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Read and Write Data to an OPC Data Access Server

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This example shows you how to use synchronous read and write operations to exchange data with an OPC server.

PREREQUISITES:

Connect to Server and Create Objects

Create an opcda client and connect that client to the OPC server.

da = opcda('localhost','Matrikon.OPC.Simulation.1');
connect(da);

Add a group to the client, and an item to the group.

grp = addgroup(da);
itm1 = additem(grp,'Random.Real8');

Perform Synchronous Read Operations

The default read operation gets values from the server cache.

r = read(itm1)

r = 

  struct with fields:

       ItemID: 'Random.Real8'
        Value: 0
      Quality: 'Bad: Out of Service'
    TimeStamp: [2016 8 30 11 55 24.4130]
        Error: ''

To force the server to read a value from the device, specify that option. This process can take a while if the OPC server is on the network or the device takes some time to produce a value.

r = read(itm1,'device')

r = 

  struct with fields:

       ItemID: 'Random.Real8'
        Value: 20.8848
      Quality: 'Good: Non-specific'
    TimeStamp: [2016 8 30 11 55 24.7220]
        Error: ''

Perform Synchronous Write Operations

Add a writable item to the group.

itm2 = additem(grp,'Bucket Brigade.Real8')

itm2 =

Summary of OPC Data Access Item Object: Bucket Brigade.Real8

   Object Parameters
      Parent        : Group0
      Access Rights : read/write

   Object Status
      Active        : on

   Data Parameters
      Data Type     : double
      Value         : 
      Quality       : Bad: Out of Service
      Timestamp     :

Write the value 10 to the item.

write(itm2,10)

Read the value back into MATLAB.

r = read(itm2,'device')

r = 

  struct with fields:

       ItemID: 'Bucket Brigade.Real8'
        Value: 10
      Quality: 'Good: Non-specific'
    TimeStamp: [2016 8 30 11 55 24.8520]
        Error: ''

Read From Multiple Items

You can read data from multiple items using the group object.

r = read(grp)

r = 

  2×1 struct array with fields:

    ItemID
    Value
    Quality
    TimeStamp
    Error

Display individual item information by indexing.

r(1)

ans = 

  struct with fields:

       ItemID: 'Random.Real8'
        Value: 20.8848
      Quality: 'Good: Non-specific'
    TimeStamp: [2016 8 30 11 55 24.7220]
        Error: ''

Extract multiple values from item.

itmIDs = {r.ItemID}
vals = [r.Value]

itmIDs =

  1×2 cell array

    'Random.Real8'    'Bucket Brigade.Real8'


vals =

   20.8848   10.0000

Write to Multiple Items

Write to multiple items, passing the values for the items in the group as a cell array.

write(grp,{1.234,5.432})

Warning: One or more items could not be written.
	Random.Real8 returned 'The item's access rights do not allow the operation.'

The previous command returns a warning, because the first item does not allow you to write data to it. However, the second has the value 5.432 written. You can verify that be reading it.

r = read(itm2,'device')

r = 

  struct with fields:

       ItemID: 'Bucket Brigade.Real8'
        Value: 5.4320
      Quality: 'Good: Non-specific'
    TimeStamp: [2016 8 30 11 55 24.9070]
        Error: ''

Clean Up

Disconnect from the server and delete the client object.

disconnect(da)
delete(da)

Deleting the client object automatically deletes the group and item objects.