Communications Standards and Serial Interface
Analog Signal:
An analog signal changes continuously e.g a sine wave, a triangular wave, a
sawtooth wave, a ramp, human voice, video signal etc
Digital Signal:
Changes only in discrete steps e.g a rectangular wave or a square wave, stair case, a
binary signal etc
Analog communications:
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Information is transferred from one place to other using analog devices and
circuits,
Consists of a TXMTR, a communication channel or lank and a RXCVR.
Signal to be transmitted is modulated at the transmit end and demodulated
at the receive end.
Modulation is the process of changing some characteristics of the signal in
proportion to a carrier signal.
AM, FM, FSK, PSK, DFSK, PCM etc are all examples of modulation
schemes used in the communication industry.
Digital communication:
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Information is transmitted using digital signals and digital devices
Also consists of a TXMTR, a link and a RXCVR
Link could be UTP or STP wire, a Coaxial cable, fiber Optic, Microwave etc.
Data Communication:
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Process of communicating information in Binary form.
Also called Computer communication, because most of the communication
occurs between computers or between computers and peripheral devices.
The term Data refers to information in Binary form represented by various
information codes such as ASCII code, BCD code, EBCDIC code etc or pure
binary data obtained from an Analog to digital converter monitoring some
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physical quantity (Temperature, pressure, humidity, motion, light changes
etc.)
Familiarize yourself with ASCII code, BCD code and EBCDIC code because
these are the most commonly used codes (P 125 and 127 text book)
Analog Data Transmission System Block diagram:
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Refer to the diagram shown
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DTE : Data Terminal Equipment – A computer, keyboard, teletype, plotter,
a front end processor, a multiplexer or any other device that generates data
to be transmitted.
UART or USART: Universal Asynchronous (or Synchronous) Receiver and
Transmitter – Commonly known as Serial Port (COM 1 or COM 2 on your
computer). For example 8250 on IBM PC or INTEL 8251. A device that
converts parallel data from the computer into a serial stream and vise versa.
It also performs a number of house keeping functions, as we shall see later.
RS-232 Connector: A 25PIN or 9 PIN D type connector that converts binary
data into bipolar logic levels. We shall study the electrical, mechanical and
functional characteristics of the RS-232 Connector in some detail shortly.
DCE or DSU: Data communication Equipment or Data Set Unit: - This
device is also called Data Set. Commonly known as MODEM (ModulatorDemodulator). Converts binary data generated by the computer into analog
form suitable for transmission over 3KHz analog telephone lines, and
converts analog signals back into digital format at the receiving end.
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INTERFACE STANDARDS FOR SERIAL COMMUNICATIONS:
RS-232 Serial data communications standards:
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Most commonly used Serial Data Communication Interface Standard
It was developed by the Electronic Industry Association (EIA) and accepted
throughout North America and now worldwide.
Also called V.28 Standards by the International Telecommunication Union
(ITU-T).
It specifies electrical, mechanical, and functional characteristics of the
physical layer (Layer 1) of the OSI Model.
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Its purpose is to make equipment and devices of various manufacturers
compatible with each other and create some order in the chaos
Uses a 25 PIN or 9 PIN connector shown in the diagram
Speed less than 20 kbps and distances up to 50 ft
Line capacitance must not exceed 2500pf
Voltage levels +,- 3V to +,- 25V unloaded and +,- 5V to +,- 15 V loaded. In
the lab you will measure about +,- 10V. See the chart below
Voltage
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RS-232
0V
TTL logic
level
Logic “ 0”
Name
+3 to +25 V
Condition
Represented
ON
+5V
Logic “ 1 “
-3V to –25 V
OFF
“ Mark”
“ Space”
Notice that the RS-232 logic levels are inverted as compared to the normal
TTL logic levels
Draw the RS-232 waveform corresponding to the following TTL Data:
0110011. Assume +,- 10 V Rs-232 levels
Also provides handshaking signals
DB-25 has 25 signal lines and DB- 9 has 9 signal lines. See pictures and draw
diagram in the space provided below
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All current PCs use only 9 pins
There are 4 categories of signals
Data lines
Control Lines
Timing lines
Secondary function lines
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Fig below shows how these lines are connected between DTE and DCE.
Table summarizes the names and direction of all signals
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Data Lines:
TXD: Data travel from DTE to DCE on TXD line
RXD: Data travel from DCE to DTE on RXD line
Control lines: Handshaking control signals to indicate readiness to transmit
and receive data.
DTR: Data Terminal Ready: (Indicates equipment readiness): Signal from the
DTE to DCE. Normally at mark. Goes to space ( or turned ON ) when ready
to transmit.
DSR: Data Set Ready: (Indicates equipment readiness) : Response from the
DCE to DTE. Normally Hi or Off or at Mark. Goes low to indicate that it is
ready to receive.
RTS: Request To Send : ( Indicates line readiness): Signal from DTE to DCE.
Normally Hi, goes low to indicate it wants to transmit.
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CTS: Clear To Send : (Indicates line readiness ): Response from the DCE
indicating it is ready to receive data. Normally Hi, goes low to indicate it is
ready to receive.
DCD: Data Carrier Detect: Signal from DCE to DTE. Normally Hi. Drops to
LOW when detects a valid carrier. Indicates line is active.
RI: Ring Indicator : Signal from DCE to DTE. Normally Hi. Goes LOW to
indicate that the DCE is receiving a ringing signal. (Indicates an incoming
Call.
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Even though the standard specifies 20Kbps ( up to 15m or 50ft), lots of
practical systems use RS-232 for up to 1.5Mbps on short lines or with
Shielded Twisted Pair wiring (STP)
A special device called a Break-Out-Box (BOB) is used to monitor serial links
for troubleshooting purposes.(Show the picture of a BOB ).
We shall use a BOB in the Lab to monitor serial link.
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EIA- 423
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Up to 4000ft
100 Kbps
+,- 3.6 V to +,- 6V
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EIA- 422
Up to 4000ft
10 Mbps
+2V= logic “0”
-2V= logic “1”High Speed connector
Good noise immunity
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V.35 :
High speed Standard
Very Expensive and complex.
Not very commonly used.
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Go to site www.sangoma.com , click on Tutorials, then RS-232 and read
about RS-232 and V.35 Standards.
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Null MODEM:
A method to connect PC to a PC.
Fig below shows how to connect to PCs together using a Null Modem Cable.
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Text Book section 8.4.3 to 8.8.7 P 184 to P 209.
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