Data communication

1. Fundamental Concepts of
Data Communications
(UART, USRT, Serial Interface, MODEM)
Chapter 22
Electronic Communications Sytems , Fifth Editon
By: Wayne Tomasi
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2. Universal Asynchronous Rx/Tx
2

3. Universal Asynchronous Rx/Tx
3

4. UART FUNCTIONS
• Parallel-to-serial at the Tx and Serial-
to-parallel conversion at the Rx
• Error detection
• Framing
• Formatting
• Status monitoring
• Voltage level conversion/matching
• Bit and character synchronization
4

5. UART Control Word
5

6. UART Transmitter
6

7. UART Receiver
7

8. UART Receiver Start-bit Verification
8

9. USRT Sampling Error
9

10. USRT FUNCTIONS
• Parallel-to-serial at the Tx and Serial-
to-parallel conversion at the Rx
• Error detection
• Synchronization
• Formatting
• Status monitoring
• Voltage level conversion/matching
• Bit and character synchronization
10

11. Serial Interface
• A communication interface between two digital
systems that transmits data as a series of voltage
pulses down a wire.
• Originally designed as the primary link between Data
Terminal Equipment and its Data Communication
Equipment.
• Now used for multiple applications requiring serial-by-
bit transmission between digital equipment and
peripherals.
• The first standardized serial interface was published as
a recommended standard RS-232 by the EIA in 1962.
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12. Serial Interface
12

13. Serial Interface
• A serial interface standard must provide
the following:
1. A specific range of voltages for transmit and
receive signal levels
2. Limitation for the electrical parameters of
the transmission line.
3. Standard cable and cable connectors.
4. Functional description of each signal on the
interface.
13

14. RS-232 Serial Interface
• Officially named Interface Between Data Terminal
Equipment and Data Communications Equipment
Employing Serial Binary Interchange.
• Introduced by the EIA at a time when there was
considerable diversity in the serial interfaces manufactured
by different data communication equipment providers.
• It specifies the mechanical, electrical, functional, and
procedural descriptions for the interface between DTEs and
DCEs.
• Similar to the ITU-T V.28 electrical specifications and the
V.24 functional description designated for transmission up
to 20 kbps over a maximum distance of 50 ft.
14

15. RS-232 Physical Characteristics
• Cable. A sheath containing 25 wires with two
connectors called the DB25P-compatible
male connector (plug) on one end and a
DB25S-compatible connector (receptacle) on
the other end designed to transmit
synchronous or asynchronous data.
• DB9P and DB9S connectors can alternately be
used for asynchronous data transmission.
15

16. • DB25 and DB9 Pinout
16

17. RS-232 Electrical Characteristics
• Driver load impedance: 3kΩ – 7k Ω
• Maximum capacitive load: 2500 pF
• Maximum data rate: 20 kbps
• Maximum length: 15 m (50 ft.)
• Logic voltages:
17
DATA SIGNALS CONTROL SIGNALS
LOGIC ‘1’ LOGIC ‘0’ ENABLE DISABLE
DRIVER -5 to -15 +5 to +15 +5 to +15 -5 to -15
TERMINATOR -3 to -25 +3 to +15 +3 to +15 -3 to -25

18. RS-232 Electrical Characteristics
18
RS-232 Equivalent Circuit

19. RS-232 Functional Characteristics
19

20. RS-232 Functional Characteristics
20

21. RS-232 Procedural Characteristics
21
Null Modem Connection

22. RS-232 Procedural Characteristics
22
Example. Draw the timing diagram for a null modem
connection with the following specifications:
• RTS/CTS Delay : 40 mS for Switched Carrier
20 mS for Cont. Carrier
• RLSD Delay : 20 mS ON, 10 mS OFF
• Primary Carrier Option : Switched Carrier and RTS
• Secondary Carrier Option : Switched Carrier and RTS
• Propagation Time (Downstream) : 20 mS
• Propagation Time (Upstream) : 20 mS
• Secondary Turnaround Time : 40 mS
• Secondary Message Length : 40 mS
• Primary Message : 60 mS
• Primary On time : 0 mS

23. Other Serial Interface Standards
• The RS-449 standard was designed in 1977 as a 37 wire
cable to replace the RS-232 interface.
• The RS-449 circuits were divided into two categories:
Category I were circuits (pins) compatible with the RS-
232 standard while the remaining circuits were under
Category II.
• The RS-449 Standard specifies 10 circuits not specified
in the RS-232 standard: LL(10), RL(14), SF(16), TM(18),
RC(20), IS(28), SS(32), NS(34), SB(36), SC(37).
• The RS-422 and RS-423 specify the electrical
specifications of the RS-449 serial interface.
23

24. Other Serial Interface Standards
• The RS-422A standard specifies a balanced
interface cable capable of spanning 15
meters at 10 Mbps data rate but could
extend to 1200 meters at slower speeds.
• The RS-423A standard specifies a balanced
interface cable capable of operating at a
maximum rate of 100 kbps and maximum
length of 90 meters.
24

25. Other Serial Interface Standards
• The RS-530 standard was introduced by
the EIA in 1987. This cable uses the same
25-pin connector of the RS-232 and uses
the electrical specifications outlined by
either the RS-422A or the RS-423A
standards.
• The RS 530 standard operates at data rates
between 20kbps and 2 Mbps.
25

