Choosing a fieldbus system

Seamless communication requires a bus system that can operate all the devices connected. In addition to networking both simple and complex terminals, controllers and computers are also linked. This requires different classