(3)  
The SELCAL decoder has the capability of responding to any combination of two tone pairs authorized for SELCAL. The 16-bit binary word, determined by the coding receptacle, prescribes which one of the combinations a particular airplane will respond to.

 B.  AIRPLANES WITH NA134-D2 DECODER; Four selector knobs on the front panel of each decoder are used to set in the airplane's four letter SELCAL code. SELCAL-1 decoder is on the left, SELCAL-2 decoder is on the right. Each selector knob has twelve positions.
 C.  AIRPLANES WITH N1401 DECODER; Four thumbwheel switches on the front panel of the decoder are used to set in the airplane's four letter SELCAL code. Each switch has twelve positions.
 D.  AIRPLANES WITH NA134-D2 DECODER; The SELCAL decoder is located on shelf No. 2 of electronic equipment rack E2.
 E.  AIRPLANES WITH N1401 DECODER; The SELCAL decoder is located on shelf No. 3 of electronic equipment rack E1.
 F.  The SELCAL decoder module is mounted on shelf No. 2 of electrical equipment rack E-1.
 5.  Operation (Fig. 2)
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A 737-300/400/500MAINTENANCE MANUAL

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SELCAL System Schematic Figure 2 (Sheet 1)
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A 737-300/400/500MAINTENANCE MANUAL

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737-300/400/500MAINTENANCE MANUAL
AIRPLANES WITH A NA134-D2 DECODER; All airplanes equipped with SELCAL are assigned a four letter SELCAL code by the FAA (or the equivalent), each being specific to a particular airplane. This code is programmed into both SELCAL decoders by means of the twin sets of four selector knobs on the decoder unit front panel. (The programming determines the four audio tones to which the vibrator reeds will respond). When the ground station transmits an airplane's four specific tones, (superimposed upon the carrier wave), the vibrator reeds in the SELCAL decoder will respond and activate the electronic chime and SELCAL alert light.The SELCAL audio output from each of the three VHF transceivers and from each of the two HF transceivers is inputted to a specific channel of the SELCAL decoder: VHF-1 to channel 1, VHF-2 to channel 2, VHF-3 to channel 3, HF-1 to channel 4, and channel 5 is not used. When any channel decodes a valid input, the corresponding call light "C" will turn on, on all the MIC SELECTOR switches for that radio on all audio selector panels. Any valid input will activate the chime.AIRPLANES WITH NA134-D2 DECODER; The received audio signal is routed to the SELCAL decoder. There the signal is amplified, limited and applied to the vibrator reeds. If the tones of the incoming audio match those of the reeds, the reeds will vibrate and actuate the vibrasponder circuits which will energize the decoder relay. The decoder relay closes and is held closed by its hold contacts. The other two contacts on the decoder relay, provide 28v dc to turn on the SELCAL alert light and apply power to the oscillator relay coil. Ground for the oscillator relay coil is through an RC oscillator network which causes the relay to energize and de-energize at regular intervals. The electronic chime, energized through the relay contacts, will sound repeatedly until deactivated.AIRPLANES WITH N1401 DECODER; Each of the four code letters, which are set by the four thumbwheels controls a single demultiplexer which channels the filter output corresponding to that code letter into an integrator. When all four integrators receive inputs, a correct SELCAL code has been received and the latch is activated. The latch causes the SELCAL indicator relay to close, which turns on the SELCAL indicator. The latch also starts the period timer which causes the chime control relay to open and close until the decoder is reset.The received audio signal from the transceiver is routed to one of five input channels in the decoder. The signal goes through a compressor amplifier which ensures a constant amplitude output, a filter which removes frequencies higher than the sampling frequency, and an analog to digital converter. The microprocessor selects the channel to be sampled by the converter and performs digital filter computations. If one of the four code frequencies assigned to the airplane is detected, the computer looks for the next assigned code until all four code frequencies are detected. Then the computer closes an electronic switch which turns on the correct indicator on the control panel and sends a repeating signal to the chime in the aural warning unit.
　
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