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Guide to DDC

Chapter 1:Input Output (IO) Basics


The following terms have been defined to help readers better understand the material covered in the Input/Output document.


The term accuracy describes the total of all deviations between a measured value and the actual value. Accuracy is usually expressed as the sum of non-linearity, repeatability and hysteresis. Accuracy may be expressed as the percent of a full-scale range or output, or in engineering units.


An address is a unique numeric or alphanumeric data (point) identifier.


These synonymous terms are used to describe data that has a value that is continuous between set limits represented by a range or span of voltage, current or resistance. The value is non-integer (real) with a resolution (number of significant digits) limited only by the measurement and analog-to-digital signal conversion technology. In typical DDC systems, analog data from an input device is converted into a value for processing within the controller. Likewise, values are converted into analog output signals for use by a controlled device, such as an actuator.

Controlled Medium

A controlled medium is a process medium of which one or more properties are made to conform to desired conditions by means of a control loop (see EMS Systems Overview Basic Control loop).


These synonymous terms are used to describe data that has a value representing one state or another. Typical values are "on/off", alarm or normal, 0 or 1, high or low, etc. In the hardware side of the DDC world, these values most commonly relate to the state of a set of switch or relay contacts (open or closed).

External Point

Data that is received by a controller from an external source, or sent by a controller to an external source, is an external point. The terms hardware, input or output may be used to describe an external point.

Global Point

Global points originate from a controller within a network that is broadcast via the network to other controllers.


Hysteresis is the maximum difference in measured value or output when a set value is approached from above, and then below the value.


The term input is used to define data flow into a controller or control function.

Internal Point

An internal point is one that resides within a digital controller that does not directly originate from input or output points. Internal points can be constants such as fixed set points created by a programmers or operators assignment. Internal points may also be created as defined by the programmer/ operator by applying logic and mathematics to other virtual, input or output points or combinations of points. The terms virtual, numeric or data may be used to describe an internal point.


Non-linearity is the maximum difference in measured value or output from a specified straight line between calibration points.


Output defines the data flow out of a controller or control function.


Point is a generic term used to describe a single item of information in a control system. Points may be further described as input, output, digital, binary, discrete, analog, modulating, internal, external, virtual or [Global]. Each unique point used by digital controllers, or in digital control systems, is typically identified by an address.

Process Medium

A process medium is a material in any phase (solid, liquid or gas) that is being used in a process. The most common types of process mediums used in commercial and industrial heating ventilating and air conditioning systems are liquid mediums (i.e., chilled water for cooling) or gaseous mediums (i.e., airflow in a duct).


Repeatability is the maximum difference in a measured value or output when a set value is approached multiple times from either above or below the value.


A sensor is a device in primary contact with a process medium. It measures particular properties of the process medium (i.e., temperature, pressure, etc.) and relates those properties to electrical signals such as voltage, current, resistance or capacitance.


Transducers accept an input of one character and produce an output of a different character. (Examples: voltage to current, voltage to pneumatic (pressure) and resistance to current.)


A transmitter is a transducer that is paired with a sensor to produce a higher-level signal (typically) than is available directly from the sensor. These sensors may be integral or remote and may include digital or analog signal processing. (Examples: temperature transmitter employing a temperature sensor. The temperature sensor varies the resistance with temperature change and the transmitter outputs a related 4-20 mA current output for use by a controller.)

Virtual Point

See Internal Point

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