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fwrite (serial)

Write binary data to device




fwrite(obj,A) writes the binary data A to the device connected to the serial port object, obj.

fwrite(obj,A,'precision') writes binary data with precision specified by precision.

precision controls the number of bits written for each value and the interpretation of those bits as integer, floating-point, or character values. If precision is not specified, uchar (an 8-bit unsigned character) is used. The supported values for precision are listed in Tips.

fwrite(obj,A,'mode') writes binary data with command-line access specified by mode. If mode is sync, A is written synchronously and the command line is blocked. If mode is async, A is written asynchronously and the command line is not blocked. If mode is not specified, the write operation is synchronous.

fwrite(obj,A,'precision','mode') writes binary data with precision specified by precision and command-line access specified by mode.


Before you can write data to the device, it must be connected to obj with the fopen function. A connected serial port object has a Status property value of open. An error is returned if you attempt to perform a write operation while obj is not connected to the device.

The ValuesSent property value is increased by the number of values written each time fwrite is issued.

An error occurs if the output buffer cannot hold all the data to be written. You can specify the size of the output buffer with the OutputBufferSize property.

If you set the FlowControl property to hardware on a serial object, and a hardware connection is not detected, fwrite returns an error message. This occurs if a device is not connected, or a connected device is not asserting that is ready to receive data. Check the remote device status and flow control settings to see if hardware flow control is causing errors in MATLAB®.

    Note:   If you want to check to see if the device is asserting that it is ready to receive data, set the FlowControl to none. Once you connect to the device, check the PinStatus structure for ClearToSend. If ClearToSend is off, there is a problem on the remote device side. If ClearToSend is on, there is a hardware FlowControl device prepared to receive data and you can execute fwrite.

Synchronous Versus Asynchronous Write Operations

By default, data is written to the device synchronously and the command line is blocked until the operation completes. You can perform an asynchronous write by configuring the mode input argument to be async. For asynchronous writes:

  • The BytesToOutput property value is continuously updated to reflect the number of bytes in the output buffer.

  • The callback function specified for the OutputEmptyFcn property is executed when the output buffer is empty.

You can determine whether an asynchronous write operation is in progress with the TransferStatus property.

Synchronous and asynchronous write operations are discussed in more detail in Writing Data.

Rules for Completing a Write Operation with fwrite

A binary write operation using fwrite completes when:

  • The specified data is written.

  • The time specified by the Timeout property passes.

      Note   The Terminator property is not used with binary write operations.

Supported Precisions

The following table shows the supported values for precision.

Data Type





8-bit unsigned character


8-bit signed character


8-bit signed or unsigned character



8-bit integer


16-bit integer


32-bit integer


8-bit unsigned integer


16-bit unsigned integer


32-bit unsigned integer


16-bit integer


32-bit integer


32- or 64-bit integer


16-bit unsigned integer


32-bit unsigned integer


32- or 64-bit unsigned integer



32-bit floating point


32-bit floating point


32-bit floating point


64-bit floating point


64-bit floating point

Introduced before R2006a

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