Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 153 of 265
‘Good’ activations are activations of the priority input that do not remain active
longer than the Monitor Time (PMT), each separated by a duration longer than the
Gap Time (PVG). The values for priority DFM reset (PDR) are as follows:
Zero means manual reset.
1 or 255 means Automatic Reset when input goes inactive.
Any other value specifies the N’th activation on which the fault will be cleared and a
priority demand accepted, e.g. the value of 16 should be used to clear the fault after
the 15
th
activation (UK default for automatic reset).
16.3 Precedence Levels
For isolated operation, the normal order of precedence for the differing levels of
priority (in descending order) shall be:
Emergency Vehicle Priority Mode
Hurry Call Mode
UTC Mode
Manual Mode
Manually Selected VA or FT Mode
CLF Mode
Bus Priority Mode
VA Mode
FT Mode
16.3.1 Servicing of Multiple Priority Requests
This section details how the facility normally handles requests from two or more Bus
Priority Units or two or more Emergency Vehicle Priority Units. The following
sections detail the interaction between a Bus Priority Unit and an Emergency
Vehicle Priority Unit.
If two or more priority demands are stored at the termination of a minimum running
period, the priority demands will be serviced in the normal cyclic order and not
necessarily in order of receipt. An exception may be made for LRVs (see section
16.7). Note that if one stage can service both requested priority phases, the
controller will move to that stage rather than service both phases one at a time. This
is the same movement strategy as used in VA.
The presence of an output from a priority vehicle detector will, while the priority
phase is running, hold the green signal. The cessation of the output will initiate a
priority extension period. If one priority unit is extending one phase, a priority
demand for another phase from another priority unit will be stored and serviced
when the proper extension or priority maximum period expires.
Each signal phase omitted or curtailed by either priority change will always be
serviced in the normal cyclic order following the priority changes. If the
compensation periods are required on a phase as a result of two separate priority
changes, these compensation periods will run concurrently.