Using C and libusb  with ADU USB Data Acquisition Products (Linux)

View the ADU series of USB based Data Acquisition Products


Communicating with USB devices via software involves a few simple steps. Unlike RS232 based devices which are connected to physical COM ports, USB devices are assigned a logical handle by operating systems when they are first plugged in. This process is known as enumeration. Once a USB device has been enumerated, it is ready for use by the host computer software. For the host application software to communicate with the USB device, it must first obtain the handle assigned to the USB device during the enumeration process. The handle can be obtained using an open function along with some specific information about the USB device. Information that can be used to obtain a handle to a USB device include, serial number, product ID, or vendor ID.

USB devices have defined interfaces which relate to their functionality. For example, a USB keyboard with built in LEDs may have an interface for sending key presses and an interface for controlling the lights on the keyboard. Interfaces as defined as a set of endpoints. Endpoints are used as communication channels to and from the device and host and can either be IN or OUT. They are defined relative to the host - OUT endpoints transport data to the device (write) and IN endpoints transport data to the host (read).

Once we obtain a USB device handle, we must claim the interface we want to use. This will allow us to read and write information to and from the USB device via our application. Once the application has finished with all communication with the USB device, the handle is closed. The handle is generally closed when the application terminates.

The sample source code outlines the basics of communicating directly with an ADU device on Linux using C and libusb. Basics of opening a USB device handle, writing and reading data, as well as closing the handle of the ADU usb device is provided as an example. An alternate way of working with ADU devices in Linux is to use the adutux kernel driver to access the device as a file descriptor (outlined here:

All source code is provided so that you may review details that are not highlighted here.

Lets have a look at the code......

libusb is a C library that provides generic access to USB devices. We will need a vendor ID and product ID in order to open the USB device. The VENDOR_ID define will always remain the same as this is OnTrak's USB vendor ID, however, PRODUCT_ID must be set to match the product that is connected via USB. See this link for a list of OnTrack product IDs:

// libusb library must be available. It can be installed on Debian/Ubuntu using apt-get install libusb-1.0-0-dev
#include <libusb-1.0/libusb.h>

#define VENDOR_ID      0x0a07  // Ontrak vendor ID. This should never change
#define PRODUCT_ID     208     // ADU208 product ID. Set this product ID to match your device.
                               // Product IDs can be found at

Low-speed and full-speed devices have different transfer sizes for the data sent between host (computer) and device (ADU).

A low-speed device writes and reads 8 byte packets, whereas a high-speed device's transfer size is 64 bytes. Be sure to uncomment the one matching your connected device. In Linux, 'dmesg' may be used to find the device speed. Uncomment the appropriate speed based on your connected device.

//#define TRANSFER_SIZE    64  
// Data transfer size is either 8 bytes (low-speed USB devices) or 64 bytes (full-speed USB devices).
// Set this to 8 or 64 depending on the ADU device you are using (see
// You can also type `dmesg` in your terminal immediately after connecting an ADU to your computer.
// This should list whether it is low-speed or full-speed.

#define TRANSFER_SIZE    8  
// ADU208 is a low-speed device, so we must use 8 byte transfers

Next, let's declare our libusb USB device handle. This handle will be used for all of our interactions with the USB device via libusb (opening, closing, reading and writing commands). We'll also initialize the libusb library and check the returned result. For information purposes, we will also set our debugging output to the maximum level.

int main( int argc, char **argv )
    struct libusb_device_handle * device_handle = NULL; // Our ADU's USB device handle
	char value_str[8]; // 8-byte buffer to store string values read from device 
			  //(7 byte string + NULL terminating character)
    int result;

    // Initialize libusb
    int result = libusb_init( NULL );
    if ( result < 0 )
        printf( "Error initializing libusb: %s\n", libusb_error_name( result ) );
        exit( -1 );

    // Set debugging output to max level

If initialization of libusb was successful, we will now attempt to open the connected USB device that matches our vendor and product ID.

itialization of libusb was successful, we will now attempt to open the connected USB device that matches our vendor and product ID.

