Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 149 of 265
be used to generate a V.A. demand for the same phase such that if Priority mode is
unavailable for any reason the phase will still be serviced.
(N.B. Both priority demands and associated VA demands are latched. An Unlatched
facility may be achieved by use of special conditioning.)
If a Priority Demand is not required, it may be disabled so that only Priority
Extensions are created. This does mean that (although not Priority demanded),
when the associated Priority phase reaches green, the controller will switch to
Priority mode as Priority extensions are present and the Priority maximum timer will
time off as normal.
If Priority mode is disabled, the associated VA demands will still be entered and
serviced, and if the standard facility (which has latched Priority demands) is used,
the Priority demands will be stored up until Priority mode is available again.
16.2.5 Priority Revertive Demand (PRE)
When a Priority Phase terminates with a Priority Extension still active, a Revertive
Priority Demand is inserted for that Priority Phase, providing the Priority Demand
facility has not been disabled.
If the Revertive Priority Demand facility is not required, it may be disabled without
affecting the Priority Demand facility.
16.2.6 Priority Change
A Priority Change is a signal change that occurs as a result of a Priority Demand. As
always, the signal change is subject to minimum green and inter-green periods.
16.2.7 Allowed VA Demands (PSA)
When a Priority Demand is being actioned all VA demands are normally disregarded
until after Priority or Emergency Vehicle mode has finished.
However, it is possible to specify phases that are to appear cyclically before the
Priority phase (if VA demands are present).
The specified phases will only run for their minimum green period regardless of any
VA extensions present.
Note: This facility is not compatible with priority phases of type 2 appearance, i.e. if
the stage in which the priority phase appears is active when a priority demand is
received, the priority phase appears immediately. At the same time unwanted
“allowed” and “enforced VA demands” are entered, which may cause unnecessary
timing of priority maximum and unnecessary stage changes when the priority phase
terminates.