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DATA COMMUNICATIONS - Categories of Data Communications


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The purpose of this Lesson is to discuss Data Communications.

Data Communications is the exchange of information between computers and other devices, including,

 Integrated Circuits,


PC Boards,

other computers,


circuits &



2 Categories of

Data Communications

1. Local

The computer & devices are directly connected.

(hardwired or internal)


2. Remote

The computer & devices are indirectly connected.

(thru telephone or satellite etc.)



These Categories of Data Communications include:

Parallel Interconnection Standards

& Serial Interconnection Standards

* * * * * * * * * *

Parallel Standards are used for local communication.

Serial Standards are used for remote communication.



Parallel Interconnection Standards include,

IEEE-488 Instrumentation Bus Standard,

S-100 Hobbyist Bus Standard

IBM-PC Bus Standard

Centronics Printer Connection Standard

And many others



Serial Interconnection Standards include,

Current Loop,








There are 3 Terms describing

Data Communications Capabilities

1. Simplex.. ..One Way Only .. .. (like a car radio)

2. Half-Duplex One Way at a Time .. (Police Radio)

3. Full-Duplex..Both Ways Simultaneously.. (telephone)



The rates of data transmission

are described in terms of;

..BPS .. .. Bits Per Second (Data Bits only)

&/or Baud Rate.. .. Data & Sync. bits per sec.. ..

.. .. .. ..Pieces of information per sec


There are

2 major types of

information modulation

commonly used


1. AM Amplitude Modulation.

& 2. FM Frequency Modulation. .




Amplitude Modulation AM

A single frequency is;

turned-on to indicate a '1',

turned-off to indicate a '0'


This Method resembles Telegraph Code



And for,

Frequency Modulation FM

2 specific types of FM

FSK - Frequency Shift Keying, which uses,

1 tone for '1'

another for '0'

PSK - Phase Shift Keying, which uses,

1 phase-shift for '1'

another for '0'



There are

2 Categories of

Serial Data Communication

1. Asynchronous

& 2. Sychronous..



Asynchronous (Below 10,000 bps)

In this category, one character is transmitted at a time, with synchronization included. (LSB 1st)

Each character is FRAMED by adding START & STOP bits, adding another 20% to the total time. (They're also called mark & space bits )



 The START bit is the1st negative edge of the frame.

A STOP bit is represented by a HI for 2 time periods at the end of each character.



Synchronous (Up to 20,000 bps)

Each user synchronizes tranmission with the other.

 In this category, many characters are sent at a time in block form.

Each block is identified by start & end characters.

 IC'S commonly used in these circuits include;

a. MC6830. ACIA

.b. 8250.UART

.c. 8251.USART

. ..d. 8291.. IEEE-488



Error Detection & Correction Methods


Just retransmit the data

& ..Parity Checking Methods

Bit-counting Techniques



Parity Checking Methods include,

a. Check Sum.. .. .. .. ..

Parity sum sent with data

b. Cyclic Redundancy Checking

A Parity Sum quotient fraction is sent with data.

c. Hamming Code.. .. .. ..

. .Similar to Check Sum with added bits.

d. Matrix Sum.. .. .. .. ..

Parity Sum set into a matrix. (LRC/VRC)




The purpose of this Lesson is to discuss Networks.


A Computer NETWORK is the interconnection of several Computers for the purpose of sharing information &/or resources.


4 Basic Network Types,

which link many microcomputers together:

1. LANs. Local Area Networks;

2. MANs. Metropolitan Area Networks

3. WANs.. Wide Area Networks

4. SNAs ..System Network Architectures;

Let's discuss each of these one at a time



Local Area Networks usually connect computers together by Common Data Transmission Medium, that are in a single building or on a single campus. (Less than 10 Kilometers.)

In NETWORK discussions, the term FILE-SERVER often arises.

A FILE-SERVER is, simply, a dedicated Microcomputer, whose function is to SERVE the members of a Network.


Metropolitan Area Networks are capable of Hi-Speed Data Communications and connect computers together, that are within the same Metropolitan Area (10 to 80 Kilometers.)


Wide Area Networks connect computers together by common carrier, that may be very far apart in Different Cities, States, or even different Countries. (Over 80 Kilometers.)


System Network Architectures links PC's to mainframe computers. (IBMs mainframe standard)

There are several of these business-dedicated Networks in use in the world, but, needless to say, they are not in the public domain.


The Network Topology

describes HOW Computers are connected together to exchange information.



The 3 connection-types are called TOPOLOGYs

The 3 most common means of network inter-connection are,

a. BUS Topology

Computers share a bus, where each may drop-off or pick-up information.

b. STAR Topology

(Parallel) .. (Token Ring) ..

