Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 10 Page 93 of 265
When the phase appears its timing periods will be considered for the stage duration.
Therefore, if the phase appears during the stage its minimum green period, and
possibly maximum green period, may affect the termination of the stage. Therefore
this type should only be used if essential. Appearance Type 3 should be used
unless the phase is a filter green arrow.
The normal use for this type of condition is a filter green arrow phase.
A demand for the filter green arrow should not be latched to ensure that a filter
green arrow demand does not exist without an associated main green demand.
The green arrow active should insert a demand for the main green phase.
6.6.4 Appearance Type 3
This is as Appearance Type 2 but the appearance of the phase after the stage has
commenced is inhibited when the ‘window period’ expires.
The window period is started when an opposing demand starts the maximum green
timer of a phase running in the stage, except in UTC mode when it commences at
the start of the stage. Any demand inserted after this will be stored and serviced
later but will be initially ignored in the same way as a type 1 phase.
The window period is normally the difference between the longest maximum green
period of the other phases in the stage and the minimum green period of the
conditional phase. This means that the phase will not appear if there is not enough
time left for its minimum green to run and so will not affect the stage termination.
Provision has been made within the software to provide 8 separate window timers,
one for each stream. This is necessary because up to 8 stages may be running
simultaneously, each demanding a different window time. Each window timer will run
the window time related to the stage within its associated stream.
If the alternative maximum green facility is used, this may affect the required window
time. There are two ways to overcome this. One is to have a compromise window
time that will suit all of the alternative maximum green periods. The other is to switch
the window times along with the alternative maximum green times.
There are two methods to achieve switched window times:
a) Alternate stages with the alternative window times are introduced and deleted
by the master time clock at the same time as the alternative maximum green
periods.
b) The conditional phase is programmed as an Appearance Type 2 and
conditioning timers are used as window timers to inhibit the appearance of
the phase. The conditioning timers are introduced and deleted by the master
time clock at the same time as the alternative maximum green periods.