High-performance RS485 library for STM32-based Arduino boards using DMA.

Designed for deterministic timing, low CPU usage, and industrial protocols (Modbus RTU, custom binary frames, etc.).

• Full-duplex RS485 using DMA (TX + RX)
• RX idle detection using UART IDLE
• Deterministic DE/RE timing
• Arduino Stream compatible
• Compatible with Arduino RS485 API (where meaningful)

• Arduino Opta (tested)
• STM32 boards with:
  • UART + DMA RX/TX
  • Optional DMAMUX (recommended)

Other STM32 targets are expected to work but require board-specific configuration.

Looking for testers for Portenta H7 / GIGA R1 / Machine Control — see STM32duino discussions/2903

• Arduino IDE: Library Manager (future)
• PlatformIO:

  lib_deps = nitrofmtl/STM32RS485DMA
  
  

#include <RS485DMA.h>


// On Opta, the RS485 object is already created.
// Do NOT instantiate RS485DMAClass yourself.
void setup() {
    RS485.begin(115200);
}

void loop() {
    if (RS485.available()) {
        int b = RS485.read();
        RS485.write(b);
    }
}

RX is enabled by default after begin().

However, because this library uses DMA, RX and TX cannot operate at the same time. When transmitting, RX DMA is temporarily disabled.

After a transmission completes, RX is not automatically re-enabled. You must explicitly call receive() to resume reception.

// Read incoming data
while (RS485.available()) {
    buffer[i++] = RS485.read();
}

// Transmit response
RS485.noReceive();
RS485.beginTransmission();
RS485.write(response, len);
RS485.endTransmission();

// Resume RX
RS485.receive();

RS485.setDelays(predelay_us, postdelay_us);
RS485.setRxIdleTime(idle_time_us);

setDelays(predelay_us, postdelay_us)

Controls driver enable timing (same behavior as ArduinoRS485):

predelay: time (µs) to wait after asserting DE before sending data

postdelay: time (µs) to wait after the last byte before deasserting DE

These values depend on the RS485 transceiver and baud rate.

Same semantics as ArduinoRS485.setDelays().

setRxIdleTime(idle_time_us) (DMA-specific)

Sets the additional idle time (in microseconds) required after the UART IDLE event before received data is considered complete.

The UART hardware detects IDLE after approximately one character time (minimum, hardware-defined). This function allows adding extra margin for noisy lines or custom protocols.

In most cases, the default value is sufficient and this function does not need to be used.

Typical usage:

Modbus RTU: ~3.5 character times

Custom protocols: protocol-dependent

Default value works for most cases

// Example: Modbus RTU at 115200 baud (~304 µs per char)
RS485.setRxIdleTime(1200);  // ≈ 4 characters

If unsure, you usually do not need to call this function.

This library provides an optional frame-based API for protocols that require deterministic frame boundaries, such as Modbus RTU or custom binary protocols.

• readFrame(buffer, bufferSize) reads a complete frame from the RX DMA buffer.
• Frames are armed on UART idle detection (IDLE event) and considered complete after a configurable post-delay.
• If a frame is not consumed in time and new data arrives, the frame is dropped (overflow).
• Frame boundaries are determined without blocking the CPU, using DMA and idle detection.

uint8_t buffer[50];
size_t len = RS485.readFrame(buffer, sizeof(buffer));

if (len > 0) {
    // Process the complete frame
    processFrame(buffer, len);
}

Notes: readFrame() returns the number of bytes available in the current frame. Frames that are not consumed before the next one starts are discarded (overflow). This API is optional; standard available() / read() calls still work for stream-based parsing.

If a new idle event (end of a frame) is detected before the previous frame has been fully consumed, the previous frame is dropped.
This ensures that:

• The DMA buffer only contains valid, complete frames.
• readFrame() always returns the most recent complete frame.
• If you need to process every byte without dropping, use available() + read() instead.

Tip: Always call readFrame() as soon as data is available to minimize dropped frames.