Direct digital control building automation

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DDC-GA component top view

One component of Direct Digital Control building automation , or DDC-GA for short , is an electronic assembly similar to a computer that is used for control and regulation tasks in building automation .


The classic DDC-GA component largely corresponds to the programmable logic controller (PLC) and has fixed internal wiring, regardless of the respective control task. However, the DDC-GA component does not come from industrial automation like the PLC, but from building automation with the focus on control. The adaptation to the machine or system to be controlled is carried out by a program that defines the desired sequence. In contrast to this, there are compact controllers that are hardwired and can fulfill a specific control task (e.g. for individual room control ).

Many modern DDC-GA components are microcontroller systems with the appropriate firmware. The basic software consists of a real-time operating system (nowadays often a Linux derivative) and DDC-GA-specific “modules” that emulate DDC-GA functions such as adders and other logic elements. Usually software for communication and programming is also included.


The hardware is designed according to the main area of ​​application, the requirements for use in control cabinets.

  • Inputs: Analog (AI) or digital inputs (DI). Analog inputs mostly accept a standard signal (0-10V, 1-10V, 0-20mA, 4-20mA, see below), digital inputs mostly 24V DC or 5V DC.
  • Outputs: Again analog (AO) or digital (DO)
  • Interface: The interface to the CAN fieldbus modules is located on the right-hand side under a flap
  • Mounting: mounting rail directly in the control cabinet


Digital inputs (you could actually call them "binary inputs", because they only know two states) are used to record switching processes. The inputs accept a voltage of 24 V direct current , rarely 5 V (for TTL signals ) for the "high" / "true" signal and 0V for "low" / "false". Sometimes they also provide an auxiliary voltage that is let through by the sensor in the field exactly at the high signal. They are therefore used for collecting potential-free contacts and polarized interfaces such as B. suitable for the S0 interface .

Analog inputs are used to record sensor values ​​or external analog control signals. The analog inputs are sometimes configurable so that a wide variety of sensor types can be used. Other types of analog inputs can only process one of the typical signals:

  • Voltage: 0 ... 10 V or 1 ... 10 V (with wire breakage detection) - unit signal / standard signal
  • Current: 20 mA or 0 ... 4 ... 20 mA (with wire breakage detection) - unit signal / standard signal
  • Temperature measurement, in the building mostly Pt100 / Pt1000 / Ni1000 or NTC10K
  • Resistors for the feedback of burner positions, valves, potentiometers etc.


  • Digital outputs (usually binary) for switching via 24V / 5V (switching of higher voltages via coupling relays) or for direct switching of 230V (then mostly with built-in relay)
  • Analog outputs with standard signal (see above) to control lighting (e.g. dimmer with 1-10V interface), valves via the valve drive , frequency converter or via additional modules for three-point control ( PWM )


DDC-GA program

DDC-GA components are mostly programmed with special programming languages , which are often graphical. The syntax and the scope of the programming language are either manufacturer-specific or cross-manufacturer (e.g. Codesys for Wago 750-88x and Wago 750-830 controllers). In contrast to the PLC , no standardized IEC / EN standard has been established to date.

However, most manufacturers have implemented graphical programming of the controllers, as shown in the following picture:

The program contains six modules. The inputs of the modules are on the left-hand side and have short designations such as E, F, G, H. The outputs are on the right-hand side of the modules and have the short designation D.

  • S1V External sensor value flow temperature of a heating circuit
  • S19 Internal calculated value of the setpoint that controller L3 should achieve
  • L3 controller with PID part
  • G4 Logical module for AND and OR link
  • K12 Internal button for setting a fixed value at the controller output
  • F25 comparison module.

The programming is traditionally done by connecting the individual modules with each other, sometimes also using a high-level language (e.g. C). By connecting the modules, the signals are passed from one module to the other and processed further.


Levels of communication between DDC-GA and BMS

The communication between the DDC-GA components usually takes place via manufacturer-specific proprietary bus systems. The manufacturer-independent bus systems EIB / KNX , LON and BACnet have been developed for building automation since 1990 . Although designed to do so, these standards cannot always guarantee that the components can communicate with one another.

The connection to the manufacturer-independent bus systems is either natively directly on the controller, which is currently one of the more cost-intensive options, or via software gateways. Using these gateways , it is then possible to open proprietary systems and use different makes in a building. Some of the established manufacturers on the market do not want this, which is why OPC , LON or BACnet gateways cannot be found in the usual price lists, but are charged with a price specific to the building project.

From industrial automation, OPC was introduced as the standard for connecting building management systems (BMS). Due to the specific properties of the OPC protocol, this system has not yet been able to establish itself. The handicap of OPC is that a permanent online connection to the controllers is necessary for data recording, since no data records can be read from the controllers. In addition, there is no reliable alarm handling , since the values ​​must be constantly queried by the OPC client in order to be aware of a change in the value.

There is currently a trend towards BACnet / IP capable DDC-GA components and control stations. In addition, many controllers now have an integrated web server , are e-mail-capable and can transmit alarms to IT systems with SNMP .

The building management system has established itself as the user interface to the automation stations in building automation. There, the controlled processes are graphically displayed for the user in an understandable way using appropriate visualization programs .

See also


  • Working group of lecturers for control engineering at universities of applied sciences with the department of supply engineering Braunschweig, Siegfried Baumgarth: Digital regulation and control in supply engineering (DDC-GA). 2nd corrected edition, Springer, Berlin 1995, ISBN 3-540-59133-8

Web links