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

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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] 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

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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 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

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 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

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LONG DISTANCE SERIAL COMMUNICATION

FRAME FORMAT

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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:

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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.

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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 8251. 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)

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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.

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2) Command

Command is used for setting the operation of the 8251. 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.

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Resetting of error flag.

Sending to break characters

Internal resetting

Hunt mode (synchronous mode)

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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 8251.

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 D0

1 1 1 1 1 1 1 0

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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 computers 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

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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

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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

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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.

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