The aircraft has two Collins model HF-9000, high frequency (HF) communication systems (HF 1 and HF 2) which operate independently from each other through a common antenna. The system provides amplitude modulated (AM) and single sideband (SSB) long-range air-to-air and air-to-ground voice communications.

There are two methods of frequency selection. The discrete frequency method allows the selection of 280,000 frequencies between 2 and 29.9999 MHz. The frequencies are selected in 100 Hz steps and 99 programmable preset channels. The channel method allows selection of 249 international telecommunications union (ITU) frequencies, 12 user programmable channels and 6 preset emergency channels.

The radio is controlled from the radio management unit (RMU), or the flight management system (FMS). Communication range on HF frequencies is much greater but less reliable when compared to VHF and UHF frequencies. HF communication range varies substantially, depending on several factors such as frequency, season, time of day, atmospheric noise, and solar activity. The radio management units (RMUs) normally tune the HF systems. The FMS CDUs provide backup tuning. RMU 1 normally tunes HF 1, and RMU 2 tunes HF 2. The HF systems are capable of operation in simplex or half-duplex modes.

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Receiver-Transmitter

The receiver-transmitter provides the signal processing required for transmitting and receiving. The unit consists of a control module, RF/IF module, frequency synthesizer, frequency standard, power supply/audio module, and a power amplifier.

The two HF transceivers (receiver/transmitters) are installed in the aft equipment compartment. HF 1 unit is installed on the left side of the aircraft at FS925L, and HF 2 unit is installed on the right side at FS925R. The HF transceivers receive and transmit the audio and RF signals. Each RT is held in position by two wing nut clamps on the single mounting rack. Electrical and fiber optic interfaces are connected to the unit front pan

HF Transceiver Mounting Rack

The HF Transceiver mounting rack supplies a stable shock-isolated mounting base. The four mount isolators have ground bonding straps installed between the isolator and the structure. A good ground bonding is important for the HF system to operate correctly (RF ground).

The Receiver-Transmitter has five primary modules that follow:

• ARINC 429 Interface
• Control
• Receiver/Exciter
• Power Supply/Audio
• Power Amplifier

ARINC 429 Interface

The ARINC 429 interface module receives control/tuning data from the RMU/FMS, reformats the data, and sends it to the control module. The control module sends system commands through the fiber-optic bus to the antenna coupler. Monitor data from the other RT modules and the remaining system is sent by the control module to the interface module. The ARINC interface module formats the data and sends it to the external bus.

Control

The control module contains a microprocessor system which includes read only memory (ROM), random access memory (RAM), nonvolatile memory (NVM), timers, and parallel input/output (I/O) devices. The control module has access to control and status lines to/from all other modules. On command from the ARINC 429 bus, the control module configures the RT and the system for the selected frequency and mode of operation. Control signals are transmitted to/from the coupler through two fiber optic cables.

Receiver/Exciter

The receiver/exciter module contains two parts: RF/IF (radio frequency/intermediate frequency) and frequency synthesizer. The RF/IF part uses a dual-conversion frequency scheme to change signals between the audio spectrum and the necessary RF frequency. The frequency synthesizer part supplies all injection frequencies for RF-to-audio and audio-to-RF frequency change. The microprocessor-compensated crystal oscillator (MCXO) supplies a stable time base reference for the frequency synthesizer.

Power Supply/Audio

The power supply part of the power supply/audio module changes the 28 VDC primary power input to controlled voltages for other RT modules.

A grounding strap is installed for LRU position identification. RT 2 has a grounding strap and RT 1 has no strap at pin X of J1.

