Existing Solutions for SOAP Based Fieldbus Access

The market today offers many fieldbus gateways that are capable of providing SOAP Web Services. As stated above, SOAP Web Services are based on open standards and therefore improve the overall compatibility between computing nodes. However, it has to be considered that SOAP is a framework which may be used for an open protocol but does not guarantee openness and compatibility by itself. As the industry concentrates on vertical integration, which forces high compatibility between devices, open standards describing SOAP Web Services for fieldbus/Internet gateways have emerged which are meant to be a solution to compatibility problems.

These standards concentrate on abstracting and representing underlying technologies and provide the following two key elements:

1. Abstraction of fieldbus specific data:

Plant floor devices, such as sensors, servos or switches provide some kind of data. This data can be abstracted into a compound of variables which can be read or written. Variables representing values of fieldbus devices can then be aggregated²³ and can be presented to the Internet by a Web Service which is deployed on a fieldbus/Internet gateway. Figure 15 depicts an aggregation of values from two fieldbus nodes into one variable.

Figure 15: Aggregation of sensor data in a variable on an Internet/fieldbus gateway

2. Managing communication:

Plant floor devices can have multiple communication capabilities²⁴ which have to be represented in some way. This means that standards have to provide message objects that can be used as a proper representation of such communications. Figure 16 shows a client which sends a standardized polling message to the gateway which fetches the requested values from the proper fieldbus node.

Figure 16: Polling of data from a Sensor with a standardized polling message