Description (AMM 23-28-00)

 

A- The SELCAL (selective calling) system allows a ground station, with tone transmitting equipment, to call a specific airplane. When the correct tone combination is received, the airplane SELCAL decoder unit signals the flight crew by a SELCAL indicator light and an electronic chime. The SELCAL system eliminates the necessity of continuous monitoring of the communication systems by the flight crew.

 

B- The SELCAL system consists of a SELCAL decoder unit and a control panel.

 

C- The component locations are shown on figure 1. The electronic chime is part of the aural warning system (AMM 31-26-00).

 

D- When the ground station operator wants to call a specific airplane using the SELCAL system, the four-tone code which has been assigned to the airplane is added to the audio transmitted from the ground station. This code is received by the VHF or HF communication system and sent to the SELCAL decoder unit. If the four-tone code received is identical to the code entered on the SELCAL decoder, the flight crew will be alerted by the SELCAL light and electronic chime.

 

E- The flight crew can reset the lights and chime by pushing the appropriate reset button on the SELCAL control panel.

 

F- AIRPLANES WITH DUAL SELCAL DECODER UNIT;

SELCAL system No. 1 obtains 28v dc from switched electronic power bus No. 1 on load control center P18. SELCAL system No. 2 obtains 28v dc from switched electronic power bus No. 2 on load control center P6.

 

G- AIRPLANES WITH FIVE CHANNEL DECODER UNIT;

The SELCAL system obtains 28v dc from electronic power bus No. 1 on load control center P18.

 

SELCAL Decoder

 

A- SELCAL DECODER NA134-D2 DESCRIPTION (DUAL TYPE DECODER)

(1) Two separate decoders, in a common case, make up the dual SELCAL decoder unit. The decoders receive the audio input from the VHF or HF transceivers (Refer to WDM 23-22-01 for specific connections). Each decoder consists of an audio amplifier, a resonant reed circuit, a dc amplifier and control relays.

 

(2) 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.

 

B- SELCAL DECODER N1401 DESCRIPTION (DUAL TYPE)

(1) Two separate decoders, in a common case, make up the dual SELCAL decoder unit. The decoders receive the audio input from the VHF or HF transceivers (Refer to WDM 23-22-01 for specific connections). Each decoder consists of integrators, an active filter network, compression amplifiers, demultiplexers and latches.

 

(2) 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.

 

C- SELCAL DECODER DESCRIPTION (FIVE CHANNEL TYPE)

(1) The SELCAL decoder is a five-channel multiplexed unit. The communication transceivers VHF, HF or ACARS provide the audio inputs to the channels (Refer to WDM 23-22-01 for specific connections). Each channel has compressor amplifiers, a selector, filters and latching circuits.

 

(2) Every channel will recognize the unique four-tone code assigned to the airplane. The assigned code is pin programmable and determined by airplane wiring to SELCAL decoder.

 

(3) The four tone code is resolved with a 16-bit binary word. Four bits of the word determine each tone. A coding receptacle is wired to implement the unique 16 bits of this code for the airplane. This receptacle is mounted external to the SELCAL decoder and connected to it with a short cable. Thus the SELCAL decoder can be replaced without affecting the code.

 

The coding receptacle is placed on the rack behind the decoder. The cable has provisions for seventeenwires. The first wire provides a common ground and is always installed. The other wires are installed at each position where a low bit (0) is required and deleted where a high bit (1) is required.

 

D- AIRPLANES WITH SELCAL DECODER INSTALLED ON E1-3;

The SELCAL decoder unit is mounted on shelf No. 3 of electrical equipment rack E-1.

 

E- AIRPLANES WITH SELCAL DECODER INSTALLED ON E2-2;

The SELCAL decoder unit is mounted on shelf No. 2 of electrical equipment rack E-2.