Dr Richard Reilly

1
Lecture 10

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• Dr Richard Reilly
• Dept. of Electronic Electrical Engineering
• Room 153,
• Engineering Building

2
SERIAL COMMUNICATION

• Computerised control and data management systems
  require extensive digital communication between
  the processor and external devices such as
•  
•  

      keyboards       printers, plotters
connected locally or at some distance      
video terminals       A/D converters      
computer networks

• Often rate in digital communication is in the
  order of
•       a few bytes per second
• Compare with communication on the data bus
  (million-bytes-per second)

3
SERIAL COMMUNICATION

• In most cases, more economical to send the 8-bits
  of a data byte
•       sequentially on a single transmission line
•       repacking serialised bits into a parallel
  byte at receiving end.
•  
• Thus information is transmitted where
•       information normally consists of more than
  one bit
•       the bits are sent along transmission line
  sequentially
• ? separated in time rather than in space
  as the case in a parallel interface.

4
SERIAL COMMUNICATION

• The basic concept of serial communications is
• a parallel to serial converter at the T/x
• and
• a serial to parallel converter at the R/x  

• If sending information sequentially one question.

How do you know the start of a byte of data ?
5
Synchronous and Asynchronous

• Need method of indicating the start of a byte or
  a word of data.
•  
• Can divide serial communications into based on
  use of clock signal

• synchronous and
• asynchronous 

• Synchronous communication
• uses clock signal as one of the communication
  signals.
• Clock signal defines the transitions from one-bit
  to the next and the centre of the bit.

6
Synchronous and Asynchronous

• Asynchronous communication does not use a clock
  signal as one of the communication signals.
• Each Rx must generate its own local estimate of
  the clock used at the T/x.
•  
•  
• Asynchronous uses less signals than synchronous
  communication but is usually slower.
•  

Asynchronous less signals -- slower
7
Synchronous Communication

• A clock signal defines the bit positions

8
Synchronous Communication

• Changes on the positive clock edge
• Þ  on negative clock edge a new bit is latched in
  at R/x side.
•  
• There is a half-a clock period to allow for
  propagation delay and ringing.

9
Synchronous Communicationwith Frame Sync

• One means of defining the bits of the serial
  stream that correspond to a byte is to use an
  additional signal known as a frame sync.
•  
• Hence a communication link could use
•  
•       TxD Transmit Data line (serial data)
•       TxFS Frame Sync Transmit
•       Clk Clock (common to both T/x and R/x)
•       RxD Receive Data
•       RxFS Frame Sync Receive
•       Gnd ground

10
Synchronous Communicationwith Frame Sync

• A byte wide Frame Sync is high during the period
  of time the data corresponds to bits of the byte.
• the R/X only clocks in the bits if TxFS is high.

11
Synchronous Communicationwith Frame Sync

• For Bit Wide Frame Sync ? Frame Sync high only
  for first bit.
• Hence R/X starts to latch in bits when it
  receives a Frame Sync pulse,
• then latches in a total of 8 bits (must use an
  internal counter).

12
Serial Connections

• Term full-duplex is used to describe transmission
  system where
•       transmission occurs in both directions
  simultaneously
•       such as a serial transmission with one
  line for each direction.

The half-duplex describes the transmission system
where       transmission occurs in both
directions but only one way at a time       as
would be constrained by a single shared line.

• If transmission is only in one direction the term
  simplex is used.

13
Serial Connections

• The T/x and R/x have to be configured for the
  same synchronous communication format.

• The bit rate
• Continuous Clock with Frame Sync
• Byte/Bit Frame Sync
• Word Length (number of bits sent each time)

• Electrical connections between the device and the
  serial interface usually conform to a standard
  known as RS-232/V24.
• this standard not only specifies the voltages but
  also the allocation of pins on the interface

14
How are the two transceivers connected ?

• The two serial ports are connected by crossing
  over T/x and R/x lines.

15
RS-232
16
RS-232 Cable Lengths
Maximum Length (meters)
Data Rate (bps)
2400
120
4800
60
9600
30
19200
15
38400
7.5
57600
5
115200
2.5
17
Asynchronous Communication

• With asynchronous serial communications the only
  connections necessary are
• TxD
• Rxd
• Gnd

• There are no clocks or Frame Syncs connected
  between the two serial ports.

• As there are no synchronising signals present
• ? must introduce one !!
• Additional bits of data are inserted in TxD and
  RxD to allow the serial port to identify the
  first bit

18
Asynchronous Communication

• The following bits are used.
•  
• START Bit This is a 0. It is sent just before
  the data bits
• DATA Bits Usually 7 or 8 data bits (LSB sent
  first)
• PARITY Bit Used for error correction
• STOP Bit This is a 1. One or two stops bits
  used after data.
•  
• Either even or odd parity can be used.
• The parity bit is chosen so that the number of
  ones in the data bits Parity is even or odd.
• e.g. 8 data bits, 1 stop bit and even parity.

19
Asynchronous Communication

• Due to these extra bits that are sent ?
• Asynchronous transmission tends to be slower than
  synchronous
• can be 20 of 30 slower
• high speed transmission tends to use synchronous
  transmission format

• The R/x must know the bit-rate (usually called
  the baud rate).
• However, the baud rate is strictly speaking the
  number of pieces of information per second.

20
Asynchronous Communication

• There are several standard rates of character
  transmission
• 110, 300, 600, 1200, 2400, 4800, 9600, 19600,
  28800, 57600 bits per second
•  
• Typically rate would be 9600 bits per second
• ? each bit has a bit-period of 104 microseconds
•  
• Using 7-bit ASCII
• ? 1711 10bits
• ? 960 characters per second.

21
Asynchronous Communication

• Asynchronous systems use an external clock that
  is 16 times faster then the Bit rate.
•  
• When the negative edge of the start pulse arrives
• ? counts 8 clock cycles to find the approximate
  centre of start pulse
•  
• The centre of the remaining bits is estimated at
  16 clock cycles intervals.
•  ? the bit is read at the middle of its bit
  period of the T/X and R/X baud rates are the
  same.
•   
• As a start bit arrives every byte
• ? the point in time at which the bit is latched
  is re-synchronised every byte.
• ? as there is a minimum of one start bit and one
  stop bit, the 1 ? 0 transition at the start bit
  is guaranteed.