Value Range

In a SLOT definition (Scaling, Limit, Offset and Transfer Function), not the entire value range of an SPN is usually employed as payload. For example, in the case of an 8-bit value (2⁸ = 256) representing an interval of 0…255 values, only the range of 0…250 is defined as valid data. In the range 251…255, properties or states of the SPN are described.

In the definition of a limit switch that is to represent a door status (OPEN/CLOSE), for example, and in which one bit would be sufficient as information, the following values are defined:

• 00 – OPEN
• 01 – CLOSE
• 10 – ERROR
• 11 – SNA (Signal Not Available)

In this way, for example, sensor failures or cable breaks can be detected and the risk of misinterpretations reduced. The signal status SNA is then used if, for example, a signal is not supported in a corresponding device variant. The tables on the right provide an excerpt from the standard. Table 1 provides a portion of the physical and ASCII values. Table 2 lists measured discrete values. Table 3 describes discrete commands.

A basic rule says: If all bits in an SPN are set to ‘1’, their status is interpreted as SNA.