Lenz Digital Plus LR101 Mode D'emploi page 17

However, for fully or partially automated operation feedback information is
indispensable. How else can a computer programme used to control a model
railway system 'see' whether a certain track is occupied or not? Feedback
information is necessary for knowing whether a train may or may not enter the
track section in question.
4 General information on the occupancy detection function for model
railway systems
Normally, the occupancy status of tracks is determined by electronically
measuring the current flowing to a track section. Each power consumer located
on the track creates a current flow. 'Current flow' is therefore synonymous with
'occupied' and 'no current flow' synonymous with 'not occupied'. Thus, from a
technical point of view, an occupancy detector is a current sensor.
If, however, the voltage is interrupted (which, in digital operation, is normally
constantly applied), so is the current flow – and occupancy detection fails.
Interruptions of this kind may be caused when the voltage is switched off or short
circuits occur. In this case, the signal output will report unoccupied tracks that are,
in fact, occupied.
Occupancy detectors that bridge a voltage interruption by means of auxiliary
voltage are unsuitable for digital operation since auxiliary voltage may be
misinterpreted by locomotive decoders. Also, such detectors require a separate
power supply which, in turn, means higher costs and extra wiring.
4.1 The exceptional ingenuity of the Digital plus feedback system
The Digital plus feedback system is a clever combination of 3 components:
The feedback module LR101, the occupancy detector (or current sensor)
LB100/LB101, and the voltage detector LB050 (which monitors the track voltage).
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