ICT3107-Microprocessor and Interfacing

Lecture Notes # 19
Outline of the Lecture
• Interfacing the Serial Port
• Basics of Serial Communication
• Asynchronous Data Communication and Data Framing
• RS232 and other Serial I/O Standards

BASICS OF SERIAL COMMUNICATION

¾ Computers transfer data in two ways: parallel and serial.
Compare and contrast between serial and parallel transmission.
• Parallel communication is fast and efficient. In parallel communication 8 or more
lines (wire conductors) are used to transfer data to a device that only a few feet
away. (e.g. printers, hard disks etc.)
• Parallel communication provides much faster transfer of data with compared to
serial communication.
• However for long distances the number of wires required as well as the
distortion on the signals in parallel wires makes parallel communication for long
distances not reliable. Also the synchronization of the parallel signals is a big
problem.
• Serial communication sends 1 bit at a time, so it is slower with compared to
parallel communication.
• For long distances serial communication provides robust way of data transfer.
Because there is no problem of synchronization.
• Serial communication requires less number of wires for communication. Serial
communication is used in modems, keyboards etc.

Serial Data Transfer Parallel Transfer

D0

Sender Receiver
Sender Receiver

D7

Serial versus Parallel Data transfer.

Asynchronous /Synchronous Methods

¾ Serial data communication uses two methods: asynchronous and synchronous.

• Synchronous communication requires common timing signals and common
clock for synchronization. This means that as well as the data, the clock signals
must also be transmitted along. In synchronous method a block of data at a time
is transmitted. 1ta line data hoile arekta line clk emne pathaite hbe.
• In asynchronous method a single byte is transferred at a time. For example,
start and stop bits are used to indicate the receipt of a byte.

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Serial Data Communication Systems

• Simplex Transmission: Transmission of data is only possible in one direction

(e.g. output only-printers)
• Half Duplex Transmission: Transmission of data is possible in both directions,
but in one direction at a time.
• Full Duplex Transmission: Transmission of data can be in both directions at a
time. Requires to wires as data lines. 2ta link or 2ta connection line lagbe
2ta channel
2ta wire
example: mobile
Transmitter Receiver
Simplex

Transmitter Receiver

Half Duplex
Receiver Transmitter

Transmitter Receiver
Full Duplex
Receiver Transmitter

Simplex, Half and Full Duplex Transfers

UART (Universal Asynchronous Receiver-Transmitter) Chip

UART chips are used for serial communication. The 8250 series, which includes the
16450, 16550, 16650, & 16750 UARTS, are the most commonly used.

• Asynchronous serial communication and data framing

¾ The data coming in the receiving end of the data line in a serial data transfer is all 1s
and 0s; it is difficult to make sense of data unless the sender and receiver agree on a
set of rules, a protocol on how the data is packed, how many bits constitute a
character, and when the data begins and ends.

Start and Stop bits

¾ Asynchronous serial data communication is widely used for character-oriented
transmission, and block-oriented data transfers use the Synchronous method.

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 ¾ In the Asynchronous method, each character is put between a start and stop bits. This
is called framing. In data framing, for asynchronous communications, the data such as
ASCII characters are packed in between a start bit and stop bit.
¾ The start bit is always one bit, but the stop bit(s) can be one or two bits. The start bit
is always a 0 (low) and the stop bit(s) is 1 (high)
¾ The example below gives the framing of the ASCII character “A”, where, binary
0100 0001, is framed in between the start bit and two stop bits. Notice that the LSB is
sent out first.
start bit - 1bit (0)
stop bit-1 or 2 bit depending on protocol (1)

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Framing of ASCII “A” (41H)

¾ Notice that the transmission begins with a start bit followed by D0, the LSB, then the
rest of the bits until the MSB (D7), and finally, the 2 stop bits indicating the end of
character “A”.
¾ In asynchronous serial communication, peripheral chips and modems can be
programmed for data that is 5, 6, 7 or 8 bits wide.
jtai pathano hy otai jno receive kore
¾ In some systems in order to maintain the data integrity, the parity bit of the
character byte is included in the data frame. actual data baade baki gulo overhead. link
establish korte eta pathatei hbe. total frame e
actual data charao start bit , stop bit, parity milay
Data Transfer Rate (Baud Rate) 12 bit but 8 bit hoilo actual data bit. one third of ur
total capacity. Total jei capacity
¾ The rate of data transfer in serial communication is stated in bps (bits per second).
Another widely used terminology for bps is the baud rate.
¾ Typical bps values are: 2,400, 4,800, 9,600, 19,200, 28,800,… etc.
dial up modem dye ISP der call dte hoto aagekar amole.

