DIN Measurement Bus

The DIN measurement bus is included with the fieldbus systems. The intelligent automation and measuring devices of the fieldbus are controlled centrally.

The DIN measurement bus was developed in 1989 by cooperation between the Federal Institute for physics and technology and the manufacturing technology / automobile industries. For safety and bit transmission, it was standardized as DIN 66348. Six years later, the standard was expanded by a section for protocols, application services and telegrams.

The first part specifies the general characteristics for serial measurement data transmission that are not directly related to the DIN measurement bus.This was developed because of a bus solution for test and measuring technology in industrial plants that was more processor-oriented and cheaper. However, the system also is used in manufacturing engineering, e.g. in computer-controlled quality assurance, for machine and production data collection, in the connection of programmable logic controllers and for statistical process control.

Because of separate transmission / reception lines and wiring of the DIN measurement buses, it can be used in devices and installations within statutory metrology. In addition to tank facilities, the bus system also can be used in technical weighing systems and in flow measuring installations.

The DIN measurement bus system has a 4-wire bus. The EIA-RS 485 interface on which the bus system is based is a serial interface. This means that the bits are sent via one line one after the other instead of simultaneously over several lines as in the case of parallel interfaces. The RS 485 only specifies the electrical characteristics.

As such, among other things, no pin assignments are defined. The RS 485 interface of the DIN measurement bus has a 15-pole D-sub-plug. Because of the 4-wire construction, there is a separate transmission and reception line. This results in the full duplex ability, i.e. the simultaneous transfer between slave and master in both directions. This way, each subscriber has his own receiver and transmitter. This is especially advantageous because of the high bus availability, low processor load of the master station, high fault tolerance, the simple construction of power amplifiers and coupling circuits, e.g. for an infrared or optical fiber.

All of the data inputs of the slaves are wired to the data output of the master. In the same way, the master is connected to all of the data outputs of the slaves. The RS 485 interface of the DIN measurement bus must be galvanically isolated from the rest of the circuit in order to suppress changes brought about by potential differences. Electric and magnetic fields can cause interference which in turn, can be prevented by shielding.

Bus access procedure

The bus access procedure from the DIN measurement bus can only function deterministically on the basis of a master/slave system. As such, the control station controls the communication in the entire system as the only subscriber. However, each participant must first have a unique address in the network. The availability of the buses can be increased by the fact that the complete system is not blocked by a defective subscriber. Instead, only the reception line of the master is limited.

However, in this case the master still can inform the other subscribers by a broadcast telegram that is sent via the transmission line. Following this, emergency services, e.g. shutting down of the current subscriber or maintenance services, can be requested or activated. If a data transmission is interrupted, then it is continued from the last position after the connection has been established again. In an emergency, the master can end the transmission with the control character EOT (end of transmission). The bus system recognizes new or missing subscribers without errors. As such, a new system initialization is not required.

Net characteristics

The system is physically designed both for 31 slaves as well as an adjustable data transmission of up to 1 Mbps. By cascading, i.e. the linking of several modules, the participants can be summed up to 961. For the full utilization of the transmission speed, the main connection line should be 500 m long, and the feed lines to the participants should not exceed 5 m. The transmission distances can be expanded by repeaters.

DIN measurement bus protocol

The communication in a DIN measurement protocol basically takes place in three phases. They are the request, the transmission and the termination.

The DIN measurement bus protocol creates the possibility for high data security. This is established by time monitoring via the arrival of the telegrams, the checking of the transmitted data via a block check character and the confirmation of the message transmission.

Here, a special feature is the enormously short status inquiry, whether concerning data for transmission to the master or for the inquiry concerning reception readiness. As such, it is possible to react to events in real-time. The data inquiry for transmission from the master to the slave is called polling. Without an exact inquiry from the master, a subscriber cannot release data.

The transmission types are divided into four variants. With this, the parameterization, i.e. the transmission from the master to the slave, and the data transmission from the slave to the master oppose each other. Furthermore, there is lateral traffic under the control of the master, as well as the circular broadcast by the master. This broadcast makes it possible for the master to inform all participants at the same time. Transmission becomes possible by ASCII characters and/or a 7-bit code. With this, orientation follows the usual representation of the interfaces on the measuring instruments.