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The antenna coupler is pressurized to prevent high-voltage arcing and moisture ingress, and provides uniform cooling medium. The antenna coupler is pressurized with 5-7 psig (34.5-48.2 kpa) of dry nitrogen or dry air. An air pressure valve on the front face provides a service and test point for the internal dry nitrogen charge. The tuner portion of the coupler varies the inductive and capacitive reactance to match the impedance of the antenna to the receiver/transmitter as different frequencies are selected. A microprocessor card performs all control and monitoring functions and communicates with the receiver/transmitter via fiber optic cables. The two HF antenna couplers are installed in a dual mounting rack on the front spar of the vertical stabilizer at FS1007.00, behind the dorsal fairing. Each unit is held in position by two wing-nut clamps on the mount. The mounting rack is attached by screws to the front spar of the vertical stabilizer. Each of the four mount isolators has a ground bonding strap between the rack and the structure. A braided flexible feedline connects the mounting rack and the couplers to the antenna. An option is available to replace the braided flexible feedline with a solid copper strap. The unit front panel contains electric and fiber optic interfaces. 

For more information regarding antenna coupler pressures, select Troubleshooting Tips from ATA 23-00-00 Communication System Description main page.

The antenna coupler has three primary modules that follow:

• Control
• RF Tuner
• Discriminator

The control module contains a microprocessor which tunes the RF tuner for correct phasing and minimum VSWR (voltage standing-wave ratio). Good tuning gives correct antenna impedance match which permits maximum power transfer. Tuning occurs in the tuner when the RT sends command signals, or when the discriminator sends error signals to the control module. Control signals are transmitted through fiber optic cables between the RT and coupler. Tuning data from used frequencies is kept in the microprocessor memory. It will be used to keep tuning time to the minimum when that frequency is used again.

The discriminator gets samples of RF input voltage and current from the coupler. From these samples, it causes dc error signals that are related to the impedance of the antenna and the power that is supplied to the antenna.

The external element is used to help in tuning the shunt type antenna. It is a network of capacitance selections in parallel. It is contained in the coupler and is supplied to the opposite coupler. The coupler has a configuration strap for the type of antenna used. Pin B of J1 connector is grounded for the shunt type HF antenna used.

HF Antenna Coupler Mounting Rack

The antenna coupler mounting rack supplies a stable shock-isolated mounting base for two antenna couplers. The mounting rack is attached to the front spar of the aircraft vertical stabilizer. Each of the four mount isolators has a ground-bonding strap installed between the isolator and the aircraft structure.

HF Antenna Flexible Feedline

The antenna flexible feedline is a braided cable that connects the dual coupler mounting rack to the HF antenna. It is attached to the rear of the coupler mounting rack. An option is available to replace the braided flexible feedline with a solid copper strap.

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The two HF systems use the same antenna. The antenna is an integral part of the leading edge of the vertical stabilizer. It is a shunt-type antenna placed in front of the load bearing spar on the vertical stabilizer. The shunt antenna is best described as a low impedance transmission line that couples RF energy to or from the aircraft structure. The whole conductive part of the airframe becomes part of the HF antenna.

The shunt, with HF currents flowing around the perimeter of the dielectric area, also radiates or receives as a loop antenna. Replacing a small section of the metal surface of the aircraft (conducting) with a dielectric (insulating) composite material basically forms the shunt antenna. The dielectric section forms an electrically small loop area, low impedance at lower HF frequencies, around which high HF currents can flow.

The HF coupler generates the large HF currents required for efficient loop operation. The antenna is connected to the antenna coupler units via a RF strap. High voltage and currents are present in this area during transmission.

Warning:
Do not transmit on HF system when personnel are working on the airframe in the tail or aft equipment area.

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The radio management unit (RMU) provides the operator interface for selecting operating mode, emission mode, power level, and operating frequency. Displayed information includes system status and fault diagnostics.

Note:
The RMU is not able to display or tune 100 Hz increments on the HF system. The RMU display and tuning starts at the 1,000 Hz increments. Tuning and display of the 100 Hz increments is carried out using the FMS CDU.

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The FMS CDU can also be used to tune the HF systems from its radio page. The CDU tunes the HF systems directly via ARINC 429 buses.

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Fiber optic cables are used for data communications between the couplers and the receiver/transmitters. These cables are immune to electromagnetic interference generated by the high power RF signals transmitted on the antenna. A fiber optic link is basically a data bus that uses light energy instead of electrical impulses to transmit information. The information is sent, in serial format, from a transmitter in one unit to a receiver in another.

