Specifications
ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 232 of 265
This test is undertaken without the knowledge of the Signal Monitor so if the signal
remains active (say due to a fault) the Signal Monitor will detect correspondence
(and possibly conflict faults) and shut down the controller.
For the ST900 Controller, when the Monitor Validation signal is active, the hardware
on each Lamp Switch card inverts a number of the actual lamp states.
For the ST900 ELV Controller, when the Monitor Validation signal is active, the
primary voltage readings (V1) of all outputs are forced so that it appears that all the
outputs are ON. The indication from the secondary monitor (V2) for each output will
continue to function normally.
33.5 Lamp Supply Checks
If the lamp supply drops below two limits, one for dim and one for bright, the
controller will log a fault and extinguish the signals.
The ST900 Controller will attempt to flash them if so configured (see the Fail to Part-
Time State option) and the supply is not too low. Normally, and for all UK
controllers, the signals are extinguished when the lamp supply drops below the
configurable thresholds set by the handset commands LDT and LBT.
The ST900 ELV Controller will only extinguish and not attempt to flash the signals
since LED Signals provide a constant light output over their complete operating
range and may operate erratically outside of these bands. In addition, there are also
two upper limits, one for dim and one for bright. If the lamp supply is detected above
these limits, the controller will log a fault and extinguish the signals.
When the lamp supply recovers the fault will be automatically cleared and the
controller will follow its ‘Signals Off to Signals On’ lamp sequence.
The ‘switch off’ and ‘switch on’ confirm times are handset configurable on the ST900
Family of Controllers using the handset commands LSF and LSN.
33.6 Secondary Processor
The heart of the controller is the processor card. This board contains the Main
Processor that performs the main controller logic. It also has a Secondary Processor
that communicates with all the Lamp Switch cards through the ‘phase bus’ and
hence is also known as the ‘Phase Bus Processor’ and also provides a secondary
check of the signals and so is also known as the ‘Signal Monitor’.
In the ST900 Controller, the Secondary Processor is the 8051 microprocessor on
the Main Processor card itself. In the ST900 ELV Controller, the Secondary
Processor is the ARM7 microprocessor on the PHS daughter card.
Each processor monitors the state of the other and can independently shutdown the
controller if a problem is detected. This extinguishes all the signals or invokes the
hardware fail flash facility, see section 34.