User's Manual

1.3.2.10 Local Control Unit Simplified Theory
Refer to Figure 1-16. The Local Control Unit (LCU) controls the normal operation of the DME/TACAN.
All operational functions are performed by the LCU and are controlled by either front panel keyboard when
in the local mode or by the Remote Maintenance Subsystem (RMS) through the parallel interface. The
LCU is controlled by the Portable Maintenance Data Terminal (PMDT), Remote Control Status Unit
(RCSU) or Remote Status Unit (RSU) through the RMS for all remote operator intervention or by the RMS
for automatic restart. The LCU receives the alarm outputs from the installed Monitor circuit card(s) and
depending on the configuration of the system, uses the results of these signals to determine alarm status. If
an alarm is detected, the LCU shuts down the system currently radiating and transfers to the standby system
in accordance with the system configuration. The LCU also provides the ability to disable monitors and
bypass alarms as required by the operator.
The LCU provides +24V to the two possible synthesizer VCO circuits, the +24V to activate the transfer
switch, and the transmit enable clock to the installed RTCs.
Status is reported by LED's visible to operator standing in front of the DME/TACAN, or through the RMS
to the PMDT/RCSU/RSU. An alarm shutdown is reported to the RMS as well as indicated by an audible
alarm.
Figure 1-16 LCU Simplified Block Diagram
1.3.2.11 Local Control Unit Block Diagram Theory
Refer to Figure 1-17.
1.3.2.11.1.1 DC to DC Converter
The LCU receives +48V from the two independent system power supplies and diode OR's the two sources
to provide input power to a DC to DC converter which supplies all required voltages for the LCU.
1.3.2.11.1.2 Power Fail Detectors
Each of the two independent +48V sources is monitored by a voltage comparator to monitor the health and
availability of power from each of the sources. These signals are used to determine voting logic for the
alarm registers and are reported back to the RMS via the parallel interface.