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15.5. Example serial code for Linux and macOS in C

The example C code below uses parts of the POSIX API provided by Linux and macOS to communicate with a Jrk G2 via serial. It demonstrates how to set the target of the Jrk by sending a “Set target” command and how to read variables using a “Get variables” command. For a very similar example that only works on Windows, see Section 15.6.

The Jrk’s input mode should be set to “Serial / I2C / USB”, or else the “Set target” command will not work. Also, you might need to change the const char * device line in the code that specifies what serial port to connect to.

If the Jrk is connected to your PC via USB, you will need to set the Jrk’s serial mode to “USB dual port” in the “Input” tab of the Jrk G2 Configuration Utility. The baud rate specified in this code and in the Jrk’s settings do not have to match because the serial bytes are transferred via USB. (You could remove the code that sets the baud rate.) Also, instead of using this code, you might consider running the Jrk G2 Command-line Utility (jrk2cmd), which uses the native USB interface, since it can take care of all of the low-level details of communication for you.

If the Jrk is connected via its RX and TX lines, you will need to set the Jrk’s serial mode to “UART” and select the baud rate you want to use in the “Input” tab of the Jrk G2 Configuration Utility. The baud rate you select in the code should match the baud rate specified in the configuration utility. The code below only supports certain standard baud rates: 4800, 9600, 19200, 38400, 115200. Due to hardware limitations, the Jrk cannot exactly produce 38400 baud or 115200 baud, but it can use similar baud rates that are close enough to work.

// Uses POSIX serial port functions to send and receive data from a Jrk G2.
// NOTE: The Jrk's input mode must be "Serial / I2C / USB".
// NOTE: The Jrk's serial mode must be set to "USB dual port" if you are
//   connecting to it directly via USB.
// NODE: The Jrk's serial mode must be set to "UART" if you are connecting to
//   it via is TX and RX lines.
// NOTE: You might need to change the 'const char * device' line below to
//   specify the correct serial port.

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <stdint.h>
#include <termios.h>

// Opens the specified serial port, sets it up for binary communication,
// configures its read timeouts, and sets its baud rate.
// Returns a non-negative file descriptor on success, or -1 on failure.
int open_serial_port(const char * device, uint32_t baud_rate)
{
  int fd = open(device, O_RDWR | O_NOCTTY);
  if (fd == -1)
  {
    perror(device);
    return -1;
  }

  // Flush away any bytes previously read or written.
  int result = tcflush(fd, TCIOFLUSH);
  if (result)
  {
    perror("tcflush failed");  // just a warning, not a fatal error
  }

  // Get the current configuration of the serial port.
  struct termios options;
  result = tcgetattr(fd, &options);
  if (result)
  {
    perror("tcgetattr failed");
    close(fd);
    return -1;
  }

  // Turn off any options that might interfere with our ability to send and
  // receive raw binary bytes.
  options.c_iflag &= ~(INLCR | IGNCR | ICRNL | IXON | IXOFF);
  options.c_oflag &= ~(ONLCR | OCRNL);
  options.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);

  // Set up timeouts: Calls to read() will return as soon as there is
  // at least one byte available or when 100 ms has passed.
  options.c_cc[VTIME] = 1;
  options.c_cc[VMIN] = 0;

  // This code only supports certain standard baud rates. Supporting
  // non-standard baud rates should be possible but takes more work.
  switch (baud_rate)
  {
  case 4800:   cfsetospeed(&options, B4800);   break;
  case 9600:   cfsetospeed(&options, B9600);   break;
  case 19200:  cfsetospeed(&options, B19200);  break;
  case 38400:  cfsetospeed(&options, B38400);  break;
  case 115200: cfsetospeed(&options, B115200); break;
  default:
    fprintf(stderr, "warning: baud rate %u is not supported, using 9600.\n",
      baud_rate);
    cfsetospeed(&options, B9600);
    break;
  }
  cfsetispeed(&options, cfgetospeed(&options));

  result = tcsetattr(fd, TCSANOW, &options);
  if (result)
  {
    perror("tcsetattr failed");
    close(fd);
    return -1;
  }

  return fd;
}

// Writes bytes to the serial port, returning 0 on success and -1 on failure.
int write_port(int fd, uint8_t * buffer, size_t size)
{
  ssize_t result = write(fd, buffer, size);
  if (result != (ssize_t)size)
  {
    perror("failed to write to port");
    return -1;
  }
  return 0;
}

