Protocols

Overview

Minimodem supports a variety of standard FSK protocols for different applications including telecommunications, amateur radio, emergency alerts, and transportation systems.

Bell System Protocols

Bell 103 (300 baud)

Classic 300 baud modem standard used in early computer communications and acoustic couplers.

Specifications

Baud rate: 300 bps
Mark frequency: 1270 Hz
Space frequency: 1070 Hz
Frequency shift: 200 Hz
Modulation: FSK
Bandwidth: 50 Hz

Framing

Data bits: 8 (ASCII)
Start bits: 1
Stop bits: 1
Total frame: 10 bits
Encoding: ASCII 8-N-1

Usage:

# Receive Bell 103
minimodem --rx 300

# Transmit Bell 103
echo "Hello World" | minimodem --tx 300

# With audio file
minimodem --rx --file recording.wav 300

Bell 103 originated at Bell Labs in 1962 and became the de facto standard for 300 baud modems worldwide.

Bell 202 (1200 baud)

High-speed protocol used for caller ID, packet radio, and faster data transmission.

Specifications

Baud rate: 1200 bps
Mark frequency: 1200 Hz
Space frequency: 2200 Hz
Frequency shift: -1000 Hz
Modulation: FSK
Bandwidth: 200 Hz

Framing

Data bits: 8 (ASCII)
Start bits: 1
Stop bits: 1
Total frame: 10 bits
Encoding: ASCII 8-N-1

Usage:

# Receive Bell 202
minimodem --rx 1200

# Transmit Bell 202
minimodem --tx 1200 < data.txt

Bell 202 is commonly used for caller ID (CLID) transmission on telephone lines between the first and second ring.

ITU-T Standards

V.21 (300 baud)

International standard similar to Bell 103 but with different frequencies.

Specifications

Baud rate: 300 bps
Mark frequency: 980 Hz
Space frequency: 1180 Hz
Frequency shift: 200 Hz
Modulation: FSK
Bandwidth: 50 Hz

Framing

Data bits: 8
Start bits: 1
Stop bits: 1
Total frame: 10 bits
Encoding: ASCII 8-N-1

Usage:

minimodem --rx V.21
minimodem --tx V.21 < message.txt

Telegraphy Protocols

RTTY (Radio Teletype)

Classic radioteletype protocol used extensively in amateur radio and maritime communications.

Specifications

Baud rate: 45.45 bps
Mark frequency: 1585 Hz (default)
Space frequency: 1415 Hz
Frequency shift: -170 Hz
Modulation: FSK
Bandwidth: 10 Hz

Framing

Data bits: 5 (Baudot)
Start bits: 1
Stop bits: 1.5
Total frame: 7.5 bits
Encoding: Baudot/ITA2

Usage:

# Receive RTTY
minimodem --rx rtty

# Transmit RTTY
minimodem --tx rtty < message.txt

# Custom RTTY frequencies
minimodem --rx --mark 2125 --space 2295 rtty

Baudot Character Set

RTTY uses the 5-bit Baudot (International Telegraph Alphabet No. 2) character set with LETTERS and FIGURES shift states:

LETTERS mode: Standard alphabet
FIGURES mode: Numbers and punctuation
Special shift characters toggle between modes
Character set defined in src/baudot.c

The 45.45 baud rate comes from mechanical teleprinter limitations: 1/45.45 ≈ 22ms per bit.

TDD/TTY (Telecommunications Device for the Deaf)

Baudot-based protocol for telecommunications accessibility.

Specifications

Baud rate: 45.45 bps
Mark frequency: 1400 Hz
Space frequency: 1800 Hz
Frequency shift: 400 Hz
Modulation: FSK
Bandwidth: 10 Hz

Framing

Data bits: 5 (Baudot)
Start bits: 1
Stop bits: 2.0
Total frame: 8 bits
Encoding: Baudot/ITA2

Usage:

minimodem --rx tdd
minimodem --tx tdd < message.txt

TDD differs from RTTY primarily in stop bit duration (2.0 vs 1.5) and frequency pair, optimized for telephone line transmission.

Emergency Alert Systems

NOAA SAME (Specific Area Message Encoding)

Emergency alert system protocol used by NOAA Weather Radio and the Emergency Alert System.

Specifications

Baud rate: 520.83 bps
Mark frequency: 2083.33 Hz
Space frequency: 1562.5 Hz
Frequency shift: -520.83 Hz
Modulation: FSK
Bandwidth: 520.83 Hz

Framing

Data bits: 8
Start bits: 0
Stop bits: 0
Preamble: 16 × 0xAB sync bytes
Encoding: ASCII (specific format)

Usage:

# Receive SAME alerts
minimodem --rx same

# Decode from recorded audio
minimodem --rx --file weather_radio.wav same

Message Format:

ZCZCZCZ-ORG-EEE-PSSCCC+TTTT-JJJHHMM-LLLLLLLL-

SAME Message Components

ZCZCZCZ: Preamble (sync bytes 0xAB)
ORG: Originator code (WXR, CIV, EAS, etc.)
EEE: Event code (tornado warning, test, etc.)
PSSCCC: Location codes (up to 31 areas)
+TTTT: Valid time period (15-minute increments)
JJJHHMM: Date/time of issue (Julian day + time)
LLLLLLLL: Station callsign

SAME uses no framing bits. The protocol relies on sync byte detection (0xAB pattern) to identify message start.

Reference: NOAA Weather Radio SAME Documentation

Caller ID

Bell 202 Caller ID

Telephone caller identification protocol using Bell 202 modulation.

