RS-485 Expansion Card

The RS-485 Expansion Card is a community-designed expansion card adding an RS-485 serial interface to any Framework laptop¹. Created by Josip Medved (medo64) and published in November 2022, it uses an FTDI FT232RL UART bridge with automatic transmit control and an SP485EN RS-485 transceiver, supporting baud rates up to 3 Mbaud¹².

Design

The card uses FTDI's FT232RL for USB-to-UART conversion, chosen for its built-in TXDEN pin (automatic flow control) and crystal-less operation¹. The RS-485 transceiver is an SP485EN in an easily hand-solderable DIP package, rated up to 5 Mbps¹. Biasing resistors (130/750 Ω) improve bus resiliency across different network configurations, and a 120 Ω termination resistor is included onboard¹².

A MOSFET keeps the transceiver fully off during USB suspend. Protection includes SM712 TVS diodes on the data lines and fast fuses on both data and ground paths¹. The card is non-isolated — disconnecting AC power (running on battery) is recommended when connecting to unknown potentials¹.

Pinout

The 4-pin JST XH connector (2.5 mm pitch) uses the last pin pulled out for keying¹²:

Pin	Signal	Description
1	GND	Ground
2	A	Negative (low for logic 1, high for logic 0)
3	B	Positive (high for logic 1, low for logic 0)
4	5V	100 mA power supply output

The 4-pin connector makes it distinctively different from Medved's RS-232 card (3-pin, GND in center), preventing accidental misconnection¹.

Specifications

Specification	Details
Interface	RS-485 (half-duplex)12
USB bridge	FTDI FT232RL12
Transceiver	SP485EN1
Max baud rate	3,000,0002
Connector	4-pin JST XH (2.5 mm pitch)12
Termination	120 Ω onboard2
Protection	SM712 TVS diodes, fast fuses on data and ground1
Isolation	Non-isolated1
PCB thickness	0.8 mm1
Source	medo64/RS485FrameCard (GitHub)2
License	MIT2

Availability

The KiCad design files are open-source (MIT) on GitHub². Boards were sponsored by PCBWay¹. Medved also offers a CAN-bus expansion card and RS-232 card using the same JST XH connector family¹³.

Related Projects

Project	Description
RS-232 Expansion Card	Medved's RS-232 card using MCP2221 + MAX232
LTE Cat 4 Cell Modem Card	Community expansion card adding cellular connectivity

Footnotes

1. RS-485 Framework Expansion Card (FTDI Edition) — Medo's Home Page (Josip Medved, November 2022) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸

2. medo64/RS485FrameCard — GitHub (MIT) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹

3. RS-232 Framework Expansion Card — Framework Community (May 2022) ↩