Radio Modems

Harris RF-3466A Universal HF Modem Harris RF-3466A

(NSN 5895-01-332-8681)

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Related TOC: (for Canadian Forces users)
Modem Software Settings
Table 2a: Recommended Modem Settings (39-tone mode)
Table 3a: Recommended Modem Settings (Robust Serial mode)

Notes: Transceiver Keyline
Terminal program (DTE) Settings
Compatibility Issues
HF Equipment  Considerations
Use of Sync or Async DTE mode of operation

General Description
The Harris RF-3466A Universal HF Modem, nomenclature MD-5063(V)2/U, is a 19" rack mount unit having the ability to combat the complexities of ionospheric propagation over HF radio circuits.  To achieve this, the RF-3466A uses specially designed waveforms to combat negative signal-to-noise ratios, fading, multipath spread and interference.   The primary modem consists of the 39-tone (multi-tone) waveform.  For compatibility, the Frequency Shift Keying (FSK) signal is supported and is interoperable with the MD-522 modem.  An optional modem and waveform includes the Robust Serial Tone waveform.

All waveforms and their variables are selected by front-panel controls.  User menus, prompting, and operational status of the modem is indicated by a front-panel mounted fluorescent matrix display.

Remote control capability is provided by an RS-232C interface.  No future improvements are planned for this modem and the modem is not Firmware upgradeable.

The 39-tone waveform is based on Time Differential Phase Shift Keying (TDPSK) of 39 tones and is combined with a Forward Error Correcting code (Reed-Solomon Code) to produce data rates ranging from 75 bps to 2400 bps with Soft Decision Decoding.

The Frequency Shift Keying (FSK) mode of operation is provided for compatibility with existing modems.  User selected mark and space frequencies over a range of 0 to 3400 Hz in 1/2 Hz resolution are accommodated.  The binary FSK waveform supports data rates of 45 bps to 1200 bps and is interoperable with the MD-522.

The optional Robust Serial waveform is based on an 8-ary PSK modulation of a single tone (spread-spectrum signalling) and includes a half-rate convolutional FEC coding process with Viterbi Soft Decision Decoding.  The data rate for the Robust Serial waveform is 75 bps.

The RF-3466A is a dual channel unit which supports a mix-and-match of waveforms on either or both channels.  This feature allows for full-duplex operation.  All signal processing is based on the TMS320 family of digital signal processors (DSP).

The interface supports both Synchronous or Asynchronous data connections.

The versatility and speed of the modem make it suitable for a variety of applications including:

Figure: RF-5710 Configurations

The RF-3466A contains transformer coupled audio input and output providing for isolation and balanced 600 ohms operation.  It is thus important that the correct audio pins (high & low) be correspondingly connected to ensure that the audio phase remains proper across the network and at all stations.

RF-3466() Interface Diagram

NOTE #1 (Keyline)
The communication program utilized with the RF-3466A modem must support the toggling (on/off) of the RS-232C RTS line as the transmit on / transmit off control function.   If the communication program does not support this function, the modem and the transceiver will be in a permanent transmit state.  Regular terminal communication programs are inadequate for use with radio modems.  You must use a terminal communication program designed for radio interfacing.

Modem Software Settings
All of the RF-3466A modem software switches are set by the front panel push buttons with the aid of the visual display.  Please pay attention the the notes bellow prior to making any mode or setting selections.

NOTE #2 (RS-232 DTE Settings)
Not all computer RS-232C ports support slow and irregular data speeds (i.e. 50 and 75 bps), it may be required to set the computer (DTE) and communication software's RS-232C data communication rates at a speeds greater than the desired over-the-air modem transmitting and receiving data rates.  The RF-3466A modem will buffer all input and output data sent and received from the computer.   Tests have proven that bit errors in the transmission and reception of FSK data may occur if the communication software is set for rates of 50 or 75 bps.  If bit errors are a problem, try setting your communication software and DTE settings in the modem to a 'standard' baud rate.  Ensure that the settings are both identical.  The software and DTE settings in table 1a are suggested for all FSK modes of operation and settings in table 1b are suggested for all serial modes of operation.

NOTICE: Your modem and DTE software configurations will have to be changed in accordance with the type of alphabet that is being transmitted (Baudot or ASCII alphabet).  Baudot is used for the FSK modes and ASCII is used for the serial modes.  Remember that your software and modem configuration must be identical.  At this point, I know that you are thinking "Instead of changing data formats, why not just leave it in Baudot and save my self a bunch of trouble?"  A valid question but, the answer is that if you want to use the 39-tone or Robust Serial modes to transmit FAX or images etc..., you will require the ASCII alphabet.