// Reads bytes from the serial port.
// Returns after all the desired bytes have been read, or if there is a
// timeout or other error.
// Returns the number of bytes successfully read into the buffer, or -1 if
// there was an error reading.
ssize_t read_port(int fd, uint8_t * buffer, size_t size)
{
  size_t received = 0;
  while (received < size)
  {
    ssize_t r = read(fd, buffer + received, size - received);
    if (r < 0)
    {
      perror("failed to read from port");
      return -1;
    }
    if (r == 0)
    {
      // Timeout
      break;
    }
    received += r;
  }
  return received;
}

// Sets the target, returning 0 on success and -1 on failure.
//
// For more information about what this command does, see the "Set Target"
// command in the "Command reference" section of the Jrk G2 user's guide.
int jrk_set_target(int fd, uint16_t target)
{
  if (target > 4095) { target = 4095; }
  uint8_t command[2];
  command[0] = 0xC0 + (target & 0x1F);
  command[1] = (target >> 5) & 0x7F;
  return write_port(fd, command, sizeof(command));
}

// Gets one or more variables from the Jrk (without clearing them).
// Returns 0 for success, -1 for failure.
int jrk_get_variable(int fd, uint8_t offset, uint8_t * buffer, uint8_t length)
{
  uint8_t command[] = { 0xE5, offset, length };
  int result = write_port(fd, command, sizeof(command));
  if (result) { return -1; }
  ssize_t received = read_port(fd, buffer, length);
  if (received < 0) { return -1; }
  if (received != length)
  {
    fprintf(stderr, "read timeout: expected %u bytes, got %zu\n",
      length, received);
    return -1;
  }
  return 0;
}

// Gets the Target variable from the jrk or returns -1 on failure.
int jrk_get_target(int fd)
{
  uint8_t buffer[2];
  int result = jrk_get_variable(fd, 0x02, buffer, sizeof(buffer));
  if (result) { return -1; }
  return buffer[0] + 256 * buffer[1];
}

// Gets the Feedback variable from the jrk or returns -1 on failure.
int jrk_get_feedback(int fd)
{
  // 0x04 is the offset of the feedback variable in the "Variable reference"
  // section of the Jrk user's guide.  The variable is two bytes long.
  uint8_t buffer[2];
  int result = jrk_get_variable(fd, 0x04, buffer, sizeof(buffer));
  if (result) { return -1; }
  return buffer[0] + 256 * buffer[1];
}

int main()
{
  // Choose the serial port name.  If the Jrk is connected directly via USB,
  // you can run "jrk2cmd --cmd-port" to get the right name to use here.
  // Linux USB example:          "/dev/ttyACM0"  (see also: /dev/serial/by-id)
  // macOS USB example:          "/dev/cu.usbmodem001234562"
  // Cygwin example:             "/dev/ttyS7"
  const char * device = "/dev/ttyACM0";

  // Choose the baud rate (bits per second).  This does not matter if you are
  // connecting to the Jrk over USB.  If you are connecting via the TX and RX
  // lines, this should match the baud rate in the Jrk's serial settings.
  uint32_t baud_rate = 9600;

  int fd = open_serial_port(device, baud_rate);
  if (fd < 0) { return 1; }

  int feedback = jrk_get_feedback(fd);
  if (feedback < 0) { return 1; }

  printf("Feedback is %d.\n", feedback);

  int target = jrk_get_target(fd);
  if (target < 0) { return 1; }
  printf("Target is %d.\n", target);

  int new_target = (target < 2048) ? 2248 : 1848;
  printf("Setting target to %d.\n", new_target);
  int result = jrk_set_target(fd, new_target);
  if (result) { return 1; }

  close(fd);
  return 0;
}

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