The receiver/transmitters and antenna couplers each contain a fiber optic interface circuit. Each circuit consists of a receiver and a transmitter. To transmit a message, the transmitter receives electrical data in the proper message format and converts it to an optical output via a light emitting diode (LED). The optical output is then coupled to the glass fiber in the fiber optic cable.

The cables consist of a glass fiber core covered by cladding, inner tubing, strength fibers and an outer jacket. An amplifier-driven LED generates the optical signal and injects it into the fiber. Alignment of the LED with the fiber core must be within a defined acceptance cone. Light energy, which enters the fiber core at an angle greater than the acceptance cone, is not reflected at the core cladding boundary and is eventually lost.

To receive a message, the optical signal on the fiber optic cable is focused on a photo transistor and converted to an electrical current. This current is a replica of the transmitter light input.

Caution:
Fiber optics cable terminations and splices require specialized tools for maintenance. The terminations must be kept clean. When the fiber optic cables are not connected, protective covers should be installed. Dust and moisture on the internal optical lenses of the connectors degrade system operation. Proper installation of the connectors is essential to reliable system operation. Closely follow the manufacturer's instructions supplied with the connectors. Only hand-tighten fiber optic connectors.

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The two HF systems are identical and independent. They may operate simultaneously in receive mode but only one can transmit at one instant. In dual HF system configuration, antenna coupler 1 and antenna coupler 2 exchange status (interlock) data. The receive signals do not go through the two couplers in this operation but go directly to the receivers. This assures that only one system at a time is connected to the antenna for the transmit mode. When one transmitter is keyed, the other system is disconnected from the antenna by an internal relay and is unable to receive or transmit.

To change back to dual receive condition, the frequency or emission mode of one of the systems must be changed. This causes the two couplers to be bypassed and the received signals to go directly to the receivers. Both systems interface with the audio integrating system. Either one can be selected for use from the pilot's or copilot's audio panel. Each HF receiver supplies an output to the SELCAL system.

Best signal reception can occur when the PTT key of the receiving system is momentarily pushed. This tunes the related antenna coupler to the received frequency, but disconnects the other HF system from the antenna. The table that follows shows the five possible dual HF system operations. 

OPERATION	HF #1	HF #2
DUAL RCV	Receive	Receive
XMT 1	Transmit	Disconnected from antenna
RTC 1	Receive through tuned coupler	Disconnected from antenna
XMT 2	Disconnected from antenna	Transmit
RTC 2	Disconnected from antenna	Receive through tuned coupler

HF Tuning

Tuning Control Interface

The RMUs and the FMS CDUs control HF tuning. During normal operation, each RMU sends ARINC 429 tuning words to both HF R/Ts and receives feedback from both units. RMU 1 sends data to receive port A on both HFs and RMU 2 sends data to receive port B on both HFs. The feedback path is from TXA on HF1 to RX3 on both RMUs and from TXA on RMU 2 to RX4 on both RMUs.

In normal operation, the FMS CDUs send their tuning instructions through the RS 422 from the CDUs to the FMS cards in the IACs. The IACs encode the tuning data and send it on the ARINC 429 buses to their respective HFs. In order to tune the opposite side HF, the FMS CDU will send the tuning information to its FMS computer to be passed-on to the other active FMS computer via ASCB. This FMS computer will then tune the HF radio via its ARINC 429 output. The FMS CDUs do not receive a feedback from the HF units.

HF Backup Tuning

The FMS CDUs allow manual selection of the HF frequencies without the possibility of modifying features normally selected on the RMU HF control page. The backup tuning is from IAC ARINC 429 outputs. Both HFs can be tuned from both active FMS CDUs. If three FMS systems are installed, FMS 3 may replace either FMS 1 or 2.

RMU HF Window

The HF window is located on the bottom right side of the radio tuning page. The window displays HF frequency selection(s), HF emission mode, HF status, and HF self-test.

