RS-232 Expansion Card
The RS-232 Expansion Card is a community-designed expansion card that adds a serial RS-232 port to any Framework laptop12. Created by Josip Medved (medo64) and first published in May 2022, the card originally used a Microchip MCP2221 USB-to-UART bridge and a Texas Instruments MAX232 RS-232 transceiver23. In November 2023, the design was reworked in KiCad to use an FTDI FT232R USB-to-serial bridge with a 5-pin connector and hardware flow control (RTS/CTS)3.
Design
The current revision (reworked November 2023) uses an FTDI FT232R to convert USB to RS-232 signal levels via a MAX232-compatible charge pump, bringing the signal to RS-232 voltage levels (~±8 V), fully compliant with the RS-232 ±3–15 V specification23. The original May 2022 design used a Microchip MCP2221 instead.
The connector is a 5-pin shrouded JST XH header (2.5 mm pitch), compatible with standard 0.1" jumper wires for testing or pre-crimped JST XH cables for permanent setups. The 5-pin revision added RTS/CTS hardware flow control that was omitted in the original 3-pin design3. Pinout (left to right, looking into the card)3:
| Pin | Signal | DB9 Equivalent |
|---|---|---|
| 1 | CTS (Clear to Send) | Pin 8 |
| 2 | RXD (Receive data) | Pin 2 |
| 3 | GND (Ground) | Pin 5 |
| 4 | TXD (Transmit data) | Pin 3 |
| 5 | RTS (Request to Send) | Pin 7 |
The original 3-pin design exposed only RXD, GND, and TXD. The reworked 5-pin design added RTS/CTS hardware flow control, making the card compatible with devices that require full handshaking3. The PCB is designed in KiCad3.
Specifications
| Specification | Details |
|---|---|
| Interface | RS-232 (5-wire: CTS, RXD, GND, TXD, RTS) — current revision; 3-wire (RXD, GND, TXD) on original23 |
| USB bridge | FTDI FT232R (current revision, Nov 2023 rework); Microchip MCP2221 (original May 2022)23 |
| RS-232 transceiver | MAX232-compatible23 |
| Max baud rate | 115,2003 |
| RS-232 signal level | ~±8 V2 |
| Connector | 5-pin JST XH (2.5 mm pitch) — current; 3-pin on original23 |
| Hardware flow control | RTS/CTS (current revision only)3 |
| PCB thickness | 0.8 mm2 |
| Isolation | Non-isolated (ground loop warnings apply)12 |
| Protection | 20 ms fast-blow fuse on ground path (original design)2 |
| Enclosure | 3D-printed case (friction-fit top)2 |
| Source | medo64/Rs232FEC (GitHub)3 |
| License | MIT3 |
Safety
The card is non-isolated — all grounds are connected together. Misconnecting a non-ground signal to the ground pin could damage the card or laptop12. The author recommends disconnecting AC power before testing connections, allowing the laptop's ground to float and relying on the MAX232's ±30 V tolerance2.
Availability
Medved sells hand-assembled, tested units enclosed in 3D-printed cases for $30 with free shipping within the continental US4. The KiCad design files and firmware are open-source (MIT) on GitHub for those who wish to manufacture their own3.
Related Projects
| Project | Description |
|---|---|
| LTE Cat 4 Cell Modem Card | Community expansion card adding cellular connectivity |
| EC Card 2 | UART-based EC debug expansion card for FW13 |