A high-performance RS485 driver with DMA support for the Arduino Due.
Designed as a drop-in replacement for the standard ArduinoRS485 library, with optional integration into ArduinoModbus.

• DMA-based transmit for minimal CPU usage
• Compatible with ArduinoRS485 API
• Works with ArduinoModbus (with a slightly modified version, via compile-time switch)
• Automatic DE/RE pin handling
• Drop-in replacement for existing RS485 sketches

Until the pull request to the official ArduinoCore-sam is accepted, you must manually patch your Arduino core:

⚠️ Required Core Patch

To use DUERS485DMA, you must make the USARTx_Handler(void) weak so the library can override it.

This patch does not change the normal functionality of the core:

• If DUERS485DMA is not used, the original functions behave as before.
• If DUERS485DMA is included, it overrides them where needed.

You can choose to make all predefined USART USARTx_Handler(void) functions weak, or only the ones you intend to use.

Note: USART2_Handler(void) was not predefined in the core, so no patch is needed there.

📂 File locations

Arduino IDE (Windows):

C:\Users\<YourName>\AppData\Local\Arduino15\packages\arduino\hardware\sam\1.6.12\variants\arduino_due_x\variant.cpp

PlatformIO:

Linux / macOS:

~/.platformio/packages/framework-arduinosam/variants/arduino_due_x/variant.cpp

Windows:

C:\Users\<YourName>\.platformio\packages\framework-arduinosam\variants\arduino_due_x\variant.cpp

Handlers defined in ArduinoCore-sam

In variant.cpp for the Arduino Due, you will find:

void USART0_Handler(void) { Serial1.IrqHandler(); } //Serial1
void USART1_Handler(void) { Serial2.IrqHandler(); } //serial2
void USART3_Handler(void) { Serial3.IrqHandler(); } //Serial3
// Note: USART2_Handler is not defined in the core, nothing to do

Before

void USART0_Handler(void) {
  // original IRQ handler code
}

After

__attribute__((weak)) void USART0_Handler(void) {
  // original IRQ handler code
}

The Arduino Due has 4 USART ports available for RS485.
Each must be enabled with a macro before including the library:

#define USE_RS485_SERIAL1 // enable RS485 on Serial1
#define USE_RS485_SERIAL2 // enable RS485 on Serial2
#define USE_RS485_SERIAL3 // enable RS485 on Serial3
#define USE_RS485_USART2  // enable RS485 on USART2

Arduino IDE

The official Arduino SAM core (DUE) has been abandoned and does not build with Arduino IDE 2.x.
This library has been tested with PlatformIO and may work with Arduino IDE 1.8.x, but support for IDE ≥ 2.0 is not guaranteed.

👉 Recommendation: Use PlatformIO, which (as of today) still supports the SAM3X8E (Arduino DUE).

1. Download or clone this repository into your libraries/ folder.

2. Restart the Arduino IDE.

3. In your sketch, enable DMA mode and define which RS485 ports you want to use before including the library:

#define USE_RS485_SERIAL1      // enable RS485 on Serial1, must be set before include
#include <DUERS485DMA.h>

⚠️ Note: Enabling RS485 on a USART port will disable the standard SerialX object for that port to prevent misusage.

1. install the library:

pio pkg install -l nitrofmtl/duers485dma

2. Add to your platformio.ini

lib_deps = 
    yourname/DUERS485DMA
build_flags = -DUSE_RS485_SERIAL1

3. include the library:

#include <DUERS485DMA.h>

This ensures the library is compiled with DMA enabled.

✅ After this, Arduino IDE and PlatformIO usage is identical.

You can define a default alias for convenience:

#define RS485 RS485_SERIAL1 //use RS485 Serial1 as default RS485 name

#define BAUDRATE 9600

void setup() {
  RS485.begin(BAUDRATE);
  //preDealy should be very short, postDelay is not need (see note)
  RS485.setDelays(50, 0);

}

void loop() {
  RS485.beginTransmission();
  RS485.write("Ping");
  RS485.endTransmission();
  delay(1000);
}

On SAM3X (Arduino Due), flush() waits for US_CSR_TXEMPTY, meaning the last stop bit is fully transmitted. Therefore, additional post-transmission delay is unnecessary and may interfere with Modbus timing. Default postDelay = 0.

DUERS485DMA can also be used with ArduinoModbus.
However, the official library does not currently support external RS485 backends.

➡️ To use with DMA, clone and install this fork:
HAVE TO CHANGE FOR BRANCH UPTADED DUERS485DMA-Compatible ArduinoModbus

Usage example:

#define USE_DUERS485DMA
#define USE_RS485_SERIAL1
#include <DUERS485DMA.h>
#include <ArduinoModbus.h>

ModbusRTUClientClass ModbusClient(RS485_SERIAL1);

void setup() {
  RS485.begin(9600);
  ModbusClient.begin(1, 9600); // nodeId = 1
}

This library exposes additional helper functions for advanced timing control and DMA-based RS485 operation. These functions are optional and are not part of the standard ArduinoRS485 API.

setTxTimeoutGuard(int timeoutUs);

This timeout prevents the application from blocking indefinitely if transmission does not complete as expected (e.g. bus contention or hardware fault).

timeoutUs: timeout in microseconds

Notes

This is a safety guard, not a protocol timing parameter.

It does not affect baud rate or frame timing.

setRXIdleTime(uint32_t idleTimeUs)

This value represents the minimum duration of RX inactivity required to consider a frame complete.

idleTimeUs: idle duration in microseconds

Typical values

Modbus RTU: getUsecForNChar(3.5f)

Custom protocols: protocol-dependent

Notes

This is distinct from the hardware RX silence timeout (RTOR).

This function does not block.

getUsecForNChar(float nChar)

This function accounts for serial mode; data bits, parity, stop bits, baud rate

Example

uint32_t modbusIdle = getUsecForNChar(3.5f);
setRXIdleTime(modbusIdle);

isRxIdle()

Returns:

true if RX frame idle time has elapsed

false otherwise

Usage

if (isRxIdle()) {
    // Safe to process received frame
}

Notes

This function does not block. It relies on RX timeout interrupts and internal timestamps.