This demo explores asynchronous read and write operations with a UDP object.
The information obtained for this demonstration was prerecorded. Therefore, you do not need an actual instrument to learn about asynchronous read and write operations using a UDP object. The instrument used was an echoserver on a Linux O/S PC. An echoserver is a service available from the O/S that returns to the sender's address and port, the same bytes it receives from the sender.
These functions are associated with reading and writing text asynchronously:
FPRINTF - Write text to instrument. READASYNC - Asynchronously read bytes from an instrument. STOPASYNC - Stop an asynchronous read and write operation.
These properties are associated with ASCII read and write asynchronous operations:
BytesAvailable - Indicate the number of bytes available in
the input buffer.
TransferStatus - Indicate what type of asynchronous operation
is in progress.
ReadAsyncMode - Specify whether data is read continuously
in the background or whether you must call
the READASYNC function to read data
asynchronously.Additionally, you can use all callback properties during asynchronous read and write operations.
The object can operate in synchronous mode or in asynchronous mode. When the object is operating synchronously, the read and write routines block the MATLAB command line until the operation has completed or a timeout occurs. When the object is operating asynchronously, the read and write routines return control immediately to the MATLAB command line.
Additionally, you can use callback properties and callback functions to perform tasks as data is being written or read. For example, you can create a callback function that notifies you when the read or write operation has finished.
To begin, create a UDP object associated with the host name daqlab11, port 7. The host name is assigned by your network administrator. Port 7 is the port number for the echoserver.
>> u = udp('daqlab11', 7);
>> uUDP Object : UDP-daqlab11
Communication Settings
RemotePort: 7
RemoteHost: daqlab11
Terminator: 'LF'% Communication State
Status: closed
RecordStatus: off Read/Write State
TransferStatus: idle
BytesAvailable: 0
ValuesReceived: 0
ValuesSent: 0Before you can perform a read or write operation, you must connect the UDP object to the instrument with the FOPEN function. If the UDP object was successfully connected, its Status property is automatically configured to open.
>> fopen(u) >> get(u, 'Status')
ans =
open
You can read data asynchronously with the UDP object in one of these two ways:
The FSCANF, FREAD, FGETL, and FGETS functions are used to bring the data from the input buffer into MATLAB. These functions operate synchronously.
To begin, read data continuously.
>> set(u, 'ReadAsyncMode', 'continuous')
Now, send a string to the echoserver.
>> fprintf(u, 'Hello net.')
Because the ReadAsyncMode property is set to 'continuous', the object is continuously asking the instrument if any data is available. The echoserver sends data as soon as it receives data. The data is then read from the instrument and is stored in the object's input buffer.
>> get(u, 'BytesAvailable')
ans =
11
You can bring the data from the object's input buffer into the MATLAB workspace with FSCANF.
>> mystring = fscanf(u)
mystring =
Hello net.
Next, read the data manually.
>> set(u, 'ReadAsyncMode', 'manual')
Now, send a string to the echoserver.
>> fprintf(u, 'Hello net.')
Once the last FPRINTF function completes, the instrument begins sending data. However, because ReadAsyncMode is set to manual, the object is not reading the data being sent from the instrument. Therefore no data is being read and placed in the input buffer.
>> get(u, 'BytesAvailable')
ans =
0
The READASYNC function can asynchronously read the data from the instrument. The READASYNC function returns control to the MATLAB command line immediately.
The READASYNC function takes two input arguments. The first argument is the instrument object and the second argument is the SIZE, the amount of data to be read from the instrument.
The READASYNC function without a specified SIZE assumes SIZE is given by the difference between the InputBufferSize property value and the BytesAvailable property value. The asynchronous read terminates when:
An error event will be generated if READASYNC terminates due to a timeout.
The object starts querying the instrument for data when the READASYNC function is called. Because all the data was sent before the READASYNC function call, no data will be stored in the input buffer and the data is lost.
>> readasync(u); >> get(u, 'BytesAvailable')
ans =
0
When the UDP object is in manual mode (the ReadAsyncMode property is configured to manual), data that is sent from the instrument to the computer is not automatically stored in the input buffer of the UDP object. Data is not stored until READASYNC or one of the blocking read functions is called.