Software Initialization

There are two types of power interruptions, a "cold" start and a "warm" start. A "cold" start occurs when the power has been off for between 5 to 10 seconds or more and the aircraft is on the ground. A "warm" start occurs if the power has been OFF for between 5 to 10 seconds or less and the aircraft is airborne.

• On a cold start, the RMU 1 reverts to:
  

  • System 1 operation
  • Radio tuning page
  • HF cold start configuration
  • Indicates that the last COMM in use was the VHF

(RMU 2 performs similar functions for HF 2).

• The cold start HF configuration reverts the HF window to:
  

  • The preset tune mode and the active tune mode to simplex mode
  • The active frequency to 2000 kHz
  • The emission mode to UV
  • The squelch to MAX
  • The transmit power to Max
    
    

• The RMU does not tune the HF to these parameters immediately; it holds these settings until it receives a valid label 037 or 351 from the HF. This indicates that the communications between the two units are valid. It then transmits these settings to the HF. If the operator changes these settings before the RMU tunes the HF, the new settings will take precedence
  
  
• On a warm start, the RMU reverts to all the previous settings for the active frequencies, modes of operation and the last COMM in use settings

HF RMU Colors

The colors used on the RMUs for HF tuning and control annunciations are green, cyan, yellow, magenta and white. The HF maintenance log page annunciations are white, yellow and green. Test colors are yellow, green and red.

HF Window Label

The window label is always present at the top left side of the HF window. It is white to indicate on side and magenta to indicate cross side.

Normal Operation Annunciators

During normal operation the HF window will display the frequency or channel, the status annunciator, and the mode of operation. This display is affected by the following keys: 1/2, SQ, PBR5, PBR6 and tuning knobs.

The 1/2 button will toggle the HF window label annunciator between HF 1 and HF 2. The onside system is displayed in white and the offside is displayed in magenta. The two systems are independent of the RMUs and can operate in different modes at the same time.

The SQ key controls the squelch level annunciated in the upper right side of the HF window.

The HF status annunciator indicates the following conditions: frequency preview Emergency or ITU channel, HF system fail status, fault status, transmit status, receive control and squelch status. These annunciations are mutually exclusive and only the highest priority is displayed. The drawing below shows an example of the HF status annunciator.

When the system is in the TEST mode a solid green line is displayed in this area. "TX INH" appears when the transmitter is keyed and the other system is transmitting. "TX", "TX MIN" and "TX MED" show the power level selected for the transmitter. "REC INH" appears when the receiver is not connected to the antenna (other system is transmitting).

HF Channel Tuning

The HF channel displays as a frequency for simplex and split modes. It displays as a channel number for emergency and ITU modes. The tuning knobs tune the channels or the frequencies.

HF Window Tuning

The HF window displays the simplex mode, the split mode, the emergency mode or the ITU mode channels. Emission modes cannot be changed on the radio tuning page.

In the simplex mode the HF window displays two configurations, the preset tune mode and the direct tune mode. The UV (upper sideband voice) is the power-on emission mode. The illustration shows the simplex mode displays and their associated pushbuttons.

Preset Tune Mode

When in the preset tune mode, the PBR5 (transfer key) swaps the preset and the active channels. The PBR6 button brings the tuning box to the preset channel if it is elsewhere.

Direct Tune Mode

Holding the PBR6 pressed for two seconds will switch the HF window to the direct tuning mode, reposition the tuning box on the active simplex channel and remove the preset window. Rotating the tuning knobs will change the contents of the tuning box.

Split Mode

In the split mode, there are two lines for the frequencies, one for the receiver and one for the transmitter. The labels "RX" and "TX" precede the frequencies. Pressing PBR5 positions the tuning box on the RX channel and PBR6 positions it on the TX channel. Holding PBR6 for two seconds has the same effect as a momentary push.

