NEC Univerge SV8100 System Hardware Manual page 227

UNIVERGE SV8100
System Hardware Manual
The switches 6 and 7 define if the terminators are active. These
should be set to ON, if the bus has no termination. If the bus is
already terminated they may be set to OFF.
Every ISDN-Bus must be terminated with a set of 100Ohm resistors
at its ends. Normally, NT's provide one set as they are assumed to
be at one end of the bus. At the other end of the bus, either the last
socket contains terminators or the attached device provides the
terminators.
The purpose of these terminators is not only to simulate an infinite
line which effectively sinks the signal in that direction. Their main
purpose on ISDN bus lines is to sink the 0's. 1's and 0's are different
in that way that 0's are actively sent to the line, and 1's are just high-
ohmic drivers. So to get rid of the 0's, there must be a load resistor to
discharge the line capacity. If there are no, or too few, terminators,
this does not work.
Issue 4.1
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