Specifications

Baud rate: 1200 bps
Mark frequency: 1200 Hz
Space frequency: 2200 Hz
Frequency shift: -1000 Hz
Timing: Between 1st and 2nd ring

Framing

Data bits: 8
Start bits: 1
Stop bits: 1
Protocol: SDMF or MDMF
Encoding: ASCII/Binary

Usage:

# Receive caller ID
minimodem --rx callerid

Caller ID mode is receive-only (--tx is not supported). The protocol uses special framing defined in databits_decode_callerid().

Minimodem automatically handles caller ID protocol decoding, including message type, length, and checksum validation.

Railway Communications

UIC-751-3 (Train/Ground)

European train control system protocol for train-to-wayside communication.

Specifications

Baud rate: 600 bps
Mark frequency: 1300 Hz
Space frequency: 1700 Hz
Frequency shift: 400 Hz
Modulation: FSK

Framing

Data bits: 39
Start bits: 8
Stop bits: 0
Frame format: Fixed pattern
Variants: Train / Ground

Usage:

# Train variant
minimodem --rx uic-train

# Ground variant
minimodem --rx uic-ground

Frame Structure:

[8 start bits: 11111111][2 fixed: 10][37 data bits][0 stop bits]

UIC mode is receive-only (--tx is not supported). Decoding uses specialized frame format validation.

Reference: EC Railway Interoperability Technical Specification

Custom Protocols

You can create custom protocols by specifying all parameters:

# Custom protocol example: 600 baud, 7-bit ASCII, 2 stop bits
minimodem --rx \
  --mark 1200 \
  --space 1800 \
  --bandwidth 100 \
  -7 \
  --startbits 1 \
  --stopbits 2 \
  600

Advanced Options

Inverted Start/Stop Bits

Some protocols use inverted framing (start=mark, stop=space):

minimodem --rx --invert-start-stop 300

MSB First Bit Order

Transmit/receive most significant bit first:

minimodem --tx --msb-first 1200

Sync Byte Protocol

Create protocols with synchronization preambles:

minimodem --tx --sync-byte 0xAA 1200

This transmits 16 sync bytes before data payload.

USOS (Baudot Shift Mode)

Control LETTERS/FIGURES shift behavior in Baudot:

# 0 = USOS (always shift)
minimodem --rx --baudot --usos 0 45.45

# 1 = Don't shift (default)
minimodem --rx --baudot --usos 1 45.45

Protocol Comparison Table

Protocol	Baud Rate	Mark Hz	Space Hz	Data Bits	Stop Bits	Use Case
Bell 103	300	1270	1070	8	1.0	Classic modems
Bell 202	1200	1200	2200	8	1.0	Fast data, Caller ID
V.21	300	980	1180	8	1.0	International standard
RTTY	45.45	1585	1415	5	1.5	Amateur radio
TDD	45.45	1400	1800	5	2.0	Accessibility
SAME	520.83	2083.33	1562.5	8	0	Emergency alerts
Caller ID	1200	1200	2200	8	1.0	Phone ID
UIC-751-3	600	1300	1700	39	0	Railway control

Binary Protocols

For raw binary data without character encoding:

# Binary output mode (hex dump format)
minimodem --rx --binary-output 1200

# Raw binary mode (specify bit count per frame)
minimodem --rx --binary-raw 16 1200

Binary modes use databits_decode_binary() instead of ASCII or Baudot decoders.

Protocol Selection Logic

Minimodem’s protocol detection in src/minimodem.c:main():

Check for named protocol (rtty, tdd, same, callerid, uic, V.21)
If numeric value, use as custom baud rate
Apply protocol-specific defaults
Override with command-line options
Calculate automatic frequencies if not specified

// Example: Named protocol detection
if ( strncasecmp(modem_mode, "rtty", 5) == 0 ) {
    bfsk_databits_decode = databits_decode_baudot;
    bfsk_databits_encode = databits_encode_baudot;
    bfsk_data_rate = 45.45;
    if ( bfsk_n_data_bits == 0 )
        bfsk_n_data_bits = 5;
    if ( bfsk_nstopbits < 0 )
        bfsk_nstopbits = 1.5;
}

Get Started

Core Concepts

Usage Guides

Advanced

​Overview

​Bell System Protocols

​Bell 103 (300 baud)

Specifications

Framing

​Bell 202 (1200 baud)

Specifications

Framing

​ITU-T Standards

​V.21 (300 baud)

Specifications

Framing

​Telegraphy Protocols

​RTTY (Radio Teletype)

Specifications

Framing

​TDD/TTY (Telecommunications Device for the Deaf)

Specifications

Framing

​Emergency Alert Systems

​NOAA SAME (Specific Area Message Encoding)

Specifications

Framing

​Caller ID

​Bell 202 Caller ID

Specifications

Framing

​Railway Communications

​UIC-751-3 (Train/Ground)

Specifications

Framing

​Custom Protocols

​Advanced Options

​Protocol Comparison Table

​Binary Protocols

​Protocol Selection Logic

​See Also

Build docs developers (and LLMs) love

Overview

Bell System Protocols

Bell 103 (300 baud)

Bell 202 (1200 baud)

ITU-T Standards

V.21 (300 baud)

Telegraphy Protocols

RTTY (Radio Teletype)

TDD/TTY (Telecommunications Device for the Deaf)

Emergency Alert Systems

NOAA SAME (Specific Area Message Encoding)

Caller ID

Bell 202 Caller ID

Railway Communications

UIC-751-3 (Train/Ground)

Custom Protocols

Advanced Options

Protocol Comparison Table

Binary Protocols

Protocol Selection Logic

See Also