• RS232 and other Serial I/O Standards

¾ RS232 is the most widely used serial I/O interfacing standard.
transistor transistor logic
¾ However the I/O voltage levels are not TTL compatible. In the RS232 a 1 is
represented by –3 to –25 V, while 0 bit is +3 to +25 V, making –3 to +3 undefined.
¾ For this reason voltage converter such as MC1488 and MC1489 are used to
convert the TTL logic levels to the RS232 voltage levels and vice versa. See Figure
below.

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RS232 Pins
¾ Serial Ports come in two "sizes". There are the D-Type 25 pin connector and the D-
Type 9 pin connector both of which are male on the back of the PC, thus you will
require a female connector on your device. Below is a table of pin connections for
the 9 pin and 25 pin D-Type connectors.

D-Type 25 pin D-Type 9-pin

only 9ta pin e kaj kore.
Serial Port Pinouts (D25 and D9 Connectors) kaj gulo only mn e rakhbo

D-Type-25 Pin D-Type-9 Pin

Abbreviation Full Name
No. No.
Pin 2 Pin 3 TD / TxD Transmit Data basic 2ta pin for serial
Pin 3 Pin 2 RD / (RxD) Receive Data communication
Pin 4 Pin 7 RTS Request To Send
Pin 5 Pin 8 CTS Clear To Send
Pin 6 Pin 6 DSR Data Set Ready
Pin 7 Pin 5 SG / (GND) Signal Ground
Pin 8 Pin 1 CD / (DCD) Data Carrier Detect
Data Terminal
Pin 20 Pin 4 DTR
Ready
Pin 22 Pin 9 RI Ring Indicator
Table 1 : D Type 9 Pin and D Type 25 Pin Connectors

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 Data exchange er aage basically they need to setup the link..link k aage establish korte hbe

Pin Functions Link establish kore holo DSR, DTR ei 2ta pin dye.
Aage 2pokkho thke link establish korbe then 2 pokkho blbe j ami data exchange korte raji
Then finally RD r TD dye receive r transmit hbe data

Abbreviation Full Name Function

TD Transmit
Serial Data Output (TXD)
Data TDTD transmit
transmit data,
data, serial
serial data
data out
RD Receive Data out
Serial Data Input (RXD)RD hlo rcv data, serial data IN
RD hlo rcv data, serialdataIN
data CTS Clear to Send This line indicates that the Modem is ready to
pathanor exchange data. onno end er modem ready
aage DSR, DCD Data Carrier When the modem detects a "Carrier" from thehoile modem
ei pin ta k set korbe
DTR link Detect modem at the other end of the phone line, this Line
establish kore becomes active. modem lineup check kore j, kono ekta numbr theke ring
hoille she connection establish korte chacche eta bujhe
DSR Data Set This tells the UART that the modem is ready to
Ready establish a link.
DTR Data
This is the opposite to DSR. This tells the Modem
Terminal
that the UART is ready to link.
RTS Pin ta 1st e request korbe,
Ready
UART RTS pin k actv kore RTS Request To This line informs the Modem that the UART is ready
modem k blbe ami data xchng
korte ready Send to exchange data.
RTS pin actv hoile modem
RI Ring Indicator Goes active when modem detects a ringing signal.
abar blbe ami xchng korte
ready..kivabe? CTS pin k actv
kore
Data Communication Classification: modem just convey kore=>DCE
¾ Devices which use serial cables for their communication are split into two
categories. These are DCE (Data Communications Equipment) and DTE (Data
data consumer Terminal Equipment.)
¾ DTE refers to terminals and computers that send and receive data; DCE refers to
communication equipment such as modems that are responsible for transferring
data.

8255 er control register k program kore serial port configure kora=> next lecture