All computers are connected to a central brain or coordinator.

c. RING Topology

(Series) .. (Cascaded Star)

Computers are daisy-chained together.


A Network, besides the Connections and users themselves, consists of,

 HARDWARE: including, .. .. ..

Network Interface Cards,

& ..Network Connecting Cables..

SOFTWARE: including, .. .. ..

Protocol Software, ..

Network Operating System Software,

& ..Network Software.. .. .. ..

to run end-user programs



Let's discuss them, one at a time


Network Interface Cards

The purpose of Network Interface Cards is to provide the physical connection for the computer to the Network.

 The Network Interface Card must,

1. Get Computer Data

2. Format Computer Data

3. Encode Computer Data

4. Get access to the Network Cable

5. Establish the lines of communications by providing the HANDSHAKING.



The 3 ways to move Data between the Computer & the Network Interface Card are,



& ..Shared Memory



The Network Interface Card & its Software determine Network SPEED.

 At the other end, the transceiver waits for the Data & the whole process happens in reverse. The Data is, ..

1. Demodulated

2. Decoded.

3.Converted (from Serial, back to Parallel)

4. Depacketized &




Connecting Cables

Network InterConnection is often done by Cable, although Microwave Transmission IS used.

 These connections are accomplished by,

Coaxial Cable,

Twisted Pair,

Fiber Optics

Each one has its advantages & disadvantages




Protocol Software,

Protocol is the set of rules, provided by Software, that governs communication between computers.

Protocols govern, Data & Handshaking,

1. Format.

2. Timing.

3. Sequence

4. Error Control



Protocols are divided into 7 layers

by the Open System Interconnection (OSI) Model to allow different Protocol Standards to work together. These are,


1. Physical .Defines Cable Characteristics

2. Data-Link .Data Blocks,Sync,Error & Flow Control

3. Network .Establishes,Maintains,Terminates Connection

4. Transport .Checks for Data OK

5. Session .Establishes Conversations

6. Presentation ..Code Conversion,Data Reformat

7. Application .Supplies Functions to Computer



Network Operating System Software,

A Network Operating System makes a collection of independent Computers act as ONE System.

Network Operating Systems provide,

1. Resource Sharing Capabilities

2. Data Integrity

3. Security

4. Administrative Tools

5. Troubleshooting Tools

6. Multi-Network Support



Network Operating Systems

being used include,

Novell NetWare

Banyan VINES






Of course, any of the dedicated Application Program software, that were discussed in earlier Lessons, can be used to process information, after it has been recovered by the System.



The purpose of this Lesson is to discuss MODEMs

MODEM stands for


A modem is a device used by computers to communicate with each other over standard telephone lines.

 Modems convert digital signals from the computer into audio frequencies within the bandwidth of a telephone.



MODEMs are available as either,


A PC Board that plugs into an Expansion Slot


A separate Unit, that can be switched or moved as needed.



Modems are also classified by speed

The speed of a MODEM can be given as Bits Per Second (bps) or as its Baud Rate.

Bits Per Second refers to Data Bits only

Baud rate is bps including start & stop bits

It is also defined as the total number of Flux Changes Per Second.



Let's see the SPEED Classifications.


Lo spd .= ..1 - 600 bps. (No Longer Used)

Med spd..= .. 600 - 2400 bps .. (Common) ..

Hi spd .= ..24K-30,000 bps .. (Most Common)


Above 19,000 bps a wide-band Modem & channel is required.

Using special data packaging techniques, the BPS actually realized may be much higher.

 We'll see HOW that can be, but first,

There are other considerations involved with MODEM information exchange. They are

1. Protocols

2. Handshaking &

3. Type of Communication




Synchronous or Asynchronous

Half or Full Duplex, etc

FM, QAM, Bits, Dibits, or Tribits



A Protocol is the established procedure for exchanging Data along with instructions that coordinate the process.

Most Protocols are error-correcting and use one or more of the error-correcting techniques already discussed.


The most-used of these include,


XMODEM..1/2 Duplex PSK.

YMODEM..1/2 Duplex PSK

MODEM .Full Duplex .QAM

KERMIT .Full Duplex .QAM

BLAST ..Full Duplex .QAM


BiMODEM Full Duplex .QAM

Sealink Full Duplex .QAM

Telink .1/2 Duplex .PSK


Almost all of these use CRC & Checksum Error Detection and will automatically request retransmission upon Error Detection.




For MODEMs, AM was never used. FM Techniques like FSK & PSK were initially used often.

