Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 259 of 265
At the start of the first stage change in which a phase can gain right of way in a
series of ripple changes, the phase delay is started on that phase if one has been
configured on that stage movement. After the delay and any inter-greens have
expired, the phase is ready to appear. If the phase is demand-dependant, the
appearance will be delayed until a demand is actually registered. Phase delays on
subsequent movements for this phase will be ignored until a full stage is reached
and the ripple change is finished. However, if the controller ripple changes to a
stage in which the phase can no longer appear, the process is repeated if the
controller then ripple changes to another stage in which the phase can appear
again.
39.5.4 Stage Restrictions and All-Red Extensions
The controller will action any stage restrictions (ignores, prevents and alternatives)
specified for the move from stage 2 to stage 3 as normal during a ripple change. For
example, if the move from stage 2 to stage 3 is configured as an ignore move, the
controller will attempt to find another stage to ripple to rather than stage 3.
However, if there is a stage restriction specified for the move from stage 1 to stage
3, the ripple change will be prevented and the controller will wait until stage 2 is
reached before moving to stage 3. This is because the ripple change from stage 1 to
stage 2 to stage 3 may occur shortly after the move from stage 1 to stage 2 started,
and the street will in effect see the move from stage 1 directly to stage 3. Since that
move was restricted in the configuration, the controller will not therefore allow the
ripple change.
If an all-red extension unit is configured on the moves from stage 1 to stage 2, or
from stage 2 to stage 3 or from stage 1 to stage 3, the ripple change will be
prevented. The controller will perform the two stage changes in turn, taking any all-
red extensions into account on each move independently.
The controller will always check for any stage restrictions or all-red extension units
configured from any stage visited during the ripple change to the suggested stage.
For example, if the controller also attempts to ripple change to stage 4 after ripple
changing from “1 to 2” to “2 to 3”, it will check all the stage moves between these
stages. Therefore, the controller will also check for a stage restriction or all-red unit
configured for the move “1 to 4” and prevent the move to stage 4 until stage 3
appears if one is found.
Note that the ripple change from “1 to 2” to “2 to 3” would have already checked the
moves “1 to 2”, “2 to 3” and “1 to 3”. And the original decision to ripple change from
“2 to 3” to “3 to 4” would have checked the moves “2 to 3”, “3 to 4” and “2 to 4”.
39.5.5 SDE/SA Facility
The SDE/SA facility will continue to function along side ripple change, with any
clearance requests from the new stage change introducing the required delays to
the inter-greens and gaining phase delays. This is true as long as the ‘gaining