The bus terminal – a coupling marvel and a quick-change artist

In the field of industrial automation technology it is necessary to connect many digital and analog inputs and outputs with each other via a wire, so that a field bus can transmit the control to these interfaces. There is a large number of such contact points, especially in the field of machine engineering and in the case of complex installations. Consequently, the task of bundling the transmitted signals is incurred so that a machine can carry out a process automatically. The bus terminal is especially suited for this. This is a higher form of the terminal strip which can process the different input and output signals. Because of this, it is now also being applied to the automation of building technology as well in the field of machine-building and plant construction. The main activity of a bus terminal is the collection of the transmitted signals and the transferring of them so that communication becomes possible. As such, it is also possible to transmit control commands via a field bus to the corresponding actuators. However, with the use of special types of such a terminal, it is possible to direct the drive of a system independently over the entire number of terminals. As such, compared to other series terminal blocks, the bus terminal has its own intelligent electronics. This means that even at this point, there is a small measure of functionality in a programmable logic controller and that simple control tasks can be executed in a decentralized way so that intervention of the actual control-enabled computer becomes insignificant. Thanks to this intelligence, these bus terminals also are called “bus-terminal controllers”.

The structure of the plug-in terminal

The name “plug-in terminal” is more correct from the engineering point of view than the common name, as it characterizes the terminal as a connection between the device and the corresponding KNX lines and EIB lines. The structure of the bus terminal has two plug-in positions on one side for coupling the device. On the other side there are twice the number of connection possibilities for the red-black wire pairs of the EIB. However, it is not necessary to connect the bus terminal to a device or a line in order to benefit from it, as it also can be free and as such, assume the job of a conventional distributor. The guide parts are another important element for the structure. In technical terms, these are also called dovetails because of their shape. One is connected to the positive pole and one is connected to the negative pole. These components make it possible to string the terminals together and as such, serve as guides (as in the case of REG devices). This ensures that the bus terminal is really locked securely. The Dehn BUStector BT24 is a special form of these terminals, in which an overvoltage conductor is integrated. The unit is installed in a switch cabinet, or more accurately on a DIN top-hat rail, into which the terminals are inserted. In the newest systems, the order of the terminals and their type does not matter as they are mutually replaceable without impairing the efficiency. As such, the bus terminal units can be reduced or expanded as required. However, normally the terminals are sorted according to the type of transmitted signal in order to take clarity into account.

Signal types of the plug-in terminal

The signals relating to the input or the output of a terminal are roughly divided into digital and analog.

  • Digital signals – these include the safe signals, the DC motor final stage, and the step motor. However, binary signals, PWM, and NAMUR also are mentioned in the list. For DC, this means 5 V, 24 V, 48 V and 120 V. For AC, the voltage values are 120 V, 230 V and 400 V.
  • Analog signals – these signals, which can process the plug-in terminal, are thermocouples, the oscilloscope, the measurement of pressure, output and the travel and/or angle, as well as the resistance sensors and bridges. Analog signals within the voltage intervals 0 to 2 V and 0 to 10 V (and the values of +/- 2 V and +/- 10 V) are registered.

The bus terminal as a milestone

Thanks to the numerous coupling possibilities, the bus terminal in automation technology is nowadays treated as a milestone and an opportunity for technical revolution. After all, now, there seems to be hardly any bus system with which this component cannot be connected. The fact, that multiple devices and field buses can be plugged into a single segment at the same time also is an indication of the correctness of this assumption. As such, it is possible to connect types of field buses such as the Profibus, Profinet IO, and Ethernet Powerlink. However, EtherCAT, CANopen and Lightbus also find their place here, and this still does not complete the list. As such, there really is no incompatible field bus. This way, even highly accurate control processes can be executed in such a way that the bus terminal can become the milestone in the field of IT and possibly already is.