Number of bytes currently in the output buffer
BytesToOutput indicates the number of bytes currently in the output buffer waiting to be written to the device. The property value is continuously updated as the output buffer is filled and emptied, and is set to 0 after the fopen function is issued.
You can make use of BytesToOutput only when writing data asynchronously. This is because when writing data synchronously, control is returned to the MATLAB® command line only after the output buffer is empty. Therefore, the BytesToOutput value is always 0. To learn how to write data asynchronously, see Writing Text Data.
Use the ValuesSent property to return the total number of values written to the device.