Manual mode should be used when a stream of data is being sent from your instrument and you only want to capture portions of the data.
Now, configure our UDP object to notify you when a terminator has been read.
>> set(u, 'ReadAsyncMode', 'continuous')
>> set(u, 'BytesAvailableFcn', {'dispcallback'})Note, the default value for the BytesAvailableFcnMode property indicates that the callback function defined by the BytesAvailableFcn property will be executed when the terminator has been read.
>> get(u, 'BytesAvailableFcnMode')
ans =
terminator
The M-file callback function DISPCALLBACK is defined below.
function dispcallback(obj, event) %DISPCALLBACK Display event information for the specified event. % % DISPCALLBACK(OBJ, EVENT) a callback function that displays % a message which contains the type of the event, the name % of the object which caused the event to occur and the % time the event occurred. % % See also INSTRCALLBACK.
callbackTime = datestr(datenum(event.Data.AbsTime)); fprintf(['A ' event.Type ' event occurred for ' obj.Name ' at ' callbackTime '.\n']);
Now, echo a string from the instrument. Once the terminator is read from the instrument and placed in the input buffer, DISPCALLBACK is executed and a message is posted to the MATLAB command window indicating that a BytesAvailable event occurred.
>> fprintf(u, 'Hello net.')
A BytesAvailable event occurred for UDP-daqlab11 at 03-Mar-2002 12:57:55.
>> get(u, 'BytesAvailable')
ans =
11
>> data = fscanf(u)
data =
Hello net.
Note: the last value read is the linefeed (10):
>> real(data)
ans =
72 101 108 108 111 32 110 101 116 46 10
Now suppose that halfway through the asynchronous read operation, you realize that the data being read from the instrument is incorrect. Rather than waiting for the asynchronous operation to complete, you can use the STOPASYNC function to stop the asynchronous read. Note that if an asynchronous write was in progress, the asynchronous write operation would also be stopped.
>> get(u, {'TransferStatus','BytesAvailable'})ans =
'read' [225]
>> stopasync(u);
>> get(u, {'TransferStatus', 'BytesAvailable'})ans =
'idle' [265]
The data that has been read from the instrument remains in the input buffer. You can use one of the synchronous read functions to bring this data into the MATLAB workspace. However, because this data represents the wrong data, the FLUSHINPUT function is called to remove all data from the input buffer.
>> flushinput(u);
>> get(u, {'TransferStatus', 'BytesAvailable'});ans =
'idle' [0]
You can perform an asynchronous write with the FPRINTF or FWRITE functions by passing 'async' as the last input argument.
For example, configure the object to notify you when an asynchronous write operation completes.
>> set(u, 'OutputEmptyFcn', {'dispcallback'});
>> fprintf(u, 'Hello net.', 'async')A OutputEmpty event occurred for UDP-daqlab11 at 03-Mar-2002 13:06:06. A BytesAvailable event occurred for UDP-daqlab11 at 03-Mar-2002 13:06:06.
UDP sends and receives data in blocks that are called datagrams. Each time you write or read data with a UDP object, you are writing or reading a datagram. In the example below, a datagram with 11 bytes (10 ASCII bytes plus the LF terminator) will be sent to the echoserver. Then the echoserver will send back a datagram containing the same 11 bytes.
Configure the object to notify you when a datagram has been received.
>> set(u, 'DatagramReceivedFcn', {'dispcallback'});
>> fprintf(u, 'Hello net.', 'async')A OutputEmpty event occurred for UDP-daqlab11 at 04-Mar-2002 17:52:29. A DatagramReceived event occurred for UDP-daqlab11 at 04-Mar-2002 17:52:29. A BytesAvailable event occurred for UDP-daqlab11 at 04-Mar-2002 17:52:29.
If the STOPASYNC function is called during an asynchronous write operation, the asynchronous write will be stopped and the data in the output buffer will automatically be flushed. If an asynchronous read operation was also in progress, it would also be stopped. However, the data in the input buffer would not be flushed automatically.
If you are finished with the UDP object, disconnect it from the instrument, remove it from memory, and remove it from the workspace.
>> fclose(u); >> delete(u); >> clear u