(b)
Receiver The TCAS 1090 MHz receiver sensitivity level is such that it has the ability to receive broadcast messages (squitters) from an intruder Mode S transponder. Data contained in these squitters allow passive tracking of aircraft beyond 100 NM with the introduction of a software upgrade option. This requires that the intruder aircraft be equipped with an ADS-B capable transponder and a GNSS receiver.
(3)
Transmitter module assembly (A3) The transmitter is a solid-state circuit that develops 1030 MHz outputs. The TCAS transmitter/receiver has a transmit/listen cycle of approximately 1s intervals. A series of increasing power mode C interrogations is first transmitted. This is the whisper-shout sequence (4-pulse group repeated with increasing power to 20 dB at 1 ms intervals). Immediately following is a Mode S interrogation modulated in DPSK ( Differential Phase-Shift Keying) format. The remainder of the 1 s-cycle is spent in listen mode.
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TCAS Computer - Block Diagram
Figure 006C
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(4)
Video processor The video processor contains circuitry needed to perform the following functions:
-analog-to-digital conversion of the sine, cosine and log sum inputs
-quantization of the log video inputs by comparing the log video against a threshold level to reduce the effect of multi-path replies
-decoding of ATCRBS Mode A and Mode C replies and elimination of phantom replies
-Mode S and address decoding and error detection/correction
-storing of real time bearing information versus range during ATCRBS
or Mode S interrogations. The video processor uses the video memory module to temporarily store the following information:
-squitter bearing and data
-interrogation bearing and data
-range
-reply edge data for range measurement.
The video controller generates outputs as required for control of the
transmitter, antenna switches and receiver.
(5)
Data processor The data processor module processes the data input from the video processor and input/output circuits. It also operates the links between the associated transponders and supplies the connector, reserved for the data recorder, mounted on the computer's front panel, via the interface circuits. The data processor module provides the computational capabilities required for processing the TCAS functions. It is based on an Intel 80386 microprocessor operated at 32 MHz. It is optimized for numeric processing and associated with an 80387 numeric data coprocessor. Program memory, which contains the preprogrammed system operating instructions is contained in EPROMs (programmable read only memory erasable by ultra-violet rays ) on memory module I. Random access memory (RAM) on memory modules I and II provides temporary storage for data that is acquired or used during system operation. Input/output circuitry permits direct communication with the transponders via an ARINC 429 high speed bus interface. It also includes a discrete I/O interface for a data recorder, a port for connection to Automatic Test Equipment (ATE) and an RS-232 interface for use with a simulator.
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(6)
Inputs/Outputs (I/O) Managed by an Intel 80C186 microprocessor whose program is contained in a Read Only Memory (ROM), the I/O module provides the main interface between the data processor and external hardware. It acquires most of the aircraft data used by the TCAS and transmits control and information signals to the other equipment. This module's activity is under control of the data processor who exchanges with it specific messages via a storage serving as a mailbox in both directions. The data is composed of discrete inputs/outputs, program pins and ARINC 429 data except those of the transponder, acquired directly by the data processor. This module also contains the synthesized voice message generation and possess the capability to process analog signals for non-digital aircraft. A validity check is performed on ARINC 429 input data. The synthesized voice generator transmits messages from a ROM containing all the information required to generate advisories. Two audio output channels (low and high level) are available. The low level audio outputs are connected to the cockpit loud speakers through the AMU.
(7)
Data loader A plug is installed next to the computer for the loading of the operational program and I/O configuration data into the TCAS computer via 2 ARINC 429 low speed buses, by means of a data loader. Presently, this operation can only be done by BENDIX.
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