Step 2: Read Data from DYNAMIXEL

This tutorial shows you how to read data from a Dynamixel using the Dynamixel Easy SDK.

NOTE: It assumes that you have already installed the SDK.

Make py file

Create a new Python source file Open it with your editor.
```
$ touch tutorial_step2.py
```

Check the Port Names

Before running the code, check the port name of the connected Dynamixel.
For General
- OpenRB-150: ttyACM0: USB ACM device
- U2D2: FTDI USB Serial Device converter now attached to ttyUSB0
```
$ sudo dmesg | grep tty
```

Source Code Description

from dynamixel_easy_sdk import *

def main():
    connector = Connector("/dev/ttyACM0", 57600)
    motor1 = connector.createMotor(1)

    present_position = motor1.getPresentPosition()
    print("Present Position:", present_position)

if __name__ == "__main__":
    main()

Import Library

Import dynamixel_easy_sdk.
```
from dynamixel_easy_sdk import *
```

Create Connector and Motor Object

Create a Connector object with port name and baud rate to manage the communication.
```
def main():
    connector = Connector("/dev/ttyACM0", 57600)
```
Create a Motor object for each Dynamixel servo you want to control with its ID, using the createMotor method of the Connector class.
```
    motor1 = connector.createMotor(1)
```

Get Present Position

You can read the present position of the motor using the getPresentPosition method.

    present_position = motor1.getPresentPosition()
    print("Present Position:", present_position)

Error Handling

When an error occurs, DxlRuntimeError is raised.

You can catch this error using a try-except block.

try:
    motor1.getPresentPosition()
except DxlRuntimeError as e:
    print(e)

DxlRuntimeError contains DxlError Enum that provides detailed information about the error.

try:
    motor1.getPresentPosition()
except DxlRuntimeError as e:
    if e.dxl_error == DxlError.SDK_COMM_TX_FAIL:
        print("Transmission failed.")
    elif e.dxl_error == DxlError.SDK_COMM_RX_FAIL:
        print("Receive failed.")

NOTE: It assumes that you have already installed and built the SDK.

Make cpp file

Create a new C++ source file Open it with your editor.
```
$ touch tutorial_step2.cpp
```

Check the Port Names

Before running the code, check the port name of the connected Dynamixel.
For General
- OpenRB-150: ttyACM0: USB ACM device
- U2D2: FTDI USB Serial Device converter now attached to ttyUSB0
```
$ sudo dmesg | grep tty
```

Source Code Description

#include "dynamixel_easy_sdk/dynamixel_easy_sdk.hpp"

int main(){

  dynamixel::Connector connector("/dev/ttyACM0", 57600);
  std::unique_ptr<dynamixel::Motor> motor1 = connector.createMotor(1);

  auto result_int32_t = motor1->getPresentPosition();
  int present_position = result_int32_t.value();
  std::cout << "Present Position: " << present_position << std::endl;
}

Add Header Files

Add dynamixel_easy_sdk/dynamixel_easy_sdk.hpp to the top of your CPP file. This class is included in the Dynamixel SDK package.
```
#include "dynamixel_easy_sdk/dynamixel_easy_sdk.hpp"
```

Create Connector and Motor Object

Create a Connector object with port name and baud rate to manage the communication.
```
int main(){
  dynamixel::Connector connector("/dev/ttyACM0", 57600);
```
Create a Motor object for each Dynamixel servo you want to control, using the createMotor method of the Connector class.
This method takes the motor ID as an argument and returns a unique pointer to a Motor instance. (shared_ptr is also available)
```
  std::unique_ptr<dynamixel::Motor> motor1 = connector.createMotor(1);
```
This process throws a DxlRuntimeError if the object creation fails.

Get Present Position

You can read the present position of the motor using the getPresentPosition method.

Return type of this method is Result<int32_t, Error>. You can get the actual position value using the value() method of the Result object.

  auto result_int32_t = motor1->getPresentPosition();
  int present_position = result_int32_t.value();
  std::cout << "Present Position: " << present_position << std::endl;
}

Error Handling

To ensure your code is robust, every method that sends a command to the motor returns a Result object that encapsulates values and errors.
This object lets you safely check for any communication or device errors before proceeding.
You can check for communication errors and device(dynamixel) errors using the Result object.

If you use value() when error occurred without checking for errors, it may throw an exception.

Example

  auto result_int32_t = motor1->getPresentPosition();
  if (!result_int32_t.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_int32_t.error()) << std::endl;
    return 1;
  }
  int present_position = result_int32_t.value();

Compile and Run

You can compile and run the code using the following commands

$ g++ tutorial_step2.cpp -o tutorial_step2 -l dxl_x64_cpp
$ ./tutorial_step2

Full Source Code With Error Handling

#include "dynamixel_easy_sdk/dynamixel_easy_sdk.hpp"

int main(){
  dynamixel::Connector connector("/dev/ttyACM0", 57600);
  std::unique_ptr<dynamixel::Motor> motor1 = connector.createMotor(1);

  auto result_int32_t = motor1->getPresentPosition();
  if (!result_int32_t.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_int32_t.error()) << std::endl;
    return 1;
  }
  int present_position = result_int32_t.value();
  std::cout << "Present Position: " << present_position << std::endl;
}