UNIT-V COMMUNICATION INTERFACE

Transcription

1 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

2 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 [citation Asynchronous transmission uses start and stop bits to signify the beginning bit needed] ASCII character would actually be transmitted using 10 bits e.g.: A " " would become " ". 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

3 must be of opposite polarity [ citation needed]. 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 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 each component [vague]. 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 become corrupted (by losing bits) [citation needed]. Ways to get around this problem include resynchronization 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

4 LONG DISTANCE SERIAL COMMUNICATION FRAME FORMAT

5 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 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:

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7 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. Operation between the 8251 and a CPU is executed by program control. Table 1 shows the operation between a CPU and the device.

8 Table 1 Operation between a CPU and 8251 PROGRAMMING THE 8251 The complete functional definition of the 8251 is programmed by the system's software. A set of control words must be sent out by the CPU to initialize the 8251 to support the desired communication format. These words must immediately follow a reset (internal/external). Control Words There are two types of control word. 1. Mode instruction (setting of function) 2. Command (setting of operation) 1) Mode Instruction Mode instruction is used for setting the function of the Mode instruction will be in "wait for write" at either internal reset or external reset. That is, the writing of a control word after resetting will be recognized as a "mode instruction." Items set by mode instruction are as follows: Synchronous/asynchronous mode Stop bit length (asynchronous mode) Character length Parity bit Baud rate factor (asynchronous mode)

9 Internal/external synchronization (synchronous mode) Number of synchronous characters (Synchronous mode) The bit configuration of mode instruction is shown in Figures 2 and 3. In the case of synchronous mode, it is necessary to write one-or two byte sync characters. If sync characters were written, a function will be set because the writing of sync characters constitutes part of mode instruction.

10 2) Command Command is used for setting the operation of the It is possible to write a command whenever necessary after writing a mode instruction and sync characters. Items to be set by command are as follows: Transmit Enable/Disable Receive Enable/Disable DTR, RTS Output of data.

11 Resetting of error flag. Sending to break characters Internal resetting Hunt mode (synchronous mode)

12 Status Word It is possible to see the internal status of the 8251 by reading a status word. The bit configuration of status word is shown in Fig. 5. SAMPLE PROGRAM OF SERIAL DATA TRANSFER Write an 8086 instruction sequence for transmitting 50 characters which are stored from the location 2010H using Sol: Assume 8251 as asynchronous mode Even parity, 2 stop bits, 8 bit char length, frequency 160 khz, and baud rate 10k D7 D6 D5 D4 D3 D2 D1 D

13 ASSUME CS:CODE CODE SEGMENT START: MOV AX,2010H MOV DS,AX //DS points to byte string segment// MOV SI,5000H //SI points to byte string charter// MOV CL,50H //length of string// MOV AL,0FEH //mode control word out to// OUT 0FEH,AL //D0-D7// MOV AX,11H //load command word// OUT 0FEH,AL //to transmit enable and error reset// WAIT: IN AL,0FEH //read status// AND AL,01H //check transmitter enable// JZ WAIT //if zero wait for transmitter is to be ready// MOV AL,[SI] //if ready first charter is tranmited// OUT 0FCH,AL INC SI //point to next byte or charter// DEC CL //decrement counter// JNZ WAIT //if CL not zero, go for next byte// MOV AH,4CH INT21H CODE ENDS END START RS232 COMMUNICATION This standard is developed by the Electronic Industry Association and the Telecommunications Industry Association (EIA/TIA). This is more popularly called simply RS-232, where RS stands for "recommended standard." RS-232 is a popular communications interface for connecting modems and data acquisition devices (i.e. GPS receivers, electronic balances, data loggers) to computers. RS-232 can be plugged straight into the computer s serial port (know as COM or Comm port). Voltage Levels are ±15VDC. Cabling Single-ended Number of Devices 1 transmit, 1 receive Communication Mode Full duplex Distance (max) 50 feet at 19.2kbps Data Rate (max) 20 kbps Input Level (min) ±3V Output Current 500mA

14 Signal Description TxD: - This pin carries data from the computer to the serial device RXD: - This pin carries data from the serial device to the computer DTR signals: - DTR is used by the computer to signal that it is ready to communicate with the serial device like modem. In other words, DTR indicates to the Dataset (i.e., the modem or DSU/CSU) that the DTE (computer) is ON. DSR: - Similarly to DTR, Data set ready (DSR) is an indication from the Dataset that it is ON. DCD: - Data Carrier Detect (DCD) indicates that carrier for the transmit data is ON. RTS: - This pin is used to request clearance to send data to a modem CTS: - This pin is used by the serial device to acknowledge the computer's RTS Signal. In most situations, RTS and CTS are constantly on throughout the communication session. Clock signals (TC, RC, and XTC): - The clock signals are only used for synchronous communications. The

15 modem or DSU extracts the clock from the data stream and provides a steady clock signal to the DTE. Note that the transmit and receive clock signals do not have to be the same, or even at the same baud rate. CD: - CD stands for Carrier Detect. Carrier Detect is used by a modem to signal that it has a made a connection with another modem, or has detected a carrier tone. In other words, this is used by the modem to signal that a carrier signal has been received from a remote modem. RI: - RI stands for Ring Indicator. A modem toggles(keystroke) the state of this line when an incoming call rings your phone. In other words, this is used by an auto answer modem to signal the receipt of a telephone ring signal The Carrier Detect (CD) and the Ring Indicator (RI) lines are only available in connections to a modem. Because most modems transmit status information to a PC when either a carrier signal is detected (i.e. when a connection is made to another modem) or when the line is ringing, these two lines are rarely used. IEEE-488 COMMUNICATIONS: The IEEE-488, General Purpose Interface Bus (GPIB), is a general purpose digital interface system that can be used to transfer data between two or more devices. The GPIB system is a parallel communication system, which can communicate with several devices through the same interface port. The devices that are connected to this bus fall into three categories: controller listener Talker all of which are connected through the IEEE-488 connector. Some of its key features are: Up to 15 devices may be connected to one bus. Total bus length may be up to 20 m. Communication is digital (as opposed to analog) and messages are sent one byte (8 bits) at a time. Data rates may be up to 1 Mbyte/sec IEEE-488 CONNECTOR

16 A Talker sends data messages to one or more Listeners, which receive the data. The Controller manages the flow of information on the GPIB by sending commands to all devices. A digital voltmeter, for example, is a Talker and is also a Listener. The GPIB is like an ordinary computer bus, except that a computer has its circuit cards interconnected via a backplane - the GPIB has stand-alone devices interconnected by standard cables. The role of the GPIB Controller is comparable to the role of a computer CPU, but a better analogy is to compare the Controller to the switching center of a city telephone system.