RS485_BRIDGE

Compact RS-485 interface / bridge board for embedded projects that need robust differential serial communication over longer cables or noisier environments.

RS485_BRIDGE is a utility communication board for connecting a microcontroller UART to an RS-485 bus. It is intended for custom PCB projects that need reliable wired communication between boards, modules, fixtures, or controller nodes.

Project Status

Hardware repository / prototype support board.

System Role

Microcontroller UART
        ↓ TX / RX / DE / RE
RS485_BRIDGE
        ↓ A / B differential bus
RS-485 cable / remote node

RS485_BRIDGE handles the physical-layer conversion between local UART logic and the external RS-485 differential pair.

Key Specs

Item	Value
Board role	UART to RS-485 bridge/interface
Communication type	Half-duplex RS-485
Local side	MCU UART/control pins
Bus side	A/B differential pair
Typical use	Wired board-to-board communication
Power	Low-voltage logic rail from host/project
Project type	Reusable communication helper board

Main Features

• RS-485 transceiver interface
• UART TX/RX local-side connection
• Driver-enable / receiver-enable control support
• A/B differential bus output
• Bus connector for external wiring
• Logic/power connector for host board
• Labeled silkscreen for fast bench wiring
• Useful companion board for STM32, ESP32, Arduino-style, and other MCU projects

Typical Connections

Local MCU side:

MCU TX   → RS485_BRIDGE DI / TX input
MCU RX   ← RS485_BRIDGE RO / RX output
MCU GPIO → DE / RE control
GND      ↔ GND
VCC      → board logic power

RS-485 bus side:

A → RS-485 A line
B → RS-485 B line
GND/shield/reference as required by system wiring

Always verify A/B polarity with the transceiver datasheet and the connected device. Some vendors label A/B opposite of others. Tiny alphabet gremlin, very real.

Control Signals

Signal	Purpose
TX / DI	UART data into RS-485 driver
RX / RO	UART data out from RS-485 receiver
DE	Driver enable
RE / nRE	Receiver enable
A	RS-485 differential bus line
B	RS-485 differential bus line
VCC	Logic/transceiver supply
GND	Shared reference

Half-Duplex Direction Control

For half-duplex RS-485, firmware usually controls transmit/receive direction:

Receive mode:  driver disabled, receiver enabled
Transmit mode: driver enabled, receiver optionally disabled

A common firmware pattern:

set DE high
send UART packet
wait for transmit complete
set DE low
return to receive mode

Bus Wiring Notes

• Use twisted pair for A/B where possible.
• Keep A/B routed as a pair.
• Add termination only where appropriate for the bus topology.
• Bias/failsafe resistors depend on the system and transceiver behavior.
• Share a ground/reference path between nodes when required by the system design.
• Avoid star wiring for longer RS-485 buses.
• Keep stubs short.

Termination / Biasing

RS-485 systems often need termination and biasing, but the correct setup depends on cable length, speed, number of nodes, and whether another device already provides those parts.

Typical concepts:

termination: resistor across A/B at bus ends
biasing: weak pull-up/pull-down network to define idle bus state

Bring-Up Checklist

1. Inspect connector orientation and soldering.
2. Confirm VCC and GND are correct before connecting the host MCU.
3. Confirm local UART logic voltage is compatible with the transceiver.
4. Verify TX/RX direction between MCU and RS485_BRIDGE.
5. Confirm DE/RE control polarity.
6. Test local UART/known packet output.
7. Connect a second RS-485 device or USB-RS485 adapter.
8. Verify A/B polarity.
9. Test receive mode.
10. Test transmit mode.
11. Add termination/bias only as needed for the final bus layout.

Design Notes

• Keep the A/B differential traces routed together.
• Keep bus connector labels obvious.
• Keep logic-side labels obvious: TX, RX, DE, RE, VCC, GND.
• Put termination/bias options where they can be populated or left open intentionally.
• Keep the board small and reusable as a communication brick for future projects.

Repository Output Images

Expected image paths for GitHub README rendering:

Outputs/
  IMG/
    3D-T.png
    3D-B.png
    SCHEMATIC.png
    L1-SIG.png
    L2-GND.png

Safety / Design Note

RS485_BRIDGE is a low-voltage communication interface board. Verify voltage levels, A/B polarity, shared ground strategy, termination, and bus wiring before connecting it to another system.

Designed & engineered by Brandon Shelly