Nortel Networks RTU-292 Front Panel Buttons and Indicators, Main Board, Audio BUS Arrangement

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.

4-19 JPS Communications, Inc.