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RTU-292 Operations Manual
4 RTU-292 Theory Of Operation
4.1 GENERAL
This section gives enough detail of the theory of operation of the RTU-292 audio and power
supply sections to allow them to be troubleshot in the field. The same level of detail is not
supplied for the DSP module and the CPU portion as they are considered impractical to field
troubleshoot because of the specialized knowledge and test equipment required. If a fault is
suspected in these sections, factory repair is required.
4.2 FRONT PANEL BUTTONS AND INDICATORS
Figure 8-1 is a schematic of the RTU-292 front panel. The control pushbuttons and associated
LEDs are arranged in three separate PC board assemblies: the Tel Line Switch Assembly, the
Handset Switch Assembly, and the Speaker Switch Assembly. Signals from each of these
assemblies are combined into one ribbon cable that plugs into the Main Board at J3. Each
control pushbutton switches +5V onto its control line when ON and open circuit when OFF.
Pulldown resistors on each of the control inputs pull the line to ground when its button is OFF.
The buttons are read by the processor on the Main Board. The processor, not the pushbuttons,
controls the unit’s audio gates as well as the front panel LEDs.
4.3 MAIN BOARD
Main Board Circuitry is detailed in Figure 8-2.
4.3.1 AUDIO BUS ARRANGEMENT
The audio switching is bus oriented. Each major bus is named after the output signal that it
carries for distribution with the major buses as follows:
Signal
Name
Source Comment
TEL U8C-11 Telephone Receive Signal.
MIC U15B-7 Amplified/AGC’d Microphone Signal.
RXA U22D-6 Amplified Radio Receiver Signal.
RING U33A-2 Telephone Ringer Signal.
DTMF J13-32 DTMF Dialer Output.
The bus signals are each gain-leveled to a nominal 0 dBm (0.775V rms) average amplitude,
except for the hybrid in/out signals, which are usually at a –12 dBm (0.194V rms). The bus
signals are routed to various destinations through analog gates controlled by signals from the
CPU on the Main Board.
JPS Communications, Inc.
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