RS-485 link on an Arduino Uno

// Two MAX485 modules on one twisted pair: RS-485 in one direction.
//
// RS-485 sends each bit as the difference between two wires, A and B, so it
// runs hundreds of meters through electrical noise. A MAX485 module turns a
// UART into that pair and back. Here one Uno does both ends: the sender on
// its hardware Serial, the receiver on SoftwareSerial, with the pair between.
//
//   sender    DI -> TX (pin 1)   DE and RE -> pin 4    VCC -> 5V, GND -> GND
//   receiver  RO -> pin 8        DE and RE -> GND      VCC -> 5V, GND -> GND
//   the pair  A -> A, B -> B
//
// Every second the sketch raises pin 4 (talk), sends "ping" and a number,
// waits for the last bit with Serial.flush(), then drops pin 4 (listen). The
// receiver hears it on pin 8 and the sketch prints what came back. The
// serial monitor shows both, because hardware Serial is also the USB port.
//
// THINGS TO TRY. Swap A and B on one module: every bit arrives inverted and
// nothing comes back. Or take the Serial.flush() out: pin 4 drops while the
// UART is still sending, and the end of each message is cut off.

#include <SoftwareSerial.h>

const int DIR = 4;              // the sender's DE and RE, tied together
SoftwareSerial bus(8, 9);       // RX on pin 8 from the receiver's RO; 9 unused

unsigned long count = 0;
unsigned long last = 0;
char line[32];
uint8_t len = 0;

void setup() {
  pinMode(DIR, OUTPUT);
  digitalWrite(DIR, LOW);
  Serial.begin(9600);
  bus.begin(9600);
  Serial.println("RS-485 link: sender on TX, receiver on pin 8");
}

void loop() {
  if (millis() - last >= 1000) {
    last = millis();
    count++;
    digitalWrite(DIR, HIGH);    // talk
    Serial.print("ping ");
    Serial.println(count);
    Serial.flush();             // wait for the last bit before letting go
    digitalWrite(DIR, LOW);     // listen
  }
  while (bus.available()) {
    char c = bus.read();
    if (c == '\r') continue;
    if (c == '\n') {
      line[len] = 0;
      if (strncmp(line, "ping ", 5) == 0) {
        Serial.print("received over the pair: ");
        Serial.println(line);
      }
      len = 0;
    } else if (len < sizeof(line) - 1) {
      line[len++] = c;
    }
  }
}