VIRTUAL EXPERIMENTS
EXPERIMENT V4
USING LABVIEW TO LEARN THE CONCEPT OF TIME DIVISION MULTIPLEXING IN
VIRTUAL EXPERIMENTATION
OBJECTIVE: - To demonstrate the concept of Time Division Multiplexing (TDM) Using
Labview
LABVIEW VIRTUAL COMPONENTS USED IN BLOCK DIAGRAM:
1.
2.
3.
4.
5.
6.
7.
Signal simulators (3).
Case Structure Unit for randomization.
Switches.
Waveform chart displays
Selector function.
Dynamic data to Scalar data converter.
Digital gates
Preamble
Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal
by separating the signal into many segments, each having a very short duration. Each
individual data stream is reassembled at the receiving end based on the timing.
The circuit that combines signals at the source (transmitting) end of a communications link is
known as a multiplexer. It accepts the input from each individual end user, breaks each signal
into segments, and assigns the segments to the composite signal in a rotating, repeating
sequence. The composite signal thus contains data from multiple senders. At the other end of
the long-distance cable, the individual signals are separated out by means of a circuit called a
demultiplexer, and routed to the proper end users. A two-way communications circuit requires
a multiplexer/demultiplexer at each end of the long-distance, high-bandwidth cable.
Fig. 1. Principles of TDM
Fig. 2 shows another look at TDM operation.
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Figure 2—Time division multiplexing.
Sometimes, when multiplexing input lines with different transmission speeds, different time
slots are assigned to each channel. In fig. 3, we see time slots assigned in the ratio 8:16:24 or
1:2:3 where a, b, c indicate data from sources A, B and C.
Figure 3—Multiplexing input lines with different transmission speeds.
A general diagram showing the various actions need for the 4-channel TDM used in part 1 of
this experiment is shown in the block diagram of fig. 4
Fig. 4 TDM Circuit block schematic
In part 1 of this experiment, we shall use four signals from four channels and multiplex them
into one channel using four switches and a summer. Only one signal can get to the output of
the multiplexer at any time depending on which counter-controlled decoder output Y that is at
logic 1. The four signals we shall be using are sine, triangular, square and sawtooth waveforms
as shown in fig.5. For this part, equal time will be allocated to each channel.
2
Fig. 5 Clock and channel signal waveforms
Experiment 1Testing the 2-bit Counter and decoder.
Fig. 6
2-bit counter and decoder
Go to page 1 of the user interface front panel.
3
a. Set the J and K inputs of the flip flops to logic 0. Then toggle the manual clock switch
several times. Run the experiment. What happens to the counter and decoder
outputs and why?
b. Change the two J inputs of the counter to logic 1 and toggle the manual clock several
times. What happens to the outputs of the counter and decoder and why?
c. Change all the J and K inputs to logic ! i.e. flip flops are in toggle modes. The counter is
now set to work as a 2-bit counter. Apply 8 manual clock pulses. Fill table 1 below:
Table 1
CLOCK
COUNTER
DECODER
Q2
Y3
Q1
Y2
Y1
Y0
0
1
2
3
4
5
6
7
8
Experiment 2
Generating channel signals.
On page 2 of the experiment, set the amplitude and frequencies of 4 signals as follows;
Table 2
1
1
2
3
4
Q2
Clock
Sine wave
Triangular
Square
Sawtooth
Amplitude
1
1
1
1
1
Frequency
1 Hz
5 Hz
5 Hz
5 Hz
5 Hz
From the chart properties, use a strip chart mode and stack to get the output shown in fig. 5.
Use a time range from 0 to 1.5 seconds. Do not use autoscale.
Run the program.
How many signal cycles occur within 2 seconds?
Experiment 3
Demonstrating Time-Division Multiplexer Action using manual clock switch
Go to page 3 of the experiment
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Fig. 7 Clock and TDM system
To use the 2-bit Counter Manual Clock2 , set the Mode selector switch to OFF. Each
manual clocking will change the conter and decoder state sequencially when the program is
run. The signals shown have been internally connected to the switches.
Manually clock the counter and complete the X spaces on Table 3. The Status for counter state
0,0 is already shown.
Table 3
CLOCK
0
1
2
3
4
etc
COUNTER
DECODER
Q2
0
0
1
1
0
Y3
0
X
X
X
X
Q1
0
1
0
1
0
Y2
0
X
X
X
X
Y1
0
X
X
X
X
Y0
1
X
X
X
X
Switch
4
Switch
3
Switch
2
Switch
1
MULTIPLEXER
OUTPUT on
Display
OFF
X
X
X
X
OFF
X
X
X
X
OFF
X
X
X
X
ON
X
X
X
X
Signal 1
X
X
X
X
etc
Experiment 4
Demonstrating Time-Division Multiplexer Action using Signal Generator clock
1. On page 3 of the experiment, set the Mode Select Switch ON so as to clock counter
with the signal generator clock. Switch OFF the randomizer switch so that the signal
generator clock has a duty cycle of 50%. From the chart properties, use a strip chart
mode and stack to get the output shown. Use a time range from 0 to 1.5 seconds. Do
not use autoscale. Then run the experiment. From the chart displays,
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a. What did you notice at the TDM output as the clock changes states?
b. At what clock edge do the signal changes take place?
2. Now switch ON the randomizer switch so as to apply clock pulses with variable
widths and run the experiment again.
What did you notice at the output of the multiplexer?
Experiment 5
Using TDM with unequal selection period for channels
Go to page 4 of the experiment.
In this experiment, we will use three signal channels. However, we will sample them at the ratio
1:2:1 which means one of the channels with have twice the time allocated to the others.
This is shown in Fig. 8.
Fig. 8 TDM with unequal selection periods
Fig. 9 shows counter and multiplexer units.
Fig. 9 Counter and Multiplexer with unequal channel selection time.
For this, the table of operation of the decoder is as shown on Table 4.
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Table 4
CLOCK
0
1
2
3
4
5
6
7
etc
COUNTER
Q2
0
0
1
1
0
0
1
1
Q1
0
1
0
1
0
1
0
1
DECODER
Y2
0
0
0
1
0
0
0
1
Y1
0
1
1
0
0
1
1
0
Multiplexer Output
Y0
1
0
0
0
1
0
0
0
Set the Mode selector switch to OFF thereby enabling the use of the Manual clock.
Clock the counter and write down the signal at the TDM output in the spaces in Table 4.
Next, Set the Mode selector switch to ON thereby allowing signal generator clock.
a. What did you notice at the output of the multipexer?
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