Emergency Mode

The emergency mode window displays the active emergency channel or the preview frequency and emission mode. The frequencies and emission modes are set and cannot be changed by the operator. When the tune box is elsewhere, pressing either PBR5 or PBR6 will position the tune box on the emergency channel. Rotation of the tuning knobs will then change the emergency channel.

ITU Frequency Preview

Once the tune box is positioned on the emergency channel, pressing and holding either PBR5 or PBR6 will cause the HF window to change to the preview mode. It will remain in that state until the button is released. The emergency channel annunciation moves to the status position and is replaced by the frequency and emission mode.

ITU Preview

Pressing and holding either PBR5 or PBR6 at this time will change the HF window to the preview mode until the buttons are released. The ITU channel display will then move to the status position and the window will display the RX and TX frequencies and the emission modes.

ITU Frequency Preview

When the HF is in the ITU mode, the first press of either PBR5 or PBR6 will bring the tune box to the ITU channel. Rotation of the tune knobs will then change the selection of the ITU channel. There are 249 international maritime channels, programmed by frequency and designated by channel number, available for tuning.

HF Control Page

This page is accessed from the "page menu" page. It provides access to the following functions:

• Active channel tuning
• Preset channel tuning
• Memory channel recall and store
• HF squelch control
• Squelch level selection
• HF mode control
• Transmitter power level selection

The HF control page is divided into an active channel window, a preset channel window and a control window. The active window format varies with the mode selected. It follows the format previously described for the radio tuning page as well as a selection of the tune mode.

The preset window displays a scratch pad channel, emission mode, and tune mode. The control window displays squelch level and power level. The transfer key exchanges the preset channel with the active channel.

Page Banner Annunciator

This annunciator is always present on the HF control page. The color is white when looking at the onside display and magenta when looking at the offside display after pressing the 1/2 button.

TEST Annunciator

When the TST button is pressed, the RMU removes all the normal control page annunciations and displays HF SYSTEM SELF-TEST.

1/2 Selection

When the 1/2 button is pressed, the display will flip between HF1 and HF2. The active channel and control windows display all the data for the system visible in the page banner. The preset channel window remains dedicated for the onside system.

Active/Preset Transfer

The PRESET label annunciator is green. Each press of PBL3, while the control page is displayed, will exchange the ACTIVE and PRESET window contents. This includes the tune mode, channel selection and the emission mode if present.

Active Tune Mode Selection

Pushbutton PBL4 selects the ACT MODE. The label is cyan. Each press of the button will increment the modes from SIMPL, to SPLIT, to EMERG, to ITU, and back to SIMPL in order. The illustration shows the HF CONTROL PAGE active channel window with the various modes displayed.

Emission Mode Selection

The emission modes can be changed using either the tune knobs or the pushbutton when the tune box is positioned on the emission mode. Each click of either knob or each push of the button will cycle the modes through the following selections: UV, LV, UD, LD, AM, CW.

Preset Memory Definition

In order to simplify the software, the preset memory channels are stored in the RMU nonvolatile memory and not in the HF receiver/ transmitter. Each RMU stores 12 preset memory channels. The channels stored in one RMU are not accessible from the offside RMU when the 1/2 button is pressed. Either HF system can be tuned to the memory channels stored in the RMU where it is displayed by using the 1/2 button.

The preset memories contain the tune mode, the RX channel, the TX channel, and the emission modes. In addition to aviation frequencies, the HF COM system can operate on frequencies of various other communication services and can be used to contact these stations in accordance with any with a mutually-adopted prior agreement.

The 249 international telecommunication union channels are preprogrammed in a permanent memory. The channel numbers are related to the first digits of the frequencies, for example: Channels 401 to 429 correspond to frequencies in the 4,000 kHz range.

Preset Window Tuning

The same procedures apply when changing values in the Preset window as did in the active window with the following exceptions. The preset window normally shows a memory location above the preset mode. When the tune box is around the preset channel and the numbers are changed, the memory channel annunciation will change to TEMP followed by the same number as the memory channel.

