User Manual

Logical I/O blocks
Value objects
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260 | 351 CM110664en_07
The value [PrVal] from the program is converted into the physical positioning value by use of a conversion
curve. This present value is then available at [PrVal] for further processing in the program while at the
same time, the raw data is transmitted to the associated I/O system, where it is converted into an electrical
signal to drive the field device.
The conversion curve is a linear function which takes the following form:
Raw Value [RwVal] = [PrVal] * Slope + Intercept
The values for slope [Slpe] and intercept [Icpt] must be defined specifically for the application concerned
in accordance with the I/O system in use and the signal type.
For slope [Slpe] and intercept [Icpt] values for SBT products, see
Slope [Slpe] and Intercept [Icpt] .
19.4 Value objects
Value objects can be seen as virtual data points which are defined in the BACnet standard and have the
same functions as the I/O blocks.
● Analog value block
● Binary value block
● Multistate value block
The only difference, in the case of value blocks, is that it is not possible to define physical connections to
sub-components or components (e.g., to I/O modules) in the plant. The value objects BVAL, AVAL and
MVAL are used in the program whenever BACnet-defined functions, such as commands, alarm generation
and runtime totalizing are required, or when a value is to be modified via an operator unit. Value blocks look
like all other blocks, and can be connected with other blocks.
Typical applications
Value objects are used typically in aggregates as command control links (PWR_CTL or CMD_CTL). The
command control mechanism passes the commands to the value object and derives the status from the
BACnet referencing system.
If
[FbAddr]
[PrVal]
[FbVal]
[PrioArr]
Feedback_Raw_Value
Analog Output
[FbVal] :=
Feedback Raw Value *Feedback Slope+ Feedback Intercept