Honeywell
COMPONENT MAINTENANCE MANUAL
PART NUMBER 964-0452
1I.B.1516A Page 58
Mar 30/01
23-12-01
(8) HF Modem, A10
HF Modem, A10, performs the necessary functions to implement both transmission and
reception of the audio waveform specified in ARINC 635. Digital message packets re-
ceived from the ADP, A2, are encoded, interleaved and scrambled to generate a stream
of binary 1’s and 0’s, all used to modulate the phase of a 1440Hz carrier. This transmit
audio is fed to a Digital-to-Analog converter via the processor, is amplified to the level
required by the Exciter, and sent through the transmit relay to the Receiver/Exciter, A1,
for downlink transmission. Audio messages received from the Receiver/Exciter, A1, are
digitized, demodulated, descrambled, decoded, and passed on to the ADP in digital
packet format. The communication path between the HF Modem and the ADP is an
RS-232, bidirectional, synchronous, serial bus, utilizing the HFDL protocol. Refer to Fig.
35 for a block diagram of the HF Modem CCA, and to Fig. 36 for the HF Modem connec-
tor pin assignments, showing the signals that interface with this assembly.
The HF Modem uses 2 Digital Signal Processors: a TMS320C30 and a TMS320C31.
Both of these processors run at a clock frequency of 33.1776MHz. The software that con-
trolls these processors is written in assembly language and resides in PROM devices.
The software is divided between the 2 processors into the "Master" (C30) and "Slave"
(C31) processor functions.
Master processor software performs all of the control functions of the HF Modem board,
including power-up self-tests, control of discretes, ADP, and HF Transceiver interfaces,
as well as all the signal processing that is required to modulate a digital message, and
the front-end signal processing that is required to demodulate an incoming audio mes-
sage. The Slave processor operates only during demodulation, and executes computa-
tionally intensive algorithms on blocks of data under the Master processor’s supervision.
The analog section of the HF Modem provides the interface to the Receiver/Exciter, A1.
The receive audio and the transmit audio are both passed through a relay circuit to place
or remove the Data Module from the audio inputs to the Receiver/Exciter, A1. When the
HF Transceiver is in internal data link mode, the relays connect the HF Modem audio cir-
cuitry directly to the data ports on the Receiver/Exciter, A1. When the HF Transceiver is
in voice mode or external data mode, the HF Modem audio circuitry is removed from the
Receiver/Exciter, A1, data ports. These relays can also be used to loop the HF Modem
output to the HF Modem input, allowing the HF Modem to perform a complete self-test of
all of it’s analog functions. After the receive audio passes through the relay, it is amplified,
placed through an Automatic
Gain Control (AGC) circuit, which is under the processor’s control, and then sent to the
Analog-to-Digital converter, which digitizes the audio signal at a sampling rate of 7200Hz.
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