Table 1a. DTE Settings (FSK Mode) Table 1b. DTE Settings (serial modes)
DTE Rate: 50 or 75 DTE Rate: 300
DTE Parity: Off DTE Parity: Even
DTE Character Size: 5 DTE Character Size: 7
DTE Stop Bits: 1 DTE Stop Bits: 1
DTE Echo Off DTE Echo: Off
DTE Flow Control: CTS DTE Flow Control: CTS
DTE Duplex: Half DTE Duplex: Half

Set the desired over-the-air bps rate from the operational mode menu option on the RF-3466A modem.

NOTE #3 (Compatibility)
For compatibility with stations equipped with the Northern Radio NR 3132 modem or the optional AFSK card in the transceiver and installations equipped with the MD-522/GRC FSK modem (AN/GRC-106) select the MD-522 mode on the RF-3466A at a symbolic rate of 50 bps (2805+/-42.5).  In the communication software, ensure that your settings reflect the selection of the Baudot (5 bit) alphabet and ensure that you change the RF-3466A's DTE settings as outlined in table 1a.

For interoperability with installations equipped with the Harris RF-5710 modem or installations with the Harris RF-5000 series of HF transceivers, select any of the available FSK modes.  Please note that the "Robust Serial" mode on the RF-3466A is not compatible with the RF-5710's "Serial" mode.  The RF-5710 modem does not feature the 39-tone (multi-tone) format and is not compatible with the RF-3466 or RF-3466A modems.  The 39-tone mode of the RF-3466A is fully compatible with the 39-tone modes of the RF-5000 series transceivers.

NOTE #4 (HF Radio Equipment Considerations)
When selecting a modem waveform and data rate, please take into consideration the type of HF radio equipment you are using.  Most waveforms from the RF-3466A have a 3 kHz bandwidth.  These waveforms may exceeds the audio bandwidth of the transceiver.  Standard HF transceivers are typically designed for a 2.7 kHz bandwidth (300 Hz to 3000 Hz).  If this is an issue with your installation, try to employ these modes at lower bit rates (i.e. 300 bps instead of 2400 bps).

Click here for a spectrum graph of the various waveforms.

Table 2a Recommended Modem Set-up (39-Tone)

Option                     Setting                
01 Data Rate 300 bps
02 Interleave Long
03 DTE Asynchronous
04 Channel A Only
05 Duplex Half
06 Key Delay 45 mSec
07 Clipping On
08 Time/Fz Div On
09 Doppler Track On
10 Acquire Enhanced

Table 2b Additional Settings when item #3 is set to Asynchronous

Option                      Setting                 
11 DTE Rate 300 bps
12 DTE Parity Even
13 DTE Char Size 7
14 DTE Stop Bits 1
15 DTE Echo Off
16 Async EOM Off
17 Flow Control CTS

Table 3a Recommended Modem Set-up (Robust Serial)

Option                     Setting                
01 Interleave 9.6 sec
02 DTE Asynchronous
03 Channel A Only
04 Duplex Half
05 Key Delay 45 mSec

Table 3b Additional Settings when item #2 is set to Asynchronous

Option                      Setting                 
06 DTE Rate 300 bps
07 DTE Parity Even
08 DTE Char Size 7
09 DTE Stop Bits 1
10 DTE Echo Off
11 Flow Control CTS

NOTE #5 (Use of Sync or Async DTE Equipment Considerations)
The use of the Synchronous or Asynchronous data port configuration is dictated by the type of equipment that is interfaced to the modem.  Modern day computer terminals, especially the ones employing a 9 pin RS-232C connector, do not support external receive (RX Clk) and transmit (TX Clk) clocks which are required for synchronous modes of operation.  It is therefore impossible to set a synchronous mode of operation on these devices.  The use of the synchronous clock mode can only be employed with terminal equipment that is designed for synchronous modes of operations.  Examples of synchronous terminal equipment are most cryptographic devices (i.e. TSEC/KG-84C), some fax machines and, specially equipped computer terminals.  Consult your equipment's technical specifications prior to employing the synchronous data port configuration.

Related links: Sounds of the Modern Commercial and Military Digital Station
Harris HF Modem Compatibility Chart

Revised: June 08, 2017
(C) 1998-2023, Richard Lacroix. All rights reserved.
Portions (C) Harris Corp.

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