MODEM COMMUNICATION TO ABB RELAYS
Page 3 of 47
Figure 2 illustrates the possible combinations of data, which may be represented by two bits. Four
possible symbols may be transmitted/received using this method (as was the case with QPSK methods).
If, for example a sine wave is split into four quadrants each part of the phase could represent each of the
two bit combinations in an analog fashion. Thus the phase from 0 – 90 degrees could represent the value
00, 90-180 degrees could represent the value 01, 180-270 degrees could represent the value 10, and
logically 270 – 360 could represent the value 11. A rapid A/D and D/A converter could determine the
phase of the conversion area and determine the value depending upon the amplitude of the signal being
converted. Thus, four symbols could be transferred in a single phase.
TWO BIT REPRESENTATION
0 0
0 1
1 0
1 1
4 Bit Combinations
00 01
01 11
0 90 90 180
180 270 270 360
DEGREES
0 90 180 270 360
WAVEFORM BIT MAP ASSIGNMENT
VERSUS FREQUENCY
Figure 2. QAM Analysis 4 Bit Analysis
Expanding this concept, Figure 3 illustrates what could occur if a 16 symbols could be transferred using
an extended sine wave interpretation. The proper designation for this encoding is 16-QAM. Thus 16 is
the number of symbols which may be expressed in one waveform. Each ¼ cycle could represent a
quadrature 00 –01- 10- 11. Each ¼ cycle could then be designated to two bit values depending upon the
phase angle location upon the cycle. QAM modem manufacturers have a quadrature plot illustrating the
phase/bit encoding which occurs in their design. This technology allows modems to transfer data at rates
of 33,600 bits per second over telephone lines designed to carry voice at 4000 hz. This is pretty
impressive in that the average cost of a 10 bit synchronous modem capable of operating at 56K bits per
second (theoretically) is $100.
FOUR BIT REPRESENTATION
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
16 Bit Combinations
0 90 90 180
180 270 270 360
DEGREES
0 90 180 270 360
WAVEFORM BIT MAP ASSIGNMENT
VERSUS FREQUENCY
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 0 0
1 1 0 1
1 1 1 0
1 1 1 1
00
01
1 0
11
1 1
10
0 1
00
00 01
QUAD QUAD
10 QUAD 11 QUAD
Figure 3. QAM – 16 Bit Encoding
Summary of Contents for REL 356
Page 23: ...ABB REL 356 Current Differential Protection 1 10 Product Overview and Specifications ...
Page 83: ...ABB REL 356 Current Differential Protection 3 36 Settings and Application ...
Page 127: ...ABB REL 356 Current Differential Protection 5 28 Testing ...
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