SIEMENS 505-6663 Communications Module

Data Formats

Because PROFIBUS DP does not enforce the use of a standard for structured data, such as floating point values, integer values, Boolean/discrete values, enumeration ordinals, etc., there exists a great variation in how data messages are formatted and interpreted amongst the vendor community. Thus, data interpretation is a complex problem for the controller. In fact, it would be virtually impossible to create a single function block that could anticipate every possible means of data representation. The data formats for the messages for a particular device are generally specified in the technical documentation that accompanies the device. The GSD file generally does not contain all of the information necessary to interpret or assemble the data messages for a particular device.

Typical Numeric Data Representation on PROFIBUS DP

Numerical data (such as real or integer numbers) is most commonly transported across PROFIBUS DP in integer format. Real numbers are converted to/from integer values through a simple linear conversion process. For example, the following equations show how a 4-20 mA value could be represented using a 16 bit unsigned integer. Bottom of raw integer value range = -20,000 = 4 mA = 0% Top of raw integer value range = 20,000 = 20 mA = 100% Each equation represents a unique point on a linear equation, and any two points define a line, from which a linear conversion equation can be derived.

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