Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 215 of 265
29.7 Uni-Directional (U/D) Facility
If only vehicles travelling in one direction over a detector are required to activate it,
whereas vehicles travelling in the opposite direction are not, the controller includes
the Uni-Directional (U/D) facility.
The facility uses two inputs that are connected to two inductive loops that partially
overlap on the carriageway, such that vehicles travelling in the required direction
activate the ‘A’ loop first, then the ‘U’ loop.
If the ‘A’ loop is activated first, the controller processes the ‘A’ input as normal.
However, if the ‘U’ loop is activated first, the ‘A’ input will be forced to remain
inactive by the controller (even if the ‘A’ loop input to the controller is actually
activated) until both inputs return inactive when the vehicle has passed.
If either loop remains active for longer than a specified time-out period, the ‘A’ input
is forced active so that the controller sees a demand, since a vehicle appears to be
stationary on one or both loops.
The handset commands associated with this facility are CUD and UDT; see the
Handset Handbook for further details.