    // Open our ADU device that matches our vendor id and product id
    device_handle = libusb_open_device_with_vid_pid( NULL, VENDOR_ID, PRODUCT_ID );
    if ( !device_handle )
        printf( "Error finding USB device\n" );
        libusb_exit( NULL );
        exit( -2 );

We should also enable auto-detaching of the kernel driver. If a kernel driver currently has an interface claimed, it will be automatically be detached when we claim that interface. When the interface is restored, the kernel driver is allowed to be re-attached. This can alternatively be manually done via libusb_detach_kernel_driver().

	libusb_set_auto_detach_kernel_driver( device_handle, 1 );

At this point we also need to claim the interface that we wish to use. ADU devices have interface 0 defined with an IN and OUT endpoint used for reading and writing, respectively.

     // Claim interface 0 on the device
    result = libusb_claim_interface( device_handle, 0 );
    if ( result < 0 )
        printf( "Error claiming interface: %s\n", libusb_error_name( result ) );
        if ( device_handle )
            libusb_close( device_handle );

        libusb_exit( NULL );
        exit( -3 );

Now that we have successfully opened our device and specified the interface we wish to use, we can use interrupt transfers to write commands to the ADU device and read the result.

Two convenience functions have been written to help properly format command packets to send to the ADU device, as well as to read a result from the ADU device: write_to_adu() and read_from_adu(). We won't be covering these functions in detail, however comments in the source code provide an explanation of what is occuring. Feel free to modify these if required.

planation of what is occuring. Feel free to modify these if required.

     // Now that we have our device and have claimed interface 0, we can write to and read from it:
    result = write_to_adu( device_handle, "RK0", 200 ); // reset relay 0

    // We aren't checking the result of write_to_adu, 
    //but it will return 0 on success and a negative number on failure.
    result = write_to_adu( device_handle, "SK0", 200 ); // close relay 0

In order to read from the ADU device, we can send a command that requests a return value (as defined in our product documentation). Such a command for the ADU208 is RPK0. This requests the value of relay 0, which we previously set with the RK0 and SK0 commands in the above code block.

It's important to note that if a command that requests a return value (such as RPK0) is sent to an ADU device, the return value must be read from the device by calling read_from_adu before issuing any other commands that request return values. Failing to do so could cause the OS to disconnect the device and require it to be reset.

     // Send a command to request value of relay 0 (should be 1 as we closed the relay with SK0)
	result = write_to_adu( device_handle, "RPK0", 200 );
    // Read the result
    int value = 0; // variable to hold the result
    result = read_from_adu( device_handle, &value, 200 );

    if ( result == 0 ) // result of 0 indicates success
        printf( "Read value: %i\n", value );

Since we are finished with the device, we should release the interface and close the device. If any kernel driver was previously attached to interface 0, it will now be reattached due to calling libusb_set_auto_detach_kernel_driver earlier in our code.

     // we are done with our device and will now release the interface we previously claimed as well as the device
    libusb_release_interface( device_handle, 0 );
    libusb_close( device_handle );

    // shutdown libusb
    libusb_exit( NULL );

    return 0;

Further Details

When newly connecting to a device, we should also read until there is nothing left to read from the device. This is to clear any pending reads that was not initiated by us:

    while ( 0 == read_from_adu( device_handle, value, 200 ) )

If you're interested in the internals of write_to_adu() and read_from_adu() as well as how to structure a command packet, the details are below.

All ADU commands have their first byte set to 0x01 and the following bytes contain the ASCII representation of the command. The ADU command packet format is described here: As described in the link, the remaining bytes in the command buffer must be null padded (0x00). We do this via a memset.

We use the OUT endpoint to send data from the host to the device. Endpoint address 0x81 is standard for IN and 0x01 is standard for OUT.

We'll use libusb_interrupt_transfer to write out command to the device on ENDPOINT_OUT (0x01). Interrupt transfers are a transfer type used for sending infrequent and small amounts of data. Compared to streaming video from a webcam, the ADU devices send small amounts of data. After sending, we check to result to make sure the transfer succeeded. The number of bytes sent should match our command buffer's size (8 bytes since we are using a low-speed device, and 64 bytes if a high speed device).

