R_A: Read analog

R_A reads present value and additional specified properties of an analog data point.

Compatible data points:

Analog input [AI]
Analog output [AO]
Analog calculated value [ACalcVal]
Analog configuration value [ACnfVal]
Analog process value [APrcVal]
Third party objects which contain the required properties.

Refer to data point objects for a detailed description of the BACnet data points.

Required properties:

Present value
State flag

Optional properties:

Reliability
Update count

Pin description (inputs)

BaObjRef
BA-Object reference Structure [BaObjRef] defines a bound connection between XFB and BACnet object. The binding occurs automatically. Manual configuration attempts are not necessary and can result in system errors.
DeviceId	Device identifier Double word (default value: 16#3FFFFF) [DeviceId] comprises object type and object instance in a single identifier. It is used to identify device object: local or remote. Local device object could also be identified by value 16#3FFFFF.
ObjectId	Object identifier Double word (default value: 16#3FFFFF) [ObjectId] comprises object type and object instance in a single identifier. It is used to identify object within a device.
ItemId	Item identifier Integer (default value: -1) [ItemId] represents an item index in the functional view node object, which references the BACnet object. [ObjectId] and [ItemId] together define an indirect access reference to the BACnet object. Value -1 indicates that direct binding is used.

Pin description (outputs)

PrVal
Present value Real (default value: 0.0) The value of the Present value property from the referenced BACnet object. [PrVal] will deliver the last valid value if [Valid] = False.

PrVal

Present value

Real (default value: 0.0)

The value of the Present value property from the referenced BACnet object.

[PrVal] will deliver the last valid value if [Valid] = False.

Valid
Valid Boolean (default value: False) An indication of whether output pin [PrVal] is reliable.
1 (True)	[PrVal] is reliable. XFB can access the BACnet object. AND Fault flag in state flag property = 0 (no fault) (Reliability = 0 (no error) or Reliability was not detected).
0 (False)	[PrVal] is not reliable. XFB cannot access the BACnet object. OR Fault flag in state flag property = 1 (fault) (Reliability ≠ 0).

Valid

Boolean (default value: False)

An indication of whether output pin [PrVal] is reliable.

1 (True)

[PrVal] is reliable.

XFB can access the BACnet object.

AND

Fault flag in state flag property = 0 (no fault)
(Reliability = 0 (no error) or Reliability was not detected).

0 (False)

[PrVal] is not reliable.

XFB cannot access the BACnet object.

Fault flag in state flag property = 1 (fault)
(Reliability ≠ 0).

UpdInd
Update indication Boolean (default value: False) An indication of whether the Present value property has been newly updated. When an update occurs, [UpdInd] toggles from 0 to 1 (True) and returns to 0 (False) on the next cycle (immediate). It is not possible to detect the toggle without an additional function block connected to [UpdInd] (for example, a counter function block). [UpdInd] will only be set to True if [Valid] is True. [UpdInd] uses the update count property if present. This makes it possible to recognize a value as having been updated in cases where the new value is the same as the previous value. For objects without update count, [UpdInd] toggles on a change to Present value.

UpdInd

Update indication

Boolean (default value: False)

An indication of whether the Present value property has been newly updated.

When an update occurs, [UpdInd] toggles from 0 to 1 (True) and returns to 0 (False) on the next cycle (immediate). It is not possible to detect the toggle without an additional function block connected to [UpdInd] (for example, a counter function block). [UpdInd] will only be set to True if [Valid] is True.

[UpdInd] uses the update count property if present. This makes it possible to recognize a value as having been updated in cases where the new value is the same as the previous value. For objects without update count, [UpdInd] toggles on a change to Present value.

ErrCode
Error code indication Integer (default value: 20) The status of the last performance of the XFB read operation.
= 0	No error.
> 0	Error, see XFB error codes.

ErrCode

Error code indication

Integer (default value: 20)

The status of the last performance of the XFB read operation.

= 0

No error.

> 0

Error, see XFB error codes.

Use case examples

NOTICE

Note
Graphics reflect examples of XFB implementations; See ABT for current examples of CFC.

Simple use: The present value is used without checking the Valid pin.

Example: Analog configuration value object used as a configuration value in the Control program (for example, PID gain).

Standard use: The present value is used only if Valid is True.

Example: Reading the analog input of a sensor.

Extended use 1: The present value is used in cases where Valid is False for specific error codes, and the error indication is overridden.

Example: Temperature sensor with an over-range reading, whose value should still be used by the control program.

Extended use 2: Extended use 2 is similar to Extended use 1, except that predefined values will be substituted for present value if certain reliability conditions are detected (over-range, under-range, loss of communication, etc.).

Example: An over-range temperature sensor reading is substituted with a predefined value for use by the control program.