Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 256 of 265
39.2 Normal Controller Operation
Normally the firmware can only decide to make a stage change while the controller
is steady in a stage and cannot make another stage change while one is already in
progress on the same stream. This restricts the operation of the junction as follows:
When a gap appears on phase B while phase A is still busy the controller remains in
stage 1. When a gap then appears on phase A, the controller will make the move
from stage 1 to stage 3. In other words, if a gap appears on phase B before one
appears on phase A, the controller would move directly to stage 3.
When a gap appears on phase A while phase B is still busy, the controller would
begin the move from stage 1 to stage 2. In other words, if a gap appears on phase A
before one appears on phase B, the controller would move to stage 2.
However, if a gap appears on
phase B a few seconds after a gap
appears on phase A, the controller
cannot make another stage move
until it has reached stage 2. But
stage 2 is not reached until phase
C appears which, in this case, is
seven seconds after phase A
terminates due to the inter-green
between the phases.
Therefore the controller is forced to leave phase B at green until stage 2 is reached,
which effectively delays the appearance of phase D by up to those seven seconds.
39.3 Ripple Change Facility
Ripple change facility allows the controller to ‘change its mind’, terminate additional
phases and start moving to a new stage, provided that the phases that are about to
appear at green are still present in the new stage.
In the above example, this would allow the controller to make the decision to move
to stage 3 while it is still making the move from stage 1 to stage 2, since phase C is
present in both stage 2 and stage 3.
In the example to the right, the
controller starts a normal stage
change from 1 to 2, terminating
phase A allowing it to bring on
phase C seven seconds later.
However, during the move from
stage 1 to stage 2, and before
phase C actually appears, the
Stage 3Stage 1
IGN B D:7
IGN A C:7
A
D
C
B
1-2 2-32
Stage 3Stage 1
IGN B D:7
IGN A C:7
A
D
C
B
1-2 2-3