// Write a command to an ADU device with a specified timeout
int write_to_adu( libusb_device_handle * _device_handle, const char * _cmd, int _timeout )
    const int command_len = strlen( _cmd ); // Get the length of the command string we are sending

    int bytes_sent = 0;

    // Buffer to hold the command we will send to the ADU device.
    // Its size is set to the transfer size for low or full speed USB devices 
    //(ADU model specific - see defines at top of file)
    unsigned char buffer[ TRANSFER_SIZE ]; 

    if ( command_len > TRANSFER_SIZE )
        printf( "Error: command is larger than our limit of %i\n", TRANSFER_SIZE );
        return -1;

    memset( buffer, 0, TRANSFER_SIZE ); // Zero out buffer to pad with null values 
     //(command buffer needs to be padded with 0s)

    buffer[0] = 0x01; // First byte of the command buffer needs to be set to a decimal value of 1

    // Copy the command ASCII bytes into our buffer, starting at the second byte 
    //(we need to leave the first byte as decimal value 1)
    memcpy( &buffer[1], _cmd, command_len ); 

    // Attempt to send the command to the OUT endpoint (0x01) with the use specified millisecond timeout
    int result = libusb_interrupt_transfer( _device_handle, 0x01, buffer, TRANSFER_SIZE, &bytes_sent, _timeout );
    printf( "Write '%s' result: %i, Bytes sent: %u\n", _cmd, result, bytes_sent );

    if ( result < 0 ) // Was the interrupt transfer successful?
        printf( "Error sending interrupt transfer: %s\n", libusb_error_name( result ) );

    return result; // Returns 0 on success, a negative number specifying the libusb error otherwise

If the interrupt transfer succeeds, we should now have a result to read from the command we sent. We can use libusb_interrupt_transfer with the ENDPOINT_IN endpoint to read the value. The parameters are the same as for sending, however after the call the specified buffer will contain the data read from the device and bytes_read will contain the number of bytes that were read and stored in the buffer.

If reading from the device was successful, let's extract the data we are interested in. It's important to note that the data we are interested in is the first 8 bytes of the data read from the device. The first byte 0x01 and is followed by an ASCII representation of the number. The remainder of the bytes are padded with 0x00 (NULL) values.

// Read a command from an ADU device with a specified timeout
int read_from_adu( libusb_device_handle * _device_handle, char * _read_str, int _read_str_len, int _timeout )
	if ( _read_str == NULL || _read_str_len < 8 )
		return -2;
    int bytes_read = 0;

    // Buffer to hold the command we will receive from the ADU device
    // Its size is set to the transfer size for low or full speed USB devices 
    //(ADU model specific - see defines at top of file)
    unsigned char buffer[ TRANSFER_SIZE ];

    memset( buffer, 0, TRANSFER_SIZE ); // Zero out buffer to pad with null values 
     //(command buffer needs to be padded with 0s)

    // Attempt to read the result from the IN endpoint (0x81) with user specified timeout
    int result = libusb_interrupt_transfer( _device_handle, 0x81, buffer, TRANSFER_SIZE, &bytes_read, _timeout );
    printf( "Read result: %i, Bytes read: %u\n", result, bytes_read );

    if ( result < 0 ) // Was the interrupt transfer successful?
        printf( "Error reading interrupt transfer: %s\n", libusb_error_name( result ) );
        return result;

	// The buffer should now hold the data read from the ADU device. The first byte will contain 0x01, the remaining bytes
	// are the returned value in string format. Let's copy the string from the read buffer, starting at index 1, to our _read_str buffer 
	memcpy( _read_str, &buffer[1], 7 );
	buffer[7] = '\0'; // null terminate the string

    return result; // returns 0 on success, a negative number specifying the libusb error otherwise

This example illustrates the basics of reading and writing to ADU devices using libusb, interrupt transfers, and endpoints.

In a larger application, you may want to use libusb's asynchronous interface as to not block the application. Using this approach, function callbacks can be set to notify you on completion of transfers. More information can be found here:

NOTE: When running the example, it must be run with root privileges in order to access the USB device.

DOWNLOAD C libusb Example (Linux)