#include <SoftwareSerial.h>

#define SER_TX_PIN            D5  // Serial Transmit pin

SoftwareSerial serialPort;
int count = 0;

void setup() {
  Serial.begin(115200);
  Serial.println("\n\nSerial Transmiter");
  serialPort.begin(9600, SWSERIAL_8N1, SER_TX_PIN, SER_TX_PIN, false, 256);
  serialPort.enableIntTx(false);

}

void loop() {
  serialPort.enableTx(true);

  for(char c='a';c<'d';c++){
    Serial.printf("->%x\n",c);
    serialPort.write(c);
    delay(1000); //1 sec
  }

}