DV4mini DMR Hotspot

A lightweight C++ hotspot bridge that connects a DV4mini USB dongle to a DMR master server using the MMDVM/Homebrew protocol.

The application moves DMR traffic in both directions:

DMR radio  <->  DV4mini  <->  dv4mini-hs  <->  DMR master

It is designed for FreeDMR-compatible/Homebrew master connections and includes configurable talkgroup filtering, RF/TX tuning, RSSI monitoring, serial handling, and automatic reconnects.

Features

• Bidirectional RF -> network and network -> RF DMR bridging
• MMDVM/Homebrew master login, authentication, configuration, and options exchange
• Windows and POSIX serial-port support at 115200 baud
• Configurable RX/TX frequency and independent frequency offsets
• Configurable DV4mini frequency byte order (MSB or LSB)
• Automatic RF receive detection for bit packing and polarity
• Configurable TX bit order and polarity
• Raw DMRD frame passthrough or sync/slot/embedded-field rewrite mode
• MS-sourced, BS-sourced, or direct-mode TX sync selection
• Firmware-buffered or host-paced network-to-RF transmission
• Configurable USB batching and repeated voice LC headers
• Primary, listen, and dial talkgroup handling
• Optional network talkgroup filtering and private-call filtering
• FreeDMR-style options generation when no explicit options string is supplied
• RSSI monitoring with an automatically learned idle baseline
• DV4mini LED status control
• Automatic DMR master reconnection
• Simulation mode for development/testing without a physical serial device
• Graceful shutdown with bridged-frame counters

Requirements

Linux / POSIX

• A C++11-capable compiler such as g++
• GNU Make or a compatible make
• Access to the DV4mini serial device, typically /dev/ttyACM0

No third-party C++ libraries are required by the project.

Windows

The included build.bat expects w64devkit in:

C:\w64devkit\bin

If your compiler is installed elsewhere, update the PATH line in build.bat or build with your own MinGW-w64 environment.

Build

Linux / POSIX

make

The resulting executable is:

dv4mini-hs

Clean the build with:

make clean

Windows

Run:

build.bat

or, from a configured MinGW-w64 shell:

mingw32-make clean
mingw32-make

The resulting executable is:

dv4mini-hs.exe

Configuration

The program reads config.ini from the current directory by default.

Start from the included example and replace the callsign, DMR ID, serial device, master address, password, frequency, and talkgroups with values appropriate for your setup.

Example:

[DV4Mini]
Device=/dev/ttyACM0
Frequency=438200000
FrequencyByteOrder=MSB
TxBitOrder=MSB
TxInvert=false
TxFrameMode=PASSTHROUGH
TxSync=MS
TxFrameIntervalMs=60
TxFeedMode=FIRMWARE
TxUsbBatchPeriods=3
TxHeaderRepeats=8
RxOffsetHz=0
TxOffsetHz=0
TxBuffer=600
Power=3
Simulation=false
RSSIMonitor=true

[Hotspot]
DMRId=1234567
Callsign=N0CALL
Latitude=0
Longitude=0
Height=0
Location=Home
Description=DV4mini hotspot
Homepage=
TxPowerDbm=0

[Master]
Host=master.example.net
Port=62031
LocalPort=62032
Password=change-me
Reconnect=10
SendOptions=true
Options=

[DMR]
ColorCode=1
TimeSlot=2
Talkgroup=262
ListenTalkgroups=262,2626,26269
DialTalkgroup=0
TalkgroupTimer=10
AllowTxToListenTalkgroups=true
AllowPrivateCalls=true
FilterNetworkTalkgroups=false

[Logging]
Level=5

On Windows, use a COM port instead, for example:

Device=COM10

[DV4Mini]

Setting	Description
Device	Serial device. Default is COM3 on Windows and /dev/ttyACM0 elsewhere.
Frequency	Base RF frequency in Hz.
FrequencyByteOrder	Frequency command byte order: MSB or LSB.
TxBitOrder	TX bit packing. MSB selects MSB-first; other values are treated as LSB-first.
TxInvert	Invert transmitted DMR bits when true.
TxFrameMode	PASSTHROUGH preserves incoming 33-byte DMRD frames. REWRITE regenerates sync/slot/embedded fields.
TxSync	Sync used in rewrite mode: MS, BS, or DIRECT.
TxFrameIntervalMs	Host-paced frame interval; clamped to 20-100 ms.
TxFeedMode	FIRMWARE uses the DV4mini transmit buffer. HOST, HOSTPACED, or PACED enables host pacing.
TxUsbBatchPeriods	Number of 72-byte TX periods per USB write; clamped to 1-3.
TxHeaderRepeats	Voice LC header repeats before audio; clamped to 1-12.
RxOffsetHz	Added to the configured base frequency for RX tuning.
TxOffsetHz	Added to the configured base frequency for TX tuning.
TxBuffer	DV4mini TX buffer size in milliseconds. Valid range: 100-1500.
Power	DV4mini firmware power level. Valid range: 0-9.
Simulation	Run the DV4mini interface in simulation mode without opening a physical serial port.
RSSIMonitor	Enable RSSI baseline learning and periodic RF-level output.

PASSTHROUGH is the recommended TX frame mode when the incoming DMRD frames already contain the correct on-air fields.

[Hotspot]

Setting	Description
DMRId	Hotspot/repeater DMR ID. Must be greater than zero.
Callsign	Hotspot callsign; maximum 8 characters.
Latitude	Latitude from -90 to 90.
Longitude	Longitude from -180 to 180.
Height	Antenna/site height value sent to the master.
Location	Location text; maximum 20 characters.
Description	Description text; maximum 20 characters.
Homepage	Homepage URL; maximum 124 characters.
TxPowerDbm	TX power metadata reported to the master in dBm.

