3. Computer Communications

• We need to be able to connect computers to transfer information.

3.1 Communication Categories

• Basic Categories

Interface to Sensors and Actuators

Uses readings from sensors to control devices in the environment
very simple protocols, but often requires specialized hardware and software.

Interface to computers

uses machine to machine connection for direct communication
can appear to be like a local area network
low speeds (eg. 300 baud, 1.2, 2.4 9.2 Kbaud)

Local Area Computer Networks

measured in distances from meters to kilometers
speed measured in Megabits/sec
medium includes, wire, fibre-optics, radio frequency
commonly available in all forms

Wide Area Computer Networks

from city wide, to global
speed measured in Kilobytes/second
medium includes, wire, fibre optics, satellite, microwave
access is costly (some mix of initial and variable costs), and geared to large corporations, or special applications.

• Classification of Interconnected processors (by Physical Size)

Table 1:

Inter-processor Distance

Processors Located in Same


0.1 m

Circuit Board

Data flow machine

1 m




10 m




100 m


1 km


10 km


Long haul network

100 km


1000 km



connection of long haul networks

10000 km


3.2 The History

Late 50’s

Bell introduces data communication networks
Texaco uses remote monitoring and control for automating polymerization plant.

Early 60’s

World wide use of data phone system to link airports for maintenance
Many uses of terminals for inputting and displaying data.

Mid 60’s

Uses of computer for large scale test monitoring
NC part programming being done remotely on time sharing mainframes.


Advent of Mini computers made it possible for companies to have multiple computers which had to communicate.


Microprocessor drops cost of computers dramatically, and automated machines become very common
Personal computers, and automated machinery start a trend to network small computers networked with large computers
3.3 Serial Communications

• Serial communication channels send data one bit at a time.

• These techniques are often slower, but are essential when transmitting using radio waves, telephone lines, or similar media.

• Serial lines have become one of the most common methods for transmitting data to instruments, and most personal computers now have two serial ports.

• Serial data can be transmitted two ways,

synchronous: all of the data bits are timed between sender and receiver.

asynchronous: the data bits arrive unannounced, without any coordination between sender and receiver. (this is the most common)

RS-232/RS-232C: the most common standard using voltage levels
RS-422: a current loop standard well suited to noisy conditions

• A typical data byte looks like the one below. The voltage/current on the line is turned on/off. The width of the bits determines the possible bits per second (bps). The value shown before is used to transmit a single byte.


• The bits shown correspond to,

before: this is a period where no bit is being sent and the line is high

start: a single bit to help get the systems synchronized

data: this byte could be 7 or 8 bits, but is almost always 8 now. The value shown here is 01001000 binary.

parity: this lets us check to see if the byte was sent properly. The most common choices here are no parity bit (just drop it out), or even parity, or odd parity. In this case there are two bits set. If we are using even parity the bit would be set. If we are using odd parity the bit would be off

stop: the stop bits allow form a natural pause at the end of the data.

after: a period of time where the line is high before the next byte.

3.3.1 RS-232

• This standard is based on a high being +3 to +15V, and an off being -3 to -15V (+/-12V is commonly used).

• The typical connection scheme is shown below. notice that the lines are crossed between connectors. Also note that in some cases the dsr/dtr lines are not used.


• The line names are,

TXD/RXD: (transmit data, receive data) these lines pass data one way from sender to receiver.

DSR/DTR: (data set ready, data terminal ready) these handshaking lines indicate when the remote machine is ready to receive data.

COM: a common ground to keep things from blowing up, etc.

• A simpler wire is also used for some devices (in low noise situations) called a null modem cable. In this cable only the data and common lines are used.


• There are typically two connectors used for these devices. (Note: the connector have very fine numbers on them)


• The main pin assignments for the DB-25 are,

2: TXD

3: RXD

6: DSR

7: COM

20: DTR

• The main pin assignments for the DB-25 are,

2: TXD

3: RXD




Other Common Connection Types




3.4 Parallel Communications

• In parallel data schemes, all of the data bits are sent at the same time.

• This generally allows a higher rate of data transfer, but only shorter distances, and with a higher number of conductors in the cable.