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• Create a flagman.
• Specify the left contact in the “on” state as the trigger of the flagman.
• Specify the right contact in the “off” state as the condition of the flagman.
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Flagman
Trigger Condition
Left Contact Right Contact
Table 7: Detecting Train Direction
If the left sensor is passed by a train coming from the left, then this event is reported to
the flagman by the trigger. The flagman then checks its condition and detects that the
right contact is turned off. Since the condition applies, the flagman is turned on as required.
If the right sensor is passed by a train coming from the right, then nothing happens because
the right contact is not part of the trigger. If the train passes the left contact a few
moments later, then this event is again reported to the flagman by the trigger. The flagman
again checks its condition and detects that the right contact is still turned on. Since
the condition does not apply, the flagman is not turned on.
By assigning operations to the flagman, it is possible to operate other elements depending
on the direction of passing trains.
Example: Detecting uncoupled Cars
The following example demonstrates how inadvertently uncoupled cars can be detected.
This mechanism is useful at the entry to hidden yards.
For this mechanism, a track occupancy detector and two additional flagman indicators
are needed. In the following, these flagmen are called “timer” and “alarm”.
Trigger
Track Occupancy Detector
Timer Alarm
Condition