Lecture Slides for the course Electronic Instrumentation-II delivered by me to Diploma in Electronics/Instrumentation Engineering.

1. BIE-601
Electronic Instrumentation-II
Unit-3 (Part-II)
Data Transmission & Telemetering
Mohd. Umar Rehman
umar.ee.amu@gmail.com
April 13, 2020
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2. Data Transmission System Revisited
A Data Transmission System (DTS) has three components:
(i) Transmitter (source)
(ii) Transmission Path (Channel/line)
(iii) Receiver (Sink)
In simple terms, a data transmission system can be drawn as follows:
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3. Serial Interface
To ensure an orderly flow of data between a DTE and a DCE, a
standard serial interface is used to interconnect them.
The serial interface coordinates the flow of data, control signals,
and timing information between the DTE and the DCE.
Before serial interfaces were standardized, every company that man-
ufactured data communications equipment used a different inter-
face configuration.
In 1962, the Electronics Industries Association (EIA), in an effort to
standardize interface equipment between data terminal equipment
and data communications equipment, agreed on a set of standards
called the RS-232 specifications (RS meaning “recommended stan-
dard”).
The official name of the RS-232 interface is Interface Between Data
Terminal Equipment and Data Communications Equipment Employing
Serial Binary Data Interchange.
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4. RS-232
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5. RS-232
The RS-232 interface standard was developed for the single pur-
pose of interfacing data terminal equipment (DTE) and data circuit
terminating equipment (DCE) employing serial binary data inter-
change.
In particular, RS-232 was developed for interfacing data terminals
to modems.
The RS-232 specifications identify the mechanical, electrical, func-
tional, and procedural descriptions for the interface between DTEs
and DCEs.
The RS-232 interface is designed for serial transmission up to
20 kbps over a maximum distance of 50 ft (∼ 15 m).
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6. Major Parts of RS-232
Mechanical characteristics of the interface
Electrical signal characteristics
Functional description of the interchange circuits.
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7. Mechanical Characteristics
The mechanical characteristic is concerned with the actual physical
connection of the DTE and DCE and involves specification of pin
assignments and genders of the connectors.
The RS-232 standard does not specify a connector type, but it is
customary to use either 25-pin D-type (DB-25) connector, which
can accommodate all 25 pins, listed in the standard.
In practice, a smaller number of pins are used; thus, as an alterna-
tive, a 9-pin D-type connector (DB-9) is often used.
The 9-pin version of the RS-232 interface is designed for transport-
ing asynchronous data between a DTE and a DCE or between two
DTEs
The 25-pin version is designed for transporting either synchronous
or asynchronous data between a DTE and a DCE.
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8. 9 Pin Configuration
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9. Electrical Signal Characteristics
The DTE and DCE must provide circuits that convert their internal
logic levels to RS-232-compatible values.
Voltage-leveling circuits convert the internal voltage levels from the
DTE and DCE to RS-232 values.
Voltages of −3 V to −25 V with respect to signal ground (pin 7) are
considered logic “1” (the marking condition)
Voltages of +3 V to +25 V are considered logic “0” (the spacing
condition).
The range of voltages between −3 V and +3 V is considered a tran-
sition region for which a signal state is not assigned.
Keep in mind that a high RS–232 voltage level actually represents a
logic 0 and a low RS-232 voltage level refers to a logic 1.
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10. Logic Levels of RS-232
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11. Functional Description of the Interchange circuits
RS-232 defines the function of the data, timing, and control signals
used at the interface of the DTE and DCE.
Functional characteristics specify the functions that are performed
by individual interchange circuits.
However, very few of the definitions are relevant to applications
for data communications for instrumentation and control.
Therefore, only a few circuit functions are defined with reference
to the DTE will be described as follows:
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12. Functional Description...Contd
Protective Ground (PG): This line ensures that the chassis of the DTE
and DCE are on the same potential.
Transmitted Data (TxD): Transmission line-signal originating from
the DTE propagates to DCE.
Received Data (RxD): Receiver line-signal originating from the DCE
propagates to DTE.
Request to Send (RTS): This signal is used to condition DCE for data
transmission. A signal originating from the DTE propagates to
DCE.
Clear to Send (CTS): This signal indicates that the DCE is ready to
receive and is the response to the asserted RTS signal. The signal is
originating from the DCE and propagates to DTE.
etc. A complete description can be found in the literature.
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13. Brief Working of RS-232
RS-232 works on the two-way communication that exchanges data
to one another. There are two devices connected to each other, DTE
& DCE which has the pins like TxD, RxD, and RTS & CTS.
Now, from DTE source, the RTS generates the request to send the
data. Then from the other side DCE, the CTS, clears the path for
receiving the data.
After clearing a path, it will give a signal to RTS of the DTE source
to send the signal. Then the bits are transmitted from DTE to DCE.
Now again from DCE source, the request can be generated by RTS
and CTS of DTE sources clears the path for receiving the data and
gives a signal to send the data.
This is the whole process through which data transmission takes
place.
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