See: The
PmaOpcDaClientGroup object
The standard
OPC server (providing data) receives the data from a data source (e.g. the PLC device - further we'll call it
DEVICE), stores them into its temporary cache memory (further we'll call it
CACHE) and provides them then to the connected clients.
For the optimization of the data transfer rate each data client (in this case the
PmaOpcDaClientGroup object) informs the
OPC server how it wants to share the specified data. By one of these optimizing parameters it is possible for the data group to set if the group is
active (see the "
Parameters" tab and the
ActiveGroup property). If the group is not active (nor other clients require these data), then the
OPC server needn't update these data in its
CACHE memory and thus it can update other data more often.
Another optimization is the selection of the data receipt mode. The
OPC client can decide whether it will receive the data automatically (and thus it wants the
OPC server to sent it the data values that were changed) or if it wants to control the data itself (read and write to the
OPC server on a request).
If the
OPC client is to receive the data automatically, then it must enable the update for the group (see the "
Parameters" tab and the
Update property). If the update is enabled, then the
OPC server sends changed data values of this group automatically to the client. The update can be further optimized by setting the parameters: the minimum update
rate and the
dead band (see
UpdateRate property and
Deadband property). It means that the
OPC server will not sent the data to the client faster that it is defined in the minimum update rate and it will not sent the data if the change of the value is less than the dead band (the change from the last sent value to the client is meant).
If the
OPC client is to read and write to the
OPC server itself, then it is advisable to disable the automatic
update. It is possible to leave the automatic update activated while writing, but this combination may cause that before the client writes the required value to the
OPC server, this value is updated from server. It means that the value being written to the
OPC server is already there.
For similar reasons it is often desirable - if you want to read the items and to write into them at the same time - rather to create two
OPC groups (two
PmaOpcDaClientGroup objects). One group will be determined for reading and the other for writing. Thus the real value states can be better controlled.
The following methods are used for reading and writing the
OPC server values: (can be used only if automated receipt or writing is not switched on)
Refresh - reading all values of the data group from the
OPC server
Read - reading the defined data block in the group from the
OPC server
Write - writing the defined data block in the group to the
OPC server
At the
Refresh and
Read methods it is possible to set whether the values have to be read from the
CACHE or the
DEVICE. It is presumed that most clients will read the data from the
CACHE. The direct access to the data from the
DEVICE is slower in general and it should be used only for diagnostic purposes or critical operations.
The
Write method writes the data always into the
DEVICE (thereby it is ensured that the data are always written for example into the PLC device and not only into the cache of the
OPC server).
At the 3 above-mentioned methods it is possible to set whether the operation has to be performed
asynchronously or
synchronously.
When calling the method in the asynchronous mode the transmission is only started but after calling the method it is not ensured that the physical transmission has terminated. This mode is suitable if it is necessary to write/read into/from the
DEVICE because the data transfer into the
DEVICE is slower in general (a few seconds) and in case of the synchronous mode the algorithms of the PROMOTIC application would be halted all the time.
Calling the methods in the synchronous mode is advisable for reading from the
CACHE of the
OPC server (located on the same computer as the client) because reading from the
CACHE is very fast and after the successful termination of this method it is certain that the data values have been updated. Writing algorithms for the synchronous mode is therefore simpler.
#vars identifier:
In the
PmaOpcDaClientGroup object, the
#vars identifier enables access to the variables (of the
PmVar type) defined on the "
Data" tab. This identifier can be used e.g. in the
Pm method, or in the "
PP - Pma object property" data binding. For example the method
pMe.
Pm("opcdata/#vars/aaa") returns the
aaa variable from the object named "
opcdata" that is of the
PmaOpcDaClientGroup type.
Data extension:
The variables in the
PmaData object can use
ExtOpcDa and register themselves directly into the
PmaOpcDaClientGroup object. Then, on the "
PmaOpcDaClientGroup > Data" tab, it is not necessary to define any variables.
Data vector transmission:
Some
OPC servers (for
Simatic,
Schneider,
Bernecker&Reiner ...) can
transfer a data array (Array) in one data item (the common limitation is that all data array items must be of the same data type). If it is necessary to receive/send such data as well, then the "
Data type" configurator has to be set to
Variant data type for the variable (this is disabled in
PmRtFree - Free runtime licence).
When
receiving data containing item of the
Array type, the PROMOTIC system modifies this item into the standard form (e.g. the
Integer values array is transformed into the
Variant values array of the
Integer type). Then one item from this array can be read e.g.
When
sending the data containing the item of the
Array type it is necessary to save a vector, that is not in the standard form, into this item.
It can be done e.g. by the
Pm.TransformValue method with the
41 parameter, e.g.: