External Interfaces Reference |
Read data asynchronously from the device
Syntax
Arguments
obj |
A serial port object. |
size |
The number of bytes to read from the device. |
Description
readasync(obj)
initiates an asynchronous read operation.
readasync(obj,size)
asynchronously reads, at most, the number of bytes given by size
. If size
is greater than the difference between the InputBufferSize
property value and the BytesAvailable
property value, an error is returned.
Remarks
Before you can read data, you must connect obj
to the device 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 read operation while obj
is not connected to the device.
You should use readasync
only when you configure the ReadAsyncMode
property to manual
. readasync
is ignored if used when ReadAsyncMode
is continuous
.
The TransferStatus
property indicates if an asynchronous read or write operation is in progress. You can write data while an asynchronous read is in progress because serial ports have separate read and write pins. You can stop asynchronous read and write operations with the stopasync
function.
You can monitor the amount of data stored in the input buffer with the BytesAvailable
property. Additionally, you can use the BytesAvailableFcn
property to execute an M-file callback function when the terminator or the specified amount of data is read.
Rules for Completing an Asynchronous Read Operation
An asynchronous read operation with readasync
completes when one of these conditions is met:
Terminator
property is read.
Timeout
property passes.
size
is not specified).
Because readasync
checks for the terminator, this function can be slow. To increase speed, you might want to configure ReadAsyncMode
to continuous
and continuously return data to the input buffer as soon as it is available from the device.
Example
This example creates the serial port object s
, connects s
to a Tektronix TDS 210 oscilloscope, configures s
to read data asynchronously only if readasync
is issued, and configures the instrument to return the peak-to-peak value of the signal on channel 1.
s = serial('COM1'); fopen(s) s.ReadAsyncMode = 'manual'; fprintf(s,'Measurement:Meas1:Source CH1') fprintf(s,'Measurement:Meas1:Type Pk2Pk') fprintf(s,'Measurement:Meas1:Value?')
Begin reading data asynchronously from the instrument using readasync
. When the read operation is complete, return the data to the MATLAB workspace using fscanf
.
Functions
Properties
BytesAvailable
, BytesAvailableFcn
, ReadAsyncMode
, Status
, TransferStatus
load (serial) | record |
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