DDC or Digital Direct Control has been a widely misunderstood yet vital industry for building control. DDC primarily refers to the control of Heating, Ventilation, Air-Conditioning and Refrigeration systems. Since all modern buildings and installations require DDC or some variety of Building Management System, in this article I will take you back to basics on DDC.

    Many years ago, I was responsible for the engineering services in a large public hospital. After being so deeply involved with the intricacies of building operation and management,  many factors led me to believe that there was a better way to control HVAC&R. As a result, I have been manufacturing, installing and supporting Australian made Direct Digital Control systems for building system management for over 20 years.

    DDC’s roots begin back in the 1970’s when main frame computers were used to monitor and crudely control plant  and follow the growth of computing with the invention of the microprocessor. Since then, the industry has grown to include smart controllers that can control plant without the need for communication, to internet based communication and even, in some cases, wireless controls.

The basis for any DDC system are points.

      What are points?

A physical point is an input or output that is wired between the controller card and the field device.

Also there can be variable or virtual points that are non-physical and only exist within a program. These points are stored in memory and are used for example as set points or results of calculations.

      Binary Point Types

A binary or digital point can be either an input or output and will operate in only two states true (on) or false (off).

Binary  Input

A binary input detects a voltage or non-voltage condition( eg ON/OFF) which can then be used to view the status of a fan via an airflow switch or an after hours push button.

Binary Output

A binary output is effectively a relay that open or closes an electrical circuit that will  turn on or off a field device such as a fan or pump.

      Analogue Point Types

An analogue point can be either an input or output and will either read a voltage signal between a certain range that will allow precise control of the state of an item as opposed to a digital point that can only be on or off.

      Analogue Input

An analogue input comes from a device with a variable signal such as a temperature or humidity sensor. The standard types used in the control industry are Active (voltage): 0-10 Volts DC , Current Loop (current): 4-20 mA  and resistive(resistance):10kohm.  Our experience has found that the resistive sensors are the cheapest to purchase yet are highest to maintain as they drift in calibration and therefore report incorrect values.

      Analogue Output

An analogue output is a driven device with a variable electrical signal such as a variable speed drive or valve actuator. The standard types used in the building control industry are 0-10 VDC or 2-10 VDC.

Although there is a tight reign on how the ways a DDC system can control field devices in a building, devices can communicate with other devices through any communication protocol.

      Communications

Protocols have been developed over the years in an attempt to give the owner and operator the flexibility to mix manufactures’ hardware into a common system. To-date none of the main line protocols have succeeded in fulfilling this much desired outcome. There are many different types of communication protocols in DDC, but all these fall under two categories: Proprietary and Open protocols.

      Proprietary Protocols

Proprietary protocols simply mean that a company has development its own internal communications protocol and its does not have to communicate with any different equipment.

      Open Protocols

There is a great number of open protocols formed for DDC, however industry only supports the major protocols. These include LonWorks, BACnet, XML and Modbus.

LonTalk is a protocol originally developed in the early 1990’s optimized for control created by Echelon Corporation for networking devices over media such as twisted pair. It is popular for the automation of various functions in industrial control, home automation, transportation, and buildings systems such as lighting and HVAC. LonTalk is part of the technology platform called  LonWorks. The system is usually installed into buildings in small segments to prevent bandwith problems.

BACnet was a communications protocol formed in 1995 for the Building Automation Industry. BACnet was to be entirely open sourced and simple for devices to communicate, while many larger companies used this protocol for this reason, soon it was apparent that simple communication between manufacturers devices was never going to be realized. In addition, since the standard has been re-published twice in the last six years, there is a great confusion over communications specifications.

Modbus is an entirely open protocol that has its roots in PLC control. It has continued to gain popularity as it is a true open source protocol, however it has been held back due to several different types of data transfer options and limitations that include network bandwidth and communication issues for larger networks.