UIC-751-3 (Eurobalise)

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Overview

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Technical Specifications

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Frequencies and Timing

• Data rate: 600 baud
• Mark frequency: 1300 Hz (binary 1)
• Space frequency: 1700 Hz (binary 0)
• Start bits: 8 (sync pattern)
• Data bits: 39 (telegram content)
• Stop bits: 0
• Total frame: 47 bits (8 start + 39 data)
• Receive-only mode (TX not supported)

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Telegram Format

[8 start bits: 11110010] [39 data bits: ddddd...ddddd]
│                         │
│                         └─ 39-bit data telegram
└─ Fixed sync pattern (0xF2)

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Protocol Variants

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Train-to-Ground (UIC-T)

minimodem --rx uic-train

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Ground-to-Train (UIC-G)

minimodem --rx uic-ground

• Mode selected by 4th character after “uic”
• uic-train or uict → train decoder
• uic-ground or uicg → ground decoder
• Different data interpretation for each direction

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Data Telegram Content

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Balise Information

• Balise Identity: Unique identifier
• Location Data: Position information
• Track Data: Line speed, gradient, signals
• Control Commands: Speed restrictions, stop commands
• Message Type: Category of information

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Typical Information Types

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Eurobalise System

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Physical Implementation

• Passive: Powered by train’s antenna
• Active: Powered by track-side equipment
• Transmission: Uplink 27.095 MHz, downlink via 4 MHz FSK

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ETCS Integration

• ETCS Level 1: Balise-based with infill
• ETCS Level 2: Radio-based with balise positioning
• ETCS Level 3: Moving block with radio

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Usage Examples

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Receiving Train Data

minimodem --rx uic-train -f train_data.wav

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Recording Balise Data

# Using audio tap from train's balise receiver
arecord -f S16_LE -r 22050 -d 60 -c 1 balise_pass.wav

# Decode recording
minimodem --rx uic-ground -f balise_pass.wav

# Monitor and log all telegrams
arecord -f S16_LE -r 22050 | \
  minimodem --rx uic-ground 2>&1 | \
  tee uic_telegrams.log

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Technical Details

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Start Bit Sequence

• Frame synchronization: Identifies telegram start
• Bit timing recovery: Locks receiver clock
• Pattern recognition: Distinct from data patterns
• Noise rejection: Reduces false triggers

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Frequency Selection

• Audio frequency range: Easy transmission/recording
• 400 Hz shift: Good discrimination
• Noise immunity: Above low-frequency rumble
• Equipment compatibility: Standard audio equipment

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No Stop Bits

• Start pattern provides synchronization
• Fixed 47-bit frame length
• Continuous transmission possible
• Reduces overhead

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Expected Bit Pattern

expect_data_string = "11110010ddddddddddddddddddddddddddddddddddddddd"
expect_n_bits = 47

• First 8 bits: 11110010 (sync)
• Next 39 bits: Data (‘d’ = any bit)
• No trailing bits

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Signal Quality Requirements

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Audio Requirements

• Sample rate: 22.05 kHz recommended minimum
• Bandwidth: 1000-2000 Hz
• SNR: > 12 dB for reliable decoding
• Frequency accuracy: ±20 Hz

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Recording Conditions

• Minimize electrical noise from train systems
• Use shielded cables
• Appropriate audio levels (-6 dB to -20 dB)
• Stable sample clock

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Applications

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Railway Operations

• Train Protection: ATP/ATC systems
• Speed Supervision: Automatic speed control
• Position Reference: Odometry correction
• Signal Aspects: Signal state transmission

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Testing and Development

• Balise Testing: Verify balise programming
• System Integration: Test train equipment
• Fault Diagnosis: Analyze transmission issues
• Training: Educational demonstrations

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Analysis and Research

• Data Logging: Record train operations
• Performance Analysis: System reliability studies
• Protocol Research: Reverse engineering
• Safety Investigations: Incident analysis

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Telegram Decoding

Bit Range | Field               | Description
----------|---------------------|---------------------------
0-9       | Balise ID           | Unique balise identifier
10-15     | Message Type        | Category of information
16-23     | Speed Limit         | Maximum permitted speed
24-31     | Distance            | Distance to next point
32-35     | Track Gradient      | Slope information
36-38     | Checksum/Parity     | Error detection

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European Rail Standards

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Related Standards

• UIC 751-3: Eurobalise transmission standard
• SUBSET-036: FFFIS for Eurobalise
• EN 50126: Railway RAMS specification
• EN 50128: Railway software standard
• EN 50129: Railway safety-related electronic systems

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ETCS Baseline

• Baseline 2: Widely deployed
• Baseline 3: Enhanced features
• Baseline 4: Future development

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Troubleshooting

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No Telegrams Decoded

• Correct mode (uic-train vs uic-ground)?
• Audio sample rate ≥ 22.05 kHz?
• Signal present in 1300-1700 Hz range?
• Audio levels appropriate?

# Visualize audio spectrum
ffmpeg -i recording.wav -filter_complex "showspectrumpic" spectrum.png

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Partial/Corrupted Telegrams

• Low SNR (noise)
• Frequency drift
• Audio clipping
• Sample rate issues

• Improve signal quality
• Use higher sample rate (44.1 kHz)
• Adjust audio input gain
• Filter out-of-band noise

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Sync Pattern Not Found

• Verify start pattern (11110010)
• Check bit timing accuracy
• Ensure adequate signal level
• May be inverted (mark/space swapped)

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Safety and Legal Considerations

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Advantages of UIC-751-3

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Reliability

• Proven technology in millions of operations
• Robust FSK modulation
• Error detection built-in
• Works in harsh electromagnetic environments

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Standardization

• Europe-wide interoperability
• Consistent implementation
• Well-documented specifications
• Mature ecosystem

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Safety

• Safety-certified design
• Fail-safe principles
• Redundancy mechanisms
• Continuous monitoring

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Comparison with Other Protocols

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References

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Standards Documents

• UIC 751-3: Eurobalise transmission system
• SUBSET-036: FFFIS for Eurobalise
• ERA Technical Documents
• CCS TSI (Control-Command and Signalling Technical Specification)

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Additional Resources

• European Railway Agency (ERA)
• UNISIG (Union Industry of Signalling)
• ETCS specifications and baselines
• National railway authority documentation

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Related Protocols

• Bell 103 - Similar low-speed FSK
• Bell 202 - Higher speed alternative
• RTTY - Low baud teletype

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See Also

• Command Reference
• Audio Configuration
• Railway Applications