Added example to demonstrate using HardwareSerial with RS485 interfaces

libraries/ESP32/examples/Serial/RS485_Echo_Demo/RS485_Echo_Demo.ino

@@ -0,0 +1,49 @@
+/*
+  This Sketch demonstrates how to use the Hardware Serial peripheral to communicate over an RS485 bus. 
+
+  Data received on the primary serial port is relayed to the bus acting as an RS485 interface and vice versa.
+
+  UART to RS485 translation hardware (e.g., MAX485, MAX33046E, ADM483) is assumed to be configured in half-duplex
+  mode with collision detection as described in
+  https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/peripherals/uart.html#circuit-a-collision-detection-circuit
+
+  To use the script open the Arduino serial monitor (or alternative serial monitor on the Arduino port). Then,
+  using an RS485 tranciver, connect another serial monitor to the RS485 port. Entering data on one terminal
+  should be displayed on the other terminal.
+*/
+
+#include <HardwareSerial.h>
+#include "hal/uart_types.h"
+
+#define RS485_RX_PIN 16
+#define RS485_TX_PIN 5
+#define RS485_RTS_PIN 37
+
+HardwareSerial RS485(2);
+
+void setup() {
+  Serial.begin(9600);
+  delay(2000);
+
+  RS485.begin(9600, SERIAL_8N1, RS485_RX_PIN, RS485_TX_PIN);
+  if(!RS485.setPins(-1, -1, -1, RS485_RTS_PIN)){
+    Serial.print("Failed to set RS485 pins");
+  }
+
+  // Certain versions of Arduino core don't define MODE_RS485_HALF_DUPLEX and so fail to compile.
+  // By using UART_MODE_RS485_HALF_DUPLEX defined in hal/uart_types.h we work around this problem.
+  // If using a newer IDF and Arduino core you can ommit including hal/uart_types.h and use MODE_RS485_HALF_DUPLEX
+  // defined in esp32-hal-uart.h (included during other build steps) instead.
+  if(!RS485.setMode(UART_MODE_RS485_HALF_DUPLEX)) {
+   Serial.print("Failed to set RS485 mode");
+  }
+}
+
+void loop() {
+  if (RS485.available()) {
+    Serial.write(RS485.read());
+  }
+  if (Serial.available()) {
+    RS485.write(Serial.read());
+  }
+}