Two Unos over one pair

// RS485 ping-pong between two Unos, MAX485 (5V) boards.
//
// Wiring, each Uno to its own board:
//   Board GND -> Uno GND     Board 5V -> Uno 5V
//   Board TX  -> Uno pin 10  (the board's TX is the chip's RO)
//   Board RX  -> Uno pin 11
//   Board DE  -> Uno pin 2   (RE is joined to DE on the board)
// Terminals: A to A, B to B, GND to GND.
// Keep both 120R fitted: two boards are the two ends.
//
// Arduino IDE: Tools > Board > Arduino AVR Boards >
// Arduino Uno, then Tools > Port. Serial Monitor 9600.
// SoftwareSerial comes with the IDE.
//
// Upload with TALKER true to one Uno, false to the other.

#include <SoftwareSerial.h>

const bool TALKER = true;
const int DIR = 2;

SoftwareSerial bus(10, 11);  // RX, TX
String line;
int count = 0;
unsigned long last = 0;

void send(const String &msg) {
  digitalWrite(DIR, HIGH);      // take the bus
  delayMicroseconds(200);
  bus.println(msg);
  bus.flush();
  digitalWrite(DIR, LOW);       // let go
}

void setup() {
  pinMode(DIR, OUTPUT);
  digitalWrite(DIR, LOW);       // listen first
  Serial.begin(9600);
  bus.begin(9600);
  Serial.println(TALKER ? "talker" : "listener");
}

void loop() {
  if (TALKER && millis() - last >= 1000) {
    last = millis();
    count++;
    send("ping " + String(count));
  }

  while (bus.available()) {
    char c = bus.read();
    if (c != '\n') {
      if (line.length() < 30) line += c;
      continue;
    }
    line.trim();
    if (!TALKER && line.startsWith("ping ")) {
      Serial.println("heard " + line);
      send("pong " + line.substring(5));
    } else if (TALKER && line.startsWith("pong ")) {
      Serial.println("reply " + line);
    }
    line = "";
  }
}