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 is based on a Microchip MCP2221 USB-to-UART bridge and a Texas Instruments MAX232 RS-232 transceiver, supporting communication up to 115,200 baud²³.

Design

The card uses an MCP2221 to convert USB to TTL UART, which is then fed into a MAX232 charge pump that brings the signal to RS-232 voltage levels (~±8 V), fully compliant with the RS-232 ±3–15 V specification²³. A 20 ms fast-blow fuse on the ground path provides basic protection².

The connector is a 3-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 setups²³. Pinout (left to right, looking into the card)³:

Pin	Signal
1	RXD (Receive data)
2	GND (Ground)
3	TXD (Transmit data)

Only the three-wire minimal RS-232 interface is exposed. This was a deliberate choice to keep the board hand-solderable and compact; hardware flow control (RTS/CTS) was omitted as the author found no devices requiring it among his test equipment¹. The PCB was manufactured by PCBWay².

Specifications

Specification	Details
Interface	RS-232 (3-wire: RXD, GND, TXD)23
USB bridge	Microchip MCP222123
RS-232 transceiver	Texas Instruments MAX23223
Max baud rate	115,2003
RS-232 signal level	~±8 V2
Connector	3-pin JST XH (2.5 mm pitch)23
PCB thickness	0.8 mm2
Isolation	Non-isolated (ground loop warnings apply)12
Protection	20 ms fast-blow fuse on ground path2
Enclosure	3D-printed case (friction-fit top)2
Source	medo64/RS232FrameCard (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

Footnotes

1. RS-232 Framework Expansion Card — Framework Community (Josip_Medved, May 2022) ↩ ↩² ↩³ ↩⁴

2. RS-232 Framework Expansion Card — Medo's Home Page (Josip Medved, May 2022) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷

3. medo64/RS232FrameCard — GitHub (MIT) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹²

4. RS-232 Framework Expansion Card store — Medo's Home Page ↩