Scalar Item Class
Data items represent a single piece of data related to a device.
A scalar item is a data item which represents a simple value type such as an integer or string. A scalar item class provides information about how to read (and write, if supported) the data associated with a scalar item. A scalar item class defines several attributes about a scalar item.
Attributes
Class attributes are part of the scalar item class; they apply to all instances of a scalar item of this class.
ID (Required)
A scalar item class’s id is a string identifying an individual scalar item class. This must be unique within the Library or Scalar Items object that contains the scalar item class.
Name (Required)
A string containing the human-readable name of the scalar item.
Description
A string that describes the purpose of a scalar item in human-readable language.
Data Type (Required*)
A scalar item class’s datatype specifies how to interpret the data stored with the scalar item.
*Data Type is not required when the scalar item class includes the extends attribute. This attribute should be excluded, and will be ignored if extends is included.
The possible types are:
number |
A numeric value, either integral or floating point. |
string |
A UTF-8 string. |
binary |
Opaque binary data. |
boolean |
A boolean value (true or false). |
enum |
A numeric integral value. |
uuid |
A universally unique identifier compliant with RFC 4122. |
Unit (Required*)
A scalar item class’s unit defines how the scalar item’s value should be interpreted. Units are only well-defined for scalar items with data type number.
*Unit is only required when the data type attribute is included, and is defined as number. This attribute should be excluded, and will be ignored if extends is included.
Unit Identifier |
Description |
---|---|
degree-celsius |
Temperature in Degrees Celsius |
degree-fahrenheit |
Temperature in Degrees Fahrenheit |
kelvin |
Temperature in Kelvin |
volt-dc |
DC Voltage in Volts |
volt-ac-peak |
Peak AC Voltage in Volts |
volt-ac-rms |
RMS AC Voltage in Volts |
amp-dc |
DC Current in Amperes |
amp-ac-peak |
Peak AC Current in Amperes |
amp-ac-rms |
RMS AC Current in Amperes |
hertz |
Frequency in Hertz |
ohm |
Resistance in Ohms |
watt |
Power in Watts |
kilogram |
Mass in Kilograms |
meter |
Length or Position in Meters |
square-meter |
Area in Square Meters |
cubic-meter |
Volume in Cubic Meters |
meter-per-second |
Velocity in Meters per Second |
meter-per-second-squared |
Acceleration in Meters per Second Squared |
newton |
Force in Newtons |
joule |
Energy in Joules |
pascal |
Pressure in Pascals |
second |
Time in Seconds |
degree |
Angle in Degrees |
steradian |
Angle in Steradians |
candela |
Luminous Intensity in Candela |
lumen |
Luminous Flux in Lumens |
lux |
Illuminance in Lux |
ire |
Chrominance in IRE |
byte |
Data Size in Bytes |
percent |
Percentage of a Maximum Value (1.0 = Maximum Value) |
Default
This attribute defines a default value for the scalar item class. It applies in different ways when combined with different data types. If this value is outside of the min/max range (either fixed or calculated) in a scalar item of this class, the nearest in range value shall be used.
If a default is also provided in a scalar item it overrides the value defined here.
Default when applied to…
When applied to… |
Default indicates… |
---|---|
number |
The number’s default value. |
string |
N/A |
binary |
N/A |
boolean |
N/A |
enum |
The default enumerated index. |
object |
N/A |
Extends
Indicates the id of the scalar item class this class extends. Scalar items may be extended for the purpose of renaming another previously defined scalar item class.
Markup
UDR Type:
scalaritemclass
Members:
Key
Value Type
Represents
id
string
name
string
description
string
datatype
string
unit
string
extends
string
Example:
{
"udrtype": "scalaritemclass",
"id": "strobe-frequency",
"name": "Strobe Frequency",
"description": "The frequency of a strobe shutter in Hertz",
"datatype": "number",
"unit": "hertz"
}