15.6. Example serial code for Windows in C

The example C code below uses the Windows API 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 works on Linux and macOS, see Section 15.5.

The Jrk’s input mode should be set to “Serial / I2C / USB”, or else the “Set target” command will not work. Also, you will 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 in the code should match the baud rate specified in the configuration utility.

// Uses Windows API 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 need to change the 'const char * device' line below to
//   specify the correct serial port.

#include <stdio.h>
#include <stdint.h>
#include <windows.h>

void print_error(const char * context)
{
  DWORD error_code = GetLastError();
  char buffer[256];
  DWORD size = FormatMessageA(
    FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_MAX_WIDTH_MASK,
    NULL, error_code, MAKELANGID(LANG_ENGLISH, SUBLANG_ENGLISH_US),
    buffer, sizeof(buffer), NULL);
  if (size == 0) { buffer[0] = 0; }
  fprintf(stderr, "%s: %s\n", context, buffer);
}

// Opens the specified serial port, configures its timeouts, and sets its
// baud rate.  Returns a handle on success, or INVALID_HANDLE_VALUE on failure.
HANDLE open_serial_port(const char * device, uint32_t baud_rate)
{
  HANDLE port = CreateFileA(device, GENERIC_READ | GENERIC_WRITE, 0, NULL,
    OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
  if (port == INVALID_HANDLE_VALUE)
  {
    print_error(device);
    return INVALID_HANDLE_VALUE;
  }

  // Flush away any bytes previously read or written.
  BOOL success = FlushFileBuffers(port);
  if (!success)
  {
    print_error("Failed to flush serial port");
    CloseHandle(port);
    return INVALID_HANDLE_VALUE;
  }

  // Configure read and write operations to time out after 100 ms.
  COMMTIMEOUTS timeouts = { 0 };
  timeouts.ReadIntervalTimeout = 0;
  timeouts.ReadTotalTimeoutConstant = 100;
  timeouts.ReadTotalTimeoutMultiplier = 0;
  timeouts.WriteTotalTimeoutConstant = 100;
  timeouts.WriteTotalTimeoutMultiplier = 0;

  success = SetCommTimeouts(port, &timeouts);
  if (!success)
  {
    print_error("Failed to set serial timeouts");
    CloseHandle(port);
    return INVALID_HANDLE_VALUE;
  }

  DCB state;
  state.DCBlength = sizeof(DCB);
  success = GetCommState(port, &state);
  if (!success)
  {
    print_error("Failed to get serial settings");
    CloseHandle(port);
    return INVALID_HANDLE_VALUE;
  }

  state.BaudRate = baud_rate;

  success = SetCommState(port, &state);
  if (!success)
  {
    print_error("Failed to set serial settings");
    CloseHandle(port);
    return INVALID_HANDLE_VALUE;
  }

  return port;
}

// Writes bytes to the serial port, returning 0 on success and -1 on failure.
int write_port(HANDLE port, uint8_t * buffer, size_t size)
{
  DWORD written;
  BOOL success = WriteFile(port, buffer, size, &written, NULL);
  if (!success)
  {
    print_error("Failed to write to port");
    return -1;
  }
  if (written != size)
  {
    print_error("Failed to write all bytes 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(HANDLE port, uint8_t * buffer, size_t size)
{
  DWORD received;
  BOOL success = ReadFile(port, buffer, size, &received, NULL);
  if (!success)
  {
    print_error("Failed to read from port");
    return -1;
  }
  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(HANDLE port, 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(port, 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(HANDLE port, uint8_t offset, uint8_t * buffer,
  uint8_t length)
{
  uint8_t command[] = { 0xE5, offset, length };
  int result = write_port(port, command, sizeof(command));
  if (result) { return -1; }
  SSIZE_T received = read_port(port, buffer, length);
  if (received < 0) { return -1; }
  if (received != length)
  {
    fprintf(stderr, "read timeout: expected %u bytes, got %ld\n",
      length, received);
    return -1;
  }
  return 0;
}

// Gets the Target variable from the jrk or returns -1 on failure.
int jrk_get_target(HANDLE port)
{
  uint8_t buffer[2];
  int result = jrk_get_variable(port, 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(HANDLE port)
{
  // 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(port, 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.
  // COM ports higher than COM9 need the \\.\ prefix, which is written as
  // "\\\\.\\" in C because we need to escape the backslashes.
  const char * device = "\\\\.\\COM7";

  // 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;

  HANDLE port = open_serial_port(device, baud_rate);
  if (port == INVALID_HANDLE_VALUE) { return 1; }

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

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

  int target = jrk_get_target(port);
  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(port, new_target);
  if (result) { return 1; }

  CloseHandle(port);
  return 0;
}

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