Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 161 of 265
request, e.g. Fire station, or it may be used as a confirm signal for a UTC
system.
17.4 Hurry Call Parallel Stage Streaming Facilities
Eight Hurry Call units are available, numbered 0 to 7. Each is assigned to one stage
and thus only affects the stream in which the stage resides.
If 2 or more hurry call units are allocated to different stages but in the same stream,
they will be actioned on the basis of hurry call unit 0 being the highest priority and
unit 7 being the lowest, see section 17.5 below.
However, if they are allocated to stages in different streams, they will be actioned
simultaneously, neither having precedence over the other nor affecting each other in
any way.
17.5 Priority of One Hurry Call Over Another
There are eight hurry call units available 0 to 7 that can be allocated over the eight
streams.
If two or more hurry calls are allocated to stages in the same stream, they are
actioned on the basis that hurry call unit 0 has the highest priority and hurry call unit
7 has the lowest. If they are allocated to stages in different streams, see section
17.4 above.
If a request for lower priority hurry call occurs while the delay period of a higher
priority hurry call is timing, the request for the lower priority hurry call is rejected (it is
not latched in by the controller).
If a request for higher priority hurry call occurs while the delay or hold period of a
lower priority hurry call is timing, the request for the lower priority hurry call is
interrupted and the controller moves to serve the higher priority hurry call.
After serving the higher priority hurry call, the controller will return to the stage
requested by the lower priority hurry call. However, if the lower priority hurry call
was interrupted during its hold period, the hold period will have continued to time
whilst the higher priority hurry call was being serviced. If it has expired by the time
the controller returns to the stage requested by the lower priority hurry call, no hold
will be applied. Thus, the controller may only stay in the stage for the duration of the
minimum greens for the phases in the stage.
Special Conditioning may be used to further enhance this priority of Hurry Calls. For
example, Special Conditioning may be written to prevent the lower priority hurry call
input whilst the higher priority hurry call prevent timer is active. This may then be
used to ensure a gap between hurry calls so that a controller is not unduly held up.