RS485

Hardware

One RS485/RS422 bus is integrated into the AutomatePro with a max data rate of 500kbs. A 120ohm termination resistor should be placed between RS485P and RS485N at both ends of the RS485 network.

Connector Pinout

Connector

Development Breakout Board

RS485 Specifications

Parameter	Value
Transceiver	THVD1400DR
Data Rate	500 kbps
RS485 Standard	TIA/EIA-485A
Duplex	Half

Software

Serial Port

/dev/ttyTHS1 is the serial port used for RS485 communication. RTS pin is used to control the direction of the transceiver. RTS is inverted and active low, RTS high enables the receiver and RTS low enables the transmitter.

Example

Python

  import serial
  import serial.rs485
  import time
  
  # Configure the serial connections
  ser = serial.rs485.RS485(
      port='/dev/ttyTHS1',
      baudrate=115200,            # Set your baud rate
      parity=serial.PARITY_NONE,
      stopbits=serial.STOPBITS_ONE,
      bytesize=serial.EIGHTBITS,
      timeout=0.1                 # Timeout for read operations
  )
  
  ser.rs485_mode = serial.rs485.RS485Settings(
    rts_level_for_tx=False,
    rts_level_for_rx=True,
    )
  def send_data(data):
      ser.write(data.encode())
      print("Sent:", data)

  def receive_data():
      incoming_data = ser.read(100)  # Read up to 100 bytes or until timeout
      if incoming_data:
          print("Received:", incoming_data.decode())  # Decode bytes to string

  if __name__ == '__main__':
      try:
          while True:
              send_data("Hello RS485")
              time.sleep(1)  # Wait a bit for a response
              receive_data()
      except Exception as e:
          print("An error occurred:", str(e))
      finally:
          ser.close()  # Always close the serial port

C++

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <linux/serial.h>

int setup_serial(const char* device, int speed) {
  int fd = open(device, O_RDWR | O_NOCTTY | O_SYNC);
  if (fd < 0) {
      perror("Error opening serial port");
      return -1;
  }

  struct termios tty;
  memset(&tty, 0, sizeof tty);
  if (tcgetattr(fd, &tty) != 0) {
      perror("Error from tcgetattr");
      close(fd);
      return -1;
  }

  cfsetospeed(&tty, speed);
  cfsetispeed(&tty, speed);

  tty.c_cflag |= (CLOCAL | CREAD);    // Ignore modem controls, enable reading
  tty.c_cflag &= ~CSIZE;
  tty.c_cflag |= CS8;                 // 8-bit characters
  tty.c_cflag &= ~PARENB;             // No parity bit
  tty.c_cflag &= ~CSTOPB;             // 1 stop bit
  tty.c_cflag &= ~CRTSCTS;            // No hardware flow control

  tty.c_iflag &= ~(IXON | IXOFF | IXANY); // Turn off s/w flow ctrl
  tty.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG); // Disable canonical mode, echo, and signal chars
  tty.c_oflag &= ~OPOST; // Prevent special interpretation of output bytes (e.g., newline chars)
  tty.c_cc[VMIN]  = 0;   // Non-blocking read
  tty.c_cc[VTIME] = 20;  // 2 seconds read timeout

  if (tcsetattr(fd, TCSANOW, &tty) != 0) {
      perror("Error from tcsetattr");
      close(fd);
      return -1;
  }

  return fd;
}

void set_rts(int fd, int level) {
  int status;
  ioctl(fd, TIOCMGET, &status);  // Get the current status of the modem lines
  if (level)
      status |= TIOCM_RTS;  // Set RTS high
  else
      status &= ~TIOCM_RTS; // Set RTS low
  ioctl(fd, TIOCMSET, &status);
}

void send_data(int fd, const char* data) {
  set_rts(fd, 0);  // Set RTS low to send
  write(fd, data, strlen(data)); // Send data
  printf("Sent: %s\n", data);
  set_rts(fd, 1);  // Set RTS high after sending
}

void receive_data(int fd) {
  char buf[100];
  int n = read(fd, buf, sizeof(buf)-1);
  if (n > 0) {
      buf[n] = '\0';
      printf("Received: %s\n", buf);
  }
}

int main() {
  int fd = setup_serial("/dev/ttyTHS1", B9600);
  if (fd < 0) {
      exit(EXIT_FAILURE);
  }

  while (1) {
      send_data(fd, "Hello RS485");
      sleep(1); // Wait a bit for a response
      receive_data(fd);
      sleep(1);
  }

  close(fd); // Always close the serial port
  return 0;
}

Troubleshooting

• No data is being sent or received: Check the wiring and termination resistors. Check the user permissions and serial port parameters such as baudrate, etc.
• Data is being sent but not received: Check whether the RTS pin is toggled correctly. RTS should be high to recieve data.
• Data is being received but not sent: Check whether the RTS pin is toggled correctly. RTS should be low to send data.

If you are using terminal emulators such as picocom, toggle the RTS pin manually using the commands offered by the terminal emulator.