SCATS Controller Technology
SCATS local controllers can operate in any of several modes. These modes can be invoked manually or automatically by the regional computer or at the local controller:
This is the real-time adaptive mode. In Masterlink mode the regional computer determines the phase sequence, the maximum phase duration, and the duration of the walk displays. The local controller may terminate any phase under the control of the local vehicle actuation timers or skip an undemanded phase, unless prohibited by instructions from the regional computer.
The regional computer controls the phase transition points in the local controller, but subject to the local controller safety interval times being satisfied (e.g. minimum green, pedestrian clearance). On completion of the transition to a new phase, the local controller times the minimum green and minimum walk intervals, and then waits for a phase termination command from the regional computer. On receipt of the command to move to the next phase, the local controller then independently times the necessary clearance intervals (e.g. yellow, all red) for the phase termination. These safety settings prevent communications errors or regional computer faults from causing the local controller to produce dangerous signal displays, such as short greens or all-red periods.
The termination of pedestrian walk signals is also under the control of the regional computer so as to allow the walk timing to be varied to match prevailing traffic conditions. As for the other settings, however, the duration of the walk signal cannot be less than the minimum time programmed into the local controller.
In the event of failure of a regional computer or loss of communications, the local controllers can revert to a form of time-based coordination known as Flexilink. In this mode, adjacent signals are synchronised by the power mains frequency or an accurate crystal controlled clock. The phase sequence and duration of each, and the duration of walk displays are determined by the current plan according to the time of day. Local vehicle actuation facilities are still operational in this mode.
The local controller may terminate any phase under the control of the local vehicle actuation timers or skip an undemanded phase, unless prohibited by instruction within the plan. Flexilink is the usual fallback mode of operation.
Signals may also operate in Isolated mode, with local vehicle actuation (by detector loops) being the sole operating strategy. In Isolated mode the sequence and the maximum duration of each phase is as specified in the local controller time settings. The local controller may terminate any phase under the control of the local vehicle actuation timers or skip an undemanded phase, unless prohibited by the local controller settings. Isolated mode may be specified as the fallback mode of operation.
The local controller invokes a pre-programmed mode usually associated with an emergency phase or local pre-emption such as a train or tram phase.
The lamp state at the local controller has been turned off using a facility key to actuate a special switch provided on the controller housing.
All lamps at the intersection have been turned to red using a facility key to actuate a special switch provided on the controller housing.
The phases at the local controller are being manually introduced using a facility key to actuate a special switch provided on the controller housing.
A technician is on-site service the controller.
The normal signal display is replaced by flashing yellow displays on all approaches, or flashing yellow and flashing red to competing approaches. Provided communications are functional, signal operation can still be centrally monitored in Flexilink, Isolated and Flashing modes. Any of the Masterlink, Flexilink, Isolated and Flashing Yellow modes may be applied by an operator using a SCATS workstation, or be programmed by time of day. Flashing Yellow is also the fall back mode if the controller has a fault.