Pressing the STO button will store this value in the same memory position. To recall a memory position, place the tune box around the memory location annunciator and rotate the tune knobs to the desired channel. Each memory position can be edited by using the procedure in the previous paragraph or by going to the HF MEMORY page from the PAGE MENU page.

Interaction with the Memory Page

If the operator leaves the control page and accesses the memory page, any change performed on the memory channel previously annunciated on the control page will be reflected upon return to the control page. If the annunciation was TEMP prior to the change on the memory page, the preset window is not affected by the change.

HF Status Annunciator

The HF status annunciators on this page are identical to the ones on the radio tuning page.

SQ and SQ Level keys

The SQ key on the bottom of the RMU is used to toggle the squelch between OFF and the previously selected squelch level. The SQ level key, PBL5, tunes the squelch level between MAX, MED, and MIN.

Transmit Power Control

The label for this function is TX POWER and the color is cyan. Each press of PBR5 will cycle through the following power levels: MIN, MED, and MAX.

HF Memory Page

The HF MEMORY page, accessed from the SYS MENU page provides the following functions:

• Active channel tuning
• Memory channel tuning
• Mode control
• HF status

There are six memory locations on the first page, and an additional six available on the second page.

This page is divided into an active channel window and memory channel windows showing three memory positions. The operations of the controls on this page are similar to the previous pages.

Load Key

This key loads the selected memory channel into the active channel position. The memory channel is selected by pressing the adjacent pushbutton. The data includes the tune mode and channels and the emission mode. The tune box repositions to the active channel along with the other data.

More Key

This key will flip through the memory channel list three positions at a time. When this key is pressed, the tune box will be repositioned to the active window.

1/2 Key

This function key has a different effect when pressed with a memory page displayed. The selected system banner and the active window will change to the cross-side system. The memory window will not be affected since the memory channels are stored in each RMU and are not available on the RSB bus.

Note:
The RMUs do not display 100 Hz increments (right of decimal). However, the FMS CDU is capable of tuning and displaying the 100 Hz increments. For example, an entry of "10000" on the RMU would be entered as "10000.0" on FMS.

SELCAL

The HF COMM systems are coupled to a SELCAL (selective calling) system that monitors the selected HF frequencies for the aircraft´s unique SELCAL code. If the code is received and detected, an indicator and audio alert are triggered to advise the crew that a ground station is calling.

Due to technical incompatibility, a single sideband suppressed carrier mode of emission can not be used to transmit SELCAL signals. Therefore, ground station transmitters switch to the AM mode when transmitting SELCAL tones. The HF receiver is designed to detect SELCAL signals transmitted in the AM mode even though the selected mode is an SSB mode.

Communication Range

Communication range on HF frequencies is much further but less reliable than on conventional VHF and UHF frequencies. It varies substantially depending on several factors such as operating frequency, season, time of day, atmospheric noise, and solar activity.

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Power-On Built-In Test

The HF system reports its power-on self-test status to the RMU. If the RMU detects a fail signal, it will annunciate ERR condition on the status annunciator on the radio tuning page, the HF control page and the HF memory page. The reported fault can be viewed on the HF maintenance log page.

Fault Indications and Display

The RMUs will detect system faults using the fault monitoring and fault reporting ARINC 429 transmissions from the HF R/T unit.

HF Maintenance Log Page

This page is accessible on the RMU, when the aircraft is on the ground, using the following sequence:

1. Select Page Menu page: PGE key
2. Select MAINTENANCE
3. Select MAINTENANCE LOG
4. Select on/side SYSTEM; 1/2 key
5. Select HF

The maintenance log is maintained in the RMU memory. The HF system only provides the current status. Each RMU records and stores maintenance data for its onside system. The RMU records one maintenance log entry for each error code listed in the log messages. Each entry is tagged with the RMU power-on count (POC) and the RMU time. The RMU logs an ER code only once per power-on count. The RMU does not log the temperature (TMP).

Number of Events

This field indicates the total number of events listed in the maintenance log. The maximum number of lines available is 16. The last entry will be followed by "END LIST".

