Implementing ESD Functionality in the ESD Server:

The actual implementation of the ESD functionality in the OPC XML-DA server is merely a translation from OPC operations into ESD commands¹⁴⁴. The following operations had to be implemented:

OPC Read/Write:

These operations could directly be mapped to their ESD counterparts. The encoding, which is part of the item name in OPC, had to be removed from the OPC address and was given as a parameter to the ESD command¹⁴⁵.

OPC Browse:

Browsing the IGUANA gateway is different by design from browsing an OPC XML-DA compliant server. OPC provides one browse operation which can be applied to all items in the address space. On the other hand, ESD has commands which provide information about specific entities, such as ``NODELIST'' for listing all available nodes, or ``ENODEINFO'' for listing all datapoints associated to a node.

Therefore the OPC browse data had to be retrieved from the IGUANA gateway by a suitable ESD command, as depicted in figure 68.

Figure 68: Required ESD Commands in an OPC Browse Request

Some ESD requests return data which has to be filtered. For instance, an OPC browse operation of the item ``/LON/GATE1'' results in an ESD ``NODELIST'' command, which lists all available nodes, regardless of whether they are associated with the fantype ``LON'' or not. Therefore unwanted data is filtered by the server.

OPC browse operations also offer retrieval of properties associated with the retrieved items. This functionality was implemented by calling the OPC ``GetProperty'' function.

OPC GetProperties:

Similar to browsing, OPC has one operation to retrieve properties from an item, while the IGUANA gateway provides two commands, namely ``NODEINFO'' and ``DPINFO'' to retrieve properties of a node/datapoint. Therefore the requested data had to be retrieved with different ESD commands, depending on the provided item address.