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384
Short range galvanic module
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The short range galvanic modem is used for point to point synchronous data transmis-
sion at 64 kbit/s at distances up to 4 km. Transmission is performed simultaneously in
both directions, full duplex, over four wires in a communication (pilot wire) line ac-
cording to figure 207.
Twisted
pair cable
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Compared to normal data transmission standards, for example V.36, X.21 etc., the short
range modem increase the operational security and admits longer distances of transmis-
sion. This is achieved by a careful choice of transmission technology, modified M-3
balanced current loop and galvanic isolation between the transmission line and the in-
ternal logic of the protection terminal.
The reach will depend on the used cable. Higher capacitance between conductors and
higher resistance will reduce the reach. The use of screened cables will increase the ca-
pacitance and thereby shorten the reach but this will often be compensated by the re-
duced noise giving a better operational security. Maximum ranges as a function of cable
parameters are given in the diagram in figure 208.
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