Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 233 of 265
In addition, there is a hardware watchdog that monitors the state of the Main
Processor and will also shutdown the controller if a problem is detected.
The requested red, amber and green states for all 32 phases are sent by the Main
Processor every mains cycle to the Phase Bus Processor and written out to the
Lamp Switch cards. The writing of data to the Lamp Switch cards is synchronised to
the mains cycle.
The red, amber and green states for all 32 phases are read from both sets of
monitors on the Lamp Switch cards and written to the Dual Port RAM every mains
cycle, for use by the Main Processor.
The Main Processor periodically checks that the secondary processor is updating
the monitored lamp states in the Dual Port RAM correctly, by performing a monitor
validation test, see section 33.4.
Both processors check the states of the greens (and reds and ambers if faults on
those colours are also configured to shutdown the controller). If the data from the
voltage monitors indicate a conflict or non-correspondence for a configurable time
(normally 300ms), the controller is shut down.