Instrument Control Toolbox

readasync

Read data asynchronously from the instrument

Syntax

• readasync(obj)
  readasync(obj,size)
  

Arguments

obj	An instrument object.
size	The number of bytes to read from the instrument.

Description

readasync(obj) initiates an asynchronous read operation.

readasync(obj,size) asynchronously reads, at most, the number of bytes specified 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 instrument with the fopen function. A connected instrument 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 instrument.

For serial port, TCP/IP, UDP, and VISA-serial objects, 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. For all instrument objects, you cannot use readasync while a read operation is in progress. For serial port and VISA-serial objects, 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:

• The terminator is read. For serial port, TCP/IP, UDP, and VISA-serial objects, the terminator is given by the Terminator property. Note that for UDP objects, DatagramTerminateMode must be off.

• For all other instrument objects except VISA-RSIB, the terminator is given by the EOSCharCode property.

• The time specified by the Timeout property passes.
• The specified number of bytes is read.
• The input buffer is filled.
• A datagram has been received (UDP objects only if DatagramTerminateMode is on)
• The EOI line is asserted (GPIB and VXI instruments only).

For serial port, TCP/IP, UDP, and VISA-serial objects, readasync can be slow because it checks for the terminator. 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 instrument.

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?')
  

Initially, there is no data in the input buffer.

• s.BytesAvailable
  ans =
       0
  

Begin reading data asynchronously from the instrument using readasync. When the read operation is complete, return the data to the MATLAB workspace using fscanf.

• readasync(s)
  s.BytesAvailable
  ans =
       15
  out = fscanf(s)
  out =
  2.0399999619E0
  fclose(s)
  

See Also

Functions

fopen, stopasync

Properties

BytesAvailable, BytesAvailableFcn, ReadAsyncMode, Status, TransferStatus

query

record