The original FSK standard was the Kansas City Standard, in which 1040 Hz is a 0 & 2240 is a 1.  It is no longer used by MODEMs.


Half-Duplex MODEMs use PSK Techniques

1 Phase Shift is a 0, Another is a 1



QAM, Dibits & Tribits

Nowadays, Full Duplex MODEMs use QAM.

QAM stands for Quadrature Amplitude Modulation.

It is a type of Phase Modulated FM, in which a Dibit, (2 Bits), are encoded together.

Quadrature means 90 degrees out of phase

Binary 00,01,10,& 11 or Decimal 0,1,2,& 3 are encoded by 1 of 4 Phase Shifts.

Tribits are encoded, similarly. Remember,

There are 2 Categories of Serial Data Communication Which MODEMs use,

1. Asynchronous

& 2. Sychronous



Asynchronous.. (Below 10,000 bps)

In this category, one character is transmitted at a time, with synchronization included (LSB 1st).


Each character is FRAMED by adding START & STOP bits, adding another 20% to the total time. (They're also called mark & space bits.)


The START bit is the 1st negative edge of the frame.

A STOP bit is represented by a HI for 2 time periods at the end of each character.



Synchronous.. (Up to 20,000 bps)

a. Each data handler person synchronizes with the other.

b. In this category, many characters are sent at a time in block form.

Each block is identified by start & end characters




In order for a Computer to exchange information with another computer, CONTACT must be established between them.

To establish contact,

1. Initialize the MODEM

2. Dial the telephone number.

3. Wait for answer carrier

4. Send originate carrier

5. Report MODEMs locked-on.



1. Initialize the MODEM

2. Answer on specified ring

3. Provide the answer carrier

4. Listen for an Originate carrier

5. Report MODEMs locked-on


In order to establish the lines of communication between one computer & another, an orderly process must be used to ensure that data is transmitted only when the other party is READY for it.

This process is known better as,





MODEM Control Signals include,

Abbrev.. .. Name.. ..Meaning

DTR..Data Terminal Ready ..Computer Ready

DSR..Data Set Ready .MODEM Ready

DCD..Data Carrier Detect ..Remote MODEM on-line & ready

RI Ring Indicator .Modem senses ringing

RTS ..Request To Send .. ..Computer to MODEM,OK/Send?

CTS ..Clear To Send.. MODEM to Computer,Go Ahead

TD .. Transmit Data .. ..Actual Outbound Data

RD . Receive Data . .Incoming Data


The MODEM may need DTR from the Computer before permitting Data Exchange.

The Communications Program may need DSR from the MODEM before permitting Data Exchange.

 The computer sends RTS and only after the MODEM responds with CTS can a computer transmit data.

 Even though that's what's supposed to happen, in reality, many office computer systems connect to MODEMs that only have 3 wires. For,

Transmit Data

Receive Data &





A MODEM has several sections, some optional

The vital circuits are,

Section..Ckt ..Purpose

I/O .INTERFACE . ..,,For V & I level adjustment

OUT ..MODULATOR . ..D-A Conversion & VCO output

IN.. ..DEMODULATOR . A-D Conversion & VCO Phase Reference

I/O ...FILTERS To remove unwanted noise

I/O AMPLIFIERS ..To ensure adequate Gain

I/O.TRANSFORMER .Telephone Line Isolation, Z-Matching




Information to be sent comes from the Computer, and is Level-adjusted by the INTERFACE Circuit.

It is then passed to the MODULATOR Circuit.

 The Modulator converts the DIGITAL Data into an Analog Voltage which is used to vary the Frequency of the Voltage Controlled Oscillator (VCO).

 The VCO Output is, then, FILTERED, AMPLIFIED & applied to the Telephone Line Isolation TRANSFORMER, where it travels to its destination.



Received information comes through the Telephone Line Isolation Transformer, is FILTERED & AMPLIFIED before being applied to the DEMODULATOR.

 The job of the DEMODULATOR is to convert the Analog Signal back into Usable Digital Data.

 This is performed by comparing the phase & frequency of the incoming signal to the phase & frequency of the local VCO.

 This Phase Comparator Output is then applied to the INTERFACE circuit, which Holds it for Computer Processing.


IC'S commonly used in Telecommunication circuits include;

IC # .. TYPE.. . USED for,

a. MC6830 ..ACIA .. ..PORTs

b. 8250 .UART .. .. ..PORTs

c. NS16450 . .UART .. .. ..MODEMs

d. NS16550 .. UART .. .. ..MODEMs

c. 8251 . ..USART .. ..PORTs

d. 8291 IEEE-488 ..Parallel Instrumentation



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