Top Entry Number

This field records the number of occurrences of the entry in the top line.

Current Power-on Count

This field annunciates the current POC for the RMU. The count is incremented when the software detects a cold start with weight-on-wheels.

Log Entry Slots

The eight lines in this window display the log entries with the latest one on the top line. If more than eight entries are listed, the tuning knobs are used to scroll through the other entries. The total number of entries possible is 16. If more events occur, the oldest entries are deleted. The RMU detects the events by decoding the ARINC 429 maintenance word label 351.

Clear/Confirm Buttons

After replacing a HF system component or the associated RMU, the maintenance log is no longer valid. If the CLEAR button (PBL5) is pressed, a CONFIRM annunciation appears next to PBR5. PBR 5 must be pressed within two seconds to replace the existing screen with a new screen. On the display, the NUMBER OF EVENTS annunciation reverts to "0", TOP reverts to "--", and END LIST appears in slot 1.

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Tuning Control Interface

The tuning modes can be divided into normal, reversion and backup. There are three ARINC 429 tuning inputs to each receiver/transmitter. These inputs are labeled port A, port B and port C.

The input from the on-side RMU, during normal operation, is received through ports A for HF 1 and B for HF 2. The receivers are strapped for BURST TUNING which means that they search for a burst input on all three ports sequentially.

During reversion tuning (direct input from RMU 1 to HF 2 or from RMU 2 to HF 1) the RMUs will change the SDI code (bits 9 and 10) in the ARINC429 word to identify the cross-side HF as the destination. At this time, HF 1 will accept the input from port B and HF 2 will accept the input from port A.

Backup tuning is used only when both RMUs have failed. For backup tuning, the FMS CDUs send tuning information to the FMS computer in the IAC on an RS-422 bus. The information is forwarded to the HF receiver in ARINC 429 format through the port C input.

When two FMS systems are installed, FMS 1 will tune HF 1 and FMS 2 will tune HF 2. If three FMS systems are installed, FMS 1 or 3 will tune HF 1, depending on the position of the FMS relay, and FMS 2 or 3 will tune HF 2 under the same conditions.

Confirmation of the ARINC 429 tuning input is returned to the RMUs in ARINC 429 format from both HF receiver/transmitters. It is also converted to RSB format and forwarded to the IACs for use by the FMSs.

Note:
The RMU is not able to display or tune the 100 Hz increments on the HF system. The RMU display and tuning starts at the 1000-Hz increments. Tuning and display of the 100 Hz increments are carried out, using the FMS CDU.

Inputs/Outputs

Inputs/outputs to the HF receiver transmitters include:

• Fiber optic cables from the antenna couplers for interfacing of the control signals
  
  
• An RF coaxial cable for interfacing of the RF signals
  
  
• Mic audio and PTT signals from the audio panels
  
  
• Receive audio and sidetone to the COMM units. The audio sent to the COM units is converted to digital audio in the COM units and sent to the audio panels on the digital audio bus
  
  
• Separate audio to the SELCAL decoder

The two HF systems may operate simultaneously in receive mode but only one can transmit at a time. The antenna couplers exchange status (interlock) data. This assures that only one system is connected to the antenna in the transmit mode. When one transmitter is keyed, the other system is disconnected from the antenna by an internal relay and is unable to receive or transmit.

The audio sent to the ICUs is converted to digital audio and sent to the audio panels on the digital audio bus. Each HF receiver supplies an output to the SELCAL system to produce aural and visual alerts when the correct SELCAL code is received.

Couplers

The HF couplers interface with the HF receiver/transmitters through fiber optic cables for control signals and coaxial cables for RF signals. There are status lines between the two units to prevent both units transmitting at the same time and to isolate the other receiver input when one is transmitting. The lines will carry a logic 1 or 0 signal to indicate the conditions of the systems. If 28 V is not present on one system, the other system will revert to single system operation.

