RS-232 loopback on an Arduino Uno

// A MAX232 and a loopback plug: the Uno's serial at RS-232 levels and back.
//
// A PC's COM port, a lab instrument or a CNC controller speaks RS-232: about
// -7 V for a 1 and +7 V for a 0. A board's 0 V and 5 V are the wrong levels,
// the wrong way round. The MAX232 makes its own plus and minus supplies from
// 5 V with a charge pump and its capacitors, and turns TTL into RS-232 and
// back.
//
//   T1IN  <- TX (pin 1)     T1OUT -> R1IN, the loopback jumper
//   R1OUT -> pin 8          C1 and C2, the pump capacitors, 1 uF each
//   VCC -> 5V, GND -> GND
//
// The jumper from T1OUT to R1IN is what a technician plugs into a port to
// test a cable: whatever goes out comes straight back. Every second the
// sketch sends "ping" and a number on Serial, and SoftwareSerial on pin 8
// hears it come back through both halves of the chip.
//
// THINGS TO TRY. Pull the jumper: nothing comes back, and R1OUT sits high,
// because an open RS-232 input reads as a mark. Or take C2 out: there is no
// negative rail, and the chip says no C1/C2.

#include <SoftwareSerial.h>

SoftwareSerial back(8, 9);      // RX on pin 8 from R1OUT; 9 unused

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

void setup() {
  Serial.begin(9600);
  back.begin(9600);
  Serial.println("RS-232 loopback: TX out through the MAX232, back on pin 8");
}

void loop() {
  if (millis() - last >= 1000) {
    last = millis();
    count++;
    Serial.print("ping ");
    Serial.println(count);
  }
  while (back.available()) {
    char c = back.read();
    if (c == '\r') continue;
    if (c == '\n') {
      line[len] = 0;
      if (strncmp(line, "ping ", 5) == 0) {
        Serial.print("came back: ");
        Serial.println(line);
      }
      len = 0;
    } else if (len < sizeof(line) - 1) {
      line[len++] = c;
    }
  }
}