USSI Microlok II Functional description
Page 16 of 27 July 2005 UM-6800A Rev1.3
only criteria for selecting the Maximum OFF thresholds (the levels below
which an input must read OFF) are that they must be below the Minimum
ON threshold, yet high enough above 0V to reject induced noise. The system
will have the ability to access Vital Inputs even when the system is running
with the Vital Outputs in an un-powered state. Since the most restrictive
state for the inputs has already been defined as the de-energized state, failed
inputs can safely be set to this state. This allows the system to react to the
failed input without causing a Critical Error. This may cause the system to
run in a downgraded state due to the OFF input state, but will cause no
safety-related failures. The failed input status is available to the Application
Logic such that the Application Developer may take other action as required
by a particular system in the case of a failed input.
•
The minimum ON threshold for inputs will be 19.6V DC for 24V DC
inputs.
•
The maximum OFF threshold for inputs will be 9.0V DC for 24V DC
inputs.
Control of inputs
A principle similar to that used for the output circuitry is employed in the
input interface circuitry. De-energizing of any input results in a more
restrictive condition. All individual input interface circuits on a given board
are forced to the more restrictive state through the closed loop vital input
monitors. The inputs are then read and verified that they can, in fact, be
forced to the more restrictive state. This is ensure that no interface circuit
malfunction have occurred that could place an input in a less restrictive
state. See the figure-1.
4.1.7. VITAL CUT-OFF RELAY
The vital cut-off relay (VCOR) contacts are used to control the power to all
card file vital outputs. The VCOR is controlled by the CPU board. The CPU
board performs continuous diagnostics, to include monitoring of all vital
output and input channels at the point of interface with external circuits.
This CPU responds to failure of a safety-critical diagnostic by commanding
the CPS on the power supply board to remove the DC supply to the VCOR
coil, thereby opening the contacts that provide battery power to the vital
output boards. This fail-safe function defaults the interlocking equipment
associated with the Microlok II system to the most restrictive state. PN-150B
vital biased relay is used for the VCOR. This relay consisting of low voltage
silver-to-silver fronts and silver-to-silver backs contacts.
4.1.8. ADDRESS SELECT PCB
The address select jumpers are used for board addressing. The jumper
settings are automatically determined by the compiler after the successful