Caution:
Fiber optic cable terminations and splices require special tools for maintenance. The terminations must be kept clean. When the fiber optic cables are not connected, protective covers should be installed. Dust and moisture on the internal optical lenses of the connectors degrade system operation. Only hand-tighten fiber optic connectors. Do not use pliers.

Both units have configuration straps to set them for shorted shunt HF antennas and RF straps on the mounting racks for proper grounding. The RF energy is transmitted directly to the mounting rack from the back-plate of the coupler(s). The coupler mounting rack feeds this signal to the antenna through an RF strap.

Power Inputs

Power for the system comes from two secondary power distribution assemblies (SPDA). SPDA 2 feeds HF 1 receiver/transmitter through one 15 A 28 VDC BUS 2 solid-state power controllers (SSPC). It also feeds the antenna coupler through one 5 A 28 VDC BUS 2 SSPC. HF system 2 is similar to HF 1, with the source from SPDA 3, 28 VDC BUS 1.

The status of the circuit protection devices may be viewed and controlled on the electrical management system (EMS) control display unit in the cockpit.

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Initiated Built-In Test – RMU

The power-on self-test (POST) for the HF is not included in the Primus II radio tests. A pilot-activated self-test (PAST) for the HF system is available on the RMU.

Pilot-Activated Self-Test (PAST)

The PAST is available for the pilot or the maintenance technician to conduct a self-test of the HF system. The test is initiated by placing the cursor inside the HF window on the main page and pressing the TST button for two seconds. Once the test is started, the RMU annunciates test in progress and the test status. The operator cannot interrupt or stop the test except with a power interruption on the system.

The operator has the choice of starting with a receiver or a transmitter test. If the HF transmitter is not active when the test is initiated, the receiver test will be initiated first.

After the test is complete, a green PASS or red FAIL will be displayed inside the HF window. The transmitter test can be started immediately following the receiver test by pressing and holding the PTT key for three seconds while the receiver test is in progress.

Warning:
Be sure all personnel are clear of HF antenna when performing transmitter tuning checks. Do not touch the RF output terminal on the antenna coupler, the antenna lead-in wire, the insulated feed through or antenna itself while microphone is keyed or during tuning cycle. Serious RF burns can result from direct contact with the above items when the system is transmitting.

Warning:
Due to the hazards of HF radiation, the "MED" and "HI" power checks should be done outside the hanger.

The test can also be initiated from the HF control or memory pages by pressing and holding the TST key for two seconds. In this instance, the screen will be blanked except for a TEST in large letters annunciated on one line and HF TEST RX or one of the other components on the line below. When the test is complete, a HF TEST PASS or FAIL will be displayed.

HF System Test Cycle

The HF system test cycle begins when the HF system ARINC 429 label 037 reports that the test is in progress to the RMU. A complete test cycle for both the RX and TX tests is normally completed within 20 seconds. When the test is successfully completed, the label 037 reports NORMAL status to the RMU and "HF PASS" is annunciated. If the test fails, the label 037 reports no computed data (NCD). The RMU then displays "HF ERR". The HF test is limited to 50 seconds by the RMU. After this time the system is returned to normal operation.

Test Operation Annunciator

The first line of the HF window displays the test in progress annunciator. The word "TEST" in yellow will be displayed when the RMUs detect the HF is in test status.

Test Status Annunciators

On the second line of the HF window, the HF test status will cycle through blank to HF TEST RX (yellow) to HF TX TEST (yellow) to either HF PASS (green) or HF ERR (red) depending on the results of the test.

Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
A49/A50	HF TRANSCEIVERS	ZONE(S) 311B	23-12-01 [ GX ] [ GXRS ] [ G5000 ]
CP4/CP5	HF ANTENNA COUPLERS	ZONE(S) DORSAL FAIRING (343)	23-12-05 [ GX ] [ GXRS ] [ G5000 ]
-	HF ANTENNA FEEDLINE	ZONE(S) DORSAL FAIRING (343)	23-12-09 [ GX ] [ GXRS ] [ G5000 ]

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