different properties (not intentionally) so some bits may arrive before others, which may corrupt the message. A parity bit can help to reduce this. However, electrical wire parallel data transmission is therefore less reliable for long distances because corrupt transmissions are far more likely. Types of Communication channels . Half-duplex . Full-duplex . Point-to-point . Multi-drop: . Bus network . Ring network . Star network . Mesh network . Wireless network Serial communications: There are two types of Serial communications Asynchronous transmission uses start and stop bits to signify the beginning bit [citation needed] http://en.wikipedia.org/wiki/ASCII ASCII character would actually be transmitted using 10 bits e.g.: A "0100 0001" would become "1 0100 0001 0". The extra one (or zero depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data is sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits Jntuhubupdates.com Jntuhubupdates.com Jntuhubupdates [ citation needed] must be of opposite polarity . This allows the receiver to recognize when the second packet of information is being sent. It is suitable for data transmitted at random intervals (e.g. keyboard to computer) large overhead (20% or more) rather low data rates (up to 115.2 kbps, practically 38.4 kbps) simplicity and availability: UART and RS232 are present in any PC used in the great majority of dial-up connections Synchronous transmission uses no start and stop bits but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signal(s) built into [vague] each component . A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in protocol (computing) for sending/receiving data, so some bytescould [citation needed] become corrupted (by losing bits) . Ways to get around this problem include re- synchronization of the clocks and use ofcheck digits to ensure the byte is correctly interpreted and received. low overhead (long frames) high rates less prone to errors Jntuhubupdates.com Jntuhubupdates.com Jntuhubupdates LONG DISTANCE SERIAL COMMUNICATION FRAME FORMAT Jntuhubupdates.com Jntuhubupdates.com Jntuhubupdates The UART performs the "overhead" tasks necessary for asynchronous serial communication. Parity bits are also often employed to ensure that the data sent has not been corrupted. The UART usually generates the start, stop, and parity bits when transmitting data, and can detect communication errors upon receiving data. 8251 USART ARCHITECTURE AND INTERFACING The 8251A is a programmable serial communication interface chip designed for synchronous and asynchronous serial data communication. It supports the serial transmission of data. It is packed in a 28 pin DIP. Block diagram of the 8251 USART (Universal Synchronous Asynchronous Receiver Transmitter) The functional block diagram of 8251A consists five sections. They are: Read/Write control logic Transmitter Receiver Data bus buffer Modem control. The functional block diagram is shown in fig: Jntuhubupdates.com Jntuhubupdates.com Jntuhubupdates Jntuhubupdates.com Jntuhubupdates.com Jntuhubupdates Read/Write control logic: The Read/Write Control logic interfaces the 8251A with CPU, determines the functions of the 8251A according to the control word written into its control register. It monitors the data flow. This section has three registers and they are control register, status register and data buffer. The active low signals RD, WR, CS and C/D(Low) are used for read/write operations with these three registers. When C/D(low) is high, the control register is selected for writing control word or reading status word. When C/D(low) is low, the data buffer is selected for read/write operation. When the reset is high, it forces 8251A into the idle mode. The clock input is necessary for 8251A for communication with CPU and this clock does not control either the serial transmission or the reception rate. Transmitter section: The transmitter section accepts parallel data from CPU and converts them into serial data. The transmitter section is double buffered, i.e., it has a buffer register to hold an 8-bit parallel data and another register called output register to convert the parallel data into serial bits. When output register is empty, the data is transferred from buffer to output register. Now the processor can again load another data in buffer register. If buffer register is empty, then TxRDY is goes to high. If output register is empty then TxEMPTY goes to high. The clock signal, TxC (low) controls the rate at which the bits are transmitted by the USART. The clock frequency can be 1,16 or 64 times the baud rate. Receiver Section: The receiver section accepts serial data and convert them into parallel data The receiver section is double buffered, i.e., it has an input register to receive serial data and convert to parallel, and a buffer register to hold the parallel data. When the RxD line goes low,the control logic assumes it as a START bit, waits for half a bit time and samples the line again. If the line is still low, then the input register accepts the following bits, forms a character and loads it into the buffer register. The CPU reads the parallel data from the buffer register. When the input register loads a parallel data to buffer register, the RxRDY line goes high. The clock signal RxC (low) controls the rate at which bits are received by the USART. During asynchronous mode, the signal SYNDET/BRKDET will indicate the break in the data transmission. During synchronous mode, the signal SYNDET/BRKDET will indicate the reception of synchronous character. MODEM Control: The MODEM control unit allows to interface a MODEM to 8251A and to establish data communication through MODEM over telephone lines. This unit takes care of handshake signals for MODEM interface.