RetroModem Bridge is a Windows serial-to-TCP modem bridge for vintage computers. It lets a retro computer use a serial connection to "dial" Telnet-accessible BBSes through a modern Windows PC.
RetroModem Bridge acts like a simple modem-style bridge between a vintage computer and the internet.
You connect your retro computer to a Windows PC using a serial cable or USB-to-serial adapter. Then, from a terminal program on the retro computer, you can dial a BBS using commands like:
ATDT darkrealms.ca:23
ATDT1
ATDT coco
RetroModem Bridge receives the AT command, opens a TCP connection from the Windows PC, and passes data back and forth between the BBS and your retro computer.
In simple terms, your Windows computer handles the modern internet connection, while your retro computer gets the experience of using a modem.
See the full feature list here:
RetroModem Bridge 2.0 Features
RetroModem Bridge was originally built and tested with a Tandy / TRS-80 Color Computer 3 using a Deluxe RS-232 Pak, but it can also work with other systems that support serial communication and terminal software.
Examples include:
Tandy / TRS-80 Color Computer
Commodore
Apple II
Atari
Amiga
DOS PCs
Other serial-capable vintage computers
For my Tandy Color Computer 3 setup, I use a USB to DB25 RS-232 serial adapter connected to a Deluxe RS-232 Pak.
USB to DB25 RS-232 Serial Adapter
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RetroModem Bridge 2.0 includes:
Modern Windows interface
Large Start Bridge and Stop controls
Slim modem light status strip
COM port and baud rate selection
Remember COM port option
BBS Directory / Dial Aliases
Telnet BBS Guide import
Live log with timestamps
Basic Hayes-style AT command support
Telnet filtering
Beginner-friendly tooltips for technical options
The BBS Directory works like a phone book.
Instead of typing a full BBS address every time, you can save BBS entries and dial them with short aliases.
Example:
ATDT1
or:
ATDT coco
Default entries include:
| Alias | Name | Host | Port | Notes |
|---|---|---|---|---|
| 1 | Dark Realms | darkrealms.ca | 23 | ANSI BBS |
| coco | CoCoNet | coconet.ddns.net | 6809 | CoCoNet BBS |
The modem light strip shows connection and serial activity.
| Light | Meaning |
|---|---|
| DTR | The vintage computer is ready |
| RTS | The vintage computer wants to send data |
| CTS | The serial adapter says it is okay to send |
| DSR | The serial device says it is ready |
| DCD | Carrier detected, usually meaning connected |
| RX | Data is being received |
| TX | Data is being sent |
| ONLINE | The bridge is connected to a BBS |
Hover over the light strip in the app for plain-English help.
| Setting | Recommended value |
|---|---|
| Baud | 19200 if supported |
| Data | 8-N-1 |
| Flow control | None |
| DTR | Enabled |
| RTS | Enabled |
| Echo | Disabled unless typed commands do not appear |
| Telnet filter | Enabled |
| Default TCP port | 23 |
- Connect the vintage computer to the Windows PC using a serial connection.
- Open RetroModem Bridge.
- Select the correct COM port.
- Select the baud rate.
- Click Start Bridge.
- Open a terminal program on the vintage computer.
- Type
ATand press Enter. - Confirm the bridge replies with
OK. - Dial a BBS.
Examples:
ATDT darkrealms.ca:23
ATDT1
ATDT coco
AT
ATI
AT&V
ATDT host:port
ATDT1
ATDT coco
ATDL
ATH
ATE0
ATE1
If no port is included, RetroModem Bridge uses the Default TCP port field.
Example:
ATDT darkrealms.ca
With Default TCP port set to 23, this connects to:
darkrealms.ca:23
Download the latest Windows ZIP from the GitHub Releases page.
Extract the ZIP, then run:
RetroModemBridge.exe
To build from source, open the project folder in Visual Studio Code and run:
dotnet restore
dotnet buildRun the publish script:
.\publish-exe.ps1The release output can then be zipped and uploaded to GitHub Releases.
To build:
Windows
.NET 8 SDK
To run the published Windows build:
Windows x64
A serial port or USB-to-serial adapter
A retro computer with serial terminal software
RetroModem Bridge is not intended to be a full hardware modem emulator. It provides the core modem-style AT command behavior needed for many vintage terminal programs to connect to Telnet BBSes over TCP.