Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 157 of 265
16.7.1 Serving Priority Demands in Order of Receipt
In general the controller will serve the Priority Demands cyclically. On receipt of one
or more ‘Priority Demands’ the controller will move to the next stage cyclically which
serves a demanded Priority Phase. If another stage further round the cycle can
serve this Priority Phase and another Priority Phase, the controller will move there
instead. This is the same movement strategy as used in VA.
However, when using Priority Mode with a LRT System, in deciding which Priority
Demand the controller will serve next, the controller needs to take into account that
one LRV approach may be used by 2 LRV’s which turn in different directions at the
intersection.
When a signal is received indicating that the LRV Request for Unit 0 is active, a
decision has to be made as to whether this Unit is one of a pair requesting ROW on
the same approach. If they are, they must be served in order of receipt relative to
each other. If they are not the same approach, they can be served cyclically. The
information specifying which LRV units have to be treated as pairs is entered at
configuration “associated priority unit”.
When the LRV Request for Unit 0 goes active, the state of the LRV request for Unit
1 is tested. If this is not active, Unit 0 will be enabled, such that when its First
Priority Delay timer has expired, it will set a flag readable by Special Conditioning.
This flag can be used during the 2nd delay period to inhibit moves other than to
specified stages if required. This is to enable immediate servicing of the Priority
Demand after the Second Priority Delay by avoiding the controller being caught in a
stage change.
After expiry of this second delay timer, the request gets passed to special
conditioning (‘B’) provided that Unit 0 request is enabled by the “first come first
served logic”, i.e. provided that a request for Unit 1 is not already present. If a
request for Unit 1 is already present, the request for Unit 0 is stopped until the
request for Unit 1 clears. If the request for Unit 0 clears before 1 clears, the request
for Unit 0 is ignored.
The Priority Inputs at ‘B’ are inputs to the special Conditioning. The outputs from the
Special Conditioning are shown at point ‘C’. The Special Conditioning outputs ‘C’
connect to the inputs of the Vehicle Priority Software. If the Priority signal does not
require conditioning, point ‘B’ can effectively be connected directly to point ‘C’
without passing through Special Conditioning.
For points ‘B’ and ‘C’ refer to Figure 29.
16.7.2 Extend All Red Option
It is also possible to use priority demand inputs to special conditioning (see ‘B’ on
Figure 29) to extend the all red period between stages should a priority phase
terminate with a priority demand input still active. This facility provides time to