[Master]

Setting	Description
Host	DMR master hostname or IP address. Required.
Port	Remote UDP port, normally 62031.
LocalPort	Local UDP port. Use 0 to let the OS choose automatically.
Password	Master-server password.
Reconnect	Seconds between reconnect attempts; minimum 1 second.
SendOptions	Send a master options string after connecting.
Options	Explicit options string. Leave empty to generate a FreeDMR-style string from the DMR settings.

When SendOptions=true and Options is empty, the program builds an options string from the selected time slot, primary/listen talkgroups, dial talkgroup, and talkgroup timer.

[DMR]

Setting	Description
ColorCode	DMR color code, 0-15.
TimeSlot	DMR time slot, 1 or 2.
Talkgroup	Primary group-call talkgroup.
ListenTalkgroups	Comma- or semicolon-separated list of additional talkgroups.
DialTalkgroup	Optional dial/dynamic talkgroup. Set to 0 to disable.
TalkgroupTimer	Timer value used when generating FreeDMR options.
AllowTxToListenTalkgroups	Allow RF-originated transmissions to talkgroups listed in ListenTalkgroups.
AllowPrivateCalls	Permit private calls in both directions.
FilterNetworkTalkgroups	When enabled, drop network group calls that are not in the local RX talkgroup list.

Talkgroup IDs are limited to the 24-bit DMR ID range (1-16777215). Invalid entries in a talkgroup list are ignored by the INI parser.

[Logging]

Level controls console diagnostics. The source uses progressively more detailed output at higher levels, including additional serial/protocol diagnostics around levels 10 and 20.

Run

With the default configuration file:

./dv4mini-hs

On Windows:

dv4mini-hs.exe

Use a different INI file with -c:

./dv4mini-hs -c my-hotspot.ini

Display command-line help:

./dv4mini-hs --help

Stop the hotspot with Ctrl+C. The program closes the master connection, flushes the DV4mini, and prints the RF-to-network and network-to-RF frame totals before exiting.

Talkgroup behavior

RF -> network

Group calls are accepted for:

• the primary Talkgroup
• entries in ListenTalkgroups when AllowTxToListenTalkgroups=true
• TG9 when DialTalkgroup is enabled

Other group calls are dropped. Private calls are dropped when AllowPrivateCalls=false.

Network -> RF

When FilterNetworkTalkgroups=true, group calls are accepted only when they match the configured local receive list. The receive list is built from the primary talkgroup, listen talkgroups, and the dial-related talkgroups when configured.

Frames for the wrong time slot or frames that cannot be transmitted while RF is busy are rejected by the bridge.

TX modes

Firmware-buffered mode

TxFeedMode=FIRMWARE

Network frames are handed to the DV4mini without host-side frame throttling. The DV4mini TX buffer is controlled by TxBuffer.

Host-paced mode

TxFeedMode=HOST
TxFrameIntervalMs=60

Network-to-RF frames are queued and released by the host at the configured interval. The queue is bounded; if it grows too large, the oldest frame is dropped.

RSSI and LEDs

With RSSIMonitor=true, the application learns an idle RSSI baseline from initial valid samples and reports changes relative to that baseline.

LED behavior reported by the application:

• Blue: idle / firmware-controlled state
• Red: TX / firmware-controlled state
• Green: decoded RF receive activity, controlled by the host

The green LED indicates decoded DMR activity, not merely RF energy.

Linux serial permissions

If the program reports:

Unable to open DV4mini device '/dev/ttyACM0'.

check that the configured device exists and that your user has permission to open it. On many Linux distributions, serial devices are assigned to a group such as dialout or uucp.

Also verify that another application is not already using the DV4mini.

Troubleshooting

Master does not connect

Check:

• Host and Port
• Password
• DMRId and Callsign
• local firewall rules for UDP traffic
• whether LocalPort conflicts with another process

The program prints each connection phase: disconnected, login, authentication, configuration, options, and connected.

Frequency is incorrect or the dongle does not tune as expected

Verify:

FrequencyByteOrder=MSB

Some firmware variants may require LSB. Also check RxOffsetHz and TxOffsetHz.

RF is visible but DMR is not decoded

Check the configured frequency and color code first. The receive parser automatically probes common bit-packing/polarity transformations, while the RSSI monitor can help distinguish RF energy from successfully decoded DMR frames.

Network traffic is connected but nothing is transmitted over RF

Check:

• TimeSlot
• FilterNetworkTalkgroups
• the primary/listen talkgroup lists
• AllowPrivateCalls
• TxFrameMode
• TxBitOrder and TxInvert
• whether the RF side is already busy

Raise Logging.Level for additional diagnostics.

Project layout

.
├── build.bat
├── config.ini
├── include/
│   ├── config.h
│   ├── dmrhotspot.h
│   ├── dv4mini.h
│   ├── homebrewclient.h
│   ├── homebrewpacket.h
│   ├── platform.h
│   ├── rs232lib.h
│   └── ...
├── resource/
│   ├── dv4mini.ico
│   └── dv4mini.rc
├── source/
│   ├── config.cpp
│   ├── dmrhotspot.cpp
│   ├── dv4mini-hotspot.cpp
│   ├── dv4mini.cpp
│   ├── homebrewclient.cpp
│   ├── homebrewpacket.cpp
│   ├── rs232lib.cpp
│   └── ...
├── LICENSE
└── makefile

Notes on radio operation

Configure frequencies, power, talkgroups, IDs, and operating parameters for your own station and network. Make sure your use of the DV4mini and DMR frequencies complies with the licensing rules and band plan that apply in your location.

Do not publish a real master password in a public Git repository. Keep secrets in a local configuration file or use a private deployment workflow.

License

This project is released under the MIT License. See LICENSE for the full text.