Transmission Mode

The direction of a signal between two devices is defined by a transmission mode.
Or
The transmission mode defines the direction of a signal between two devices.There are three types of transmission mode given as below;

Simplex Mode:
In the simplex mode the communication is unidirectional. Only one can send data and other can receive. No any other possibility is available. A remote is an example of simplex mode.

Half Duplex Mode:
In half duplex  mode both devices can send and receive data, but not at the same time. At a time one device send data and other receives. Walkie talkie is an example of half duplex mode.

Full Duplex Mode:
In full duplex mode both devices can send and receive data at the same time. It has two way traffic at the same time. The telephone network is an example of full duplex mode.

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Components of Data Communication

Data communication consists of five basic components.

1. Messsage

The message is the information or data to be communicated. i can be text, numbers, pictures, sound or video or combination of these.

2. Sender

The sender is the device that sends the message. It can be a computer, work station, camera etc..

3. Receiver

The receiver is the device that receives the message.It can be a computer, work station, television etc..

4. Medium

The transmission medium is the path by which message trevels from sender to receiver. It can be a wire, coaxial cale, fibre optics, laser or radio waves etc..

5. Protocol

A protocol is a set of rules that governs the data communication. It represents the agreement of communicating devices. Without a protocol devices can not communicate to each other.

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Millman's Theorem

Millman’s theorem provides a shortcut for finding the common voltage across any number of parallel branches with different voltage sources. A typical example is shown in fig. 10-16 For all the branches, the ends at point Y are connected to chassis ground. Furthermore, the opposite ends of all the branches are also connected to the point X. The voltage V/x y, therefore, is the common voltage across all the branches.











Finding the value of V/x y gives the net effect of the all source in determining the voltage at X with respect to chassis ground. To calculate this voltage
 v/x y=V1/R1 + V2/R2 + V3/R3 ……etc
1/R1 + 1/R2 +1/R3
This formula is derived from converting the voltage source to current sources and combining the results. The numerator with V/R terms is the sum of the parallel current sources. The denominator with 1/R terms is the sum of the parallel conductances. The net V/x y then is the form of I/G or 1*R, which is in units of voltage.
CALCULATING V/XY
For the values in Fig.10-16,
Vxy =32/4 + 0/2 – 8/4
1/4 + 1/2 + 1/4
=8 + 0 -2
1
Vxy=6 V
Note the branches 3, V3 is considered negative because it world make point X negative. However, all the resistances are positive. The positive answer for Vxy means that point X is positive with respect to Y.
In branch 2, V2 is zero because this branch has no voltage source however, R2 is still used in the denominator.
This method can be used for any number of branches, but they must all be in parallel, without any series resistance between the branches. In a branch with several resistance, they can be combined as one Rt. When a branch has more than one voltage source, they can be combined algebraically for one Vt. Read More!

Alternating Current Application

Figure 16-1 shows the out put from an voltage generator, with the reversals between positive and negative polarities and the variations in amplitude. In Fig. 16-1a, the wave from shown simulates an ac voltage as it would appear on the screen of an oscilloscope, which is an important test instrument for ac voltages. The oscilloscope shows a picture of any as voltage connected to its input terminals, while indicting the amplitude. The details of how to use the oscilloscope for ac voltage measurements are explained in App. D. “Using the oscilloscope”.
In Fig.16-1b the Read More!

The 555 Timer IC

The 555 Timer is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. The IC was designed and invented by Hans R. Camenzind. It was designed in 1970 and introduced in 1971 by Signetics (later acquired by Philips). The original name was the SE555/NE555 and was called "The IC Time Machine".

The 555 gets its name from the three 5-k Ohm resistors used in typical early implementations. It is still in wide use, thanks to its ease of use, low price and good stability. As of 2003[update], 1 billion units are manufactured every year.

The 555 timer is one of the most popular and versatile integrated circuits ever produced. It includes 23 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).

The 555 has three operating modes:

* Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, Frequency Divider,Capacitance Measurement, Pulse Width Modulation (PWM) etc

* Astable - Free Running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.

* Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.


The connection of the pins is as follows:

Nr. Name Purpose

1 GND Ground, low level (0V)
2 TR A short pulse high → low on the trigger starts the timer
3 Q During a timing interval, the output stays at +VCC
4 R A timing interval can be interrupted by applying a reset pulse to low (0V)
5 CV Control voltage allows access to the internal voltage divider (2/3 VCC)
6 THR The threshold at which the interval ends (it ends if U.thr → 2/3 VCC)
7 DIS Connected to a capacitor whose discharge time will influence the timing interval
8 V+, VCC The positive supply voltage which must be between 3 and 15 V


In the astable mode, the high time from each pulse is given by

high = 0.693.(R1 + R2).C

and the low time from each pulse is given by

low = 0.693.R2.C

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.


Specifications

These specifications apply to the NE555. Other 555 timers can have better specifications depending on the grade (military, medical, etc).

* Supply voltage (VCC) 4.5 to 15 V
* Supply current (VCC = +5 V) 3 to 6 mA
* Supply current (VCC = +15 V) 10 to 15 mA
* Output current (maximum) 200 mA
* Power dissipation 600 mW
* Operating temperature 0 to 70 °C Read More!