26. Data Communication Modems
• Data Communication Modems. Devices principally
designed to interface computers, computer networks and
other digital terminal equipment to analog
communication facilities necessary when transmitting
signals over long distances.
• Modems perform digital to analog conversion prior to
modulation at the transmitter and analog to digital
conversion after demodulation at the receiver.
• Telephone Loop or POTS Modems. Modems used to
interface DTEs through a serial interface to standard
voice-band telephone lines at data rates from 300 bps to
56 kbps.
26

27. Data Communication Modems
• Data Communication Modem Block Diagram
27

28. Data Communication Modems
• Modem Specifications.
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29. Data Communication Modems
• Modem Synchronization. Achieved by sending a special
internally generated bit pattern called the training
sequence to synchronize/ train the receive modem at the
other end of the communications channel. The training
sequence accomplishes one or more of the following:
– Initializes the communication channel (i.e. disabling echo and
AGC gain)
– Verifies continuity
– Initializes receive modem descrambler circuits
– Initializes receive modem automatic equalizers
– Synchronize receive carrier to the transmit carrier
– Synchronize receive clock to the transmit clock
29

30. Data Communication Modems
• Modem Equalizers. Circuits designed to reduce the phase delay
and amplitude distortion inherently present on telephone
communications channels.
• Compromise Equalizers. Located at the transmitter modem to
provide pre-equalization by shaping the transmitted signal
(altering delay and gain characteristics) before the signal reaches
the telephone line. May affect: 1. Amplitude Only, 2. Delay
Only, 3. Amplitude and Delay, or 4. Neither Amplitude nor
Delay.
• Adaptive Equalizers. Located at the receiver modem to provide
post-equalization to the signal by automatically adjusting gain
and delay characteristics to compensate for phase and amplitude
impairments introduced by the communication channel.
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31. Data Communication Modems
• ITU-T Modem Recommendations. V-series of recommendations
published by the ITU-T in the late 1980s as transmission
standards for data modems outside the United States.
• GROUP WORK: Make a comparative table of the important V-
series standards mentioned by Tomasi namely: V.29, V.32,
V.32bis, V.32terbo, V.32(Vfast), V.33, V.34+, V.42, V.90, V.92.
Specify important parameters such as: year of publication,
transmission speed, maximum cable length, type of
modulation, error coding, etc.
31

32. Data Communication Modems
• ITU-T V-Series Standards Summary: (Alcantara, Austria, Datic, Ismael, Quiray)
32
ITU-T
Standard
Data
Rate
(bps)
No.
of
Bits
Baud
Rate
Operating
Mode
Modulation Carrier
Frequency
Error Coding/
Other
Characteristics
V.29
(1988)
9600 4 2400 Full
Duplex
16 QAM 1700 Pingpong;
Statistical
Duplexing
V.32
(1988)
9600
2400
4800
5 2400 Full
Duplex
QAM/
TCM
1800 Trellis Encoding;
Echo Cancellation
V.32bis
(1991)
14400;
12200
9600
4800
6 2400 Full
Duplex
QAM/
TCM
1800 Fall-forward and Fall-
back Features;
Data Compression
V.32
Terbo
(1993)
19200
14400;
12200
9600
4800
6 2400 Full
Duplex
QAM/
TCM
1800 Adaptive Speed
Levelling; Fall-forward
and Fall-back features;
Data Compression

33. Data Communication Modems
• ITU-T V-Series Standards Summary: (Alcantara, Austria, Datic, Ismael, Quiray)
33
ITU-T
Standard
Data
Rate
(bps)
No.
of
Bits
Baud
Rate
Operating
Mode
Modulation Carrier
Frequency
Error Coding/
Other
Characteristics
V.32
V.fast
(1994)
28800 8 3429 FDX/ HDX PCM 1800 Nonlinear Coding;
Multidimensional
Coding; Constellation
Shaping; Reduced
Decoder Complexity;
Precoding of Data; Line
Probing
V.33
(1988)
14400 6/1
Info/
redun
dant
2400 FDX 128 QAM
64 QAM
TCM
1800 Trellis Coding
V.34+
(1996)
28800
31200
33600
5 3429 FDX/
HDX
Assymetric
TCM 1800 MSE Fallback or Fall
Forward control; Echo
Cancellation

34. Data Communication Modems
• ITU-T V-Series Standards Summary: (Alcantara, Austria, Datic, Ismael, Quiray)
34
ITU-T
Standard
Data
Rate
(bps)
No.
of
Bits
Baud
Rate
Operating
Mode
Modulation Carrier
Frequency
Error Coding/
Other
Characteristics
V.42
(1988)
28800 6 2400 FDX QAM 1800 Asynchronous to
Synchronous
Conversion; Error
Detection and
Correction; LAPM
V.90
(1998)
56000
(Rx)
33600
(Tx)
5
to
8
3200
or
8000
FDX PCM/
TCM
1800 Asymmetrical Data
Transmission
V.92
(2000)
56000
(Rx)
48000
(Tx)
5
to
8
8000 FDX PCM 3000 Asymmetrical Data
Transmission; Faster call
set-up capabilities;
Incorporation of a hold
option