User Manual

Logical I/O b
locks
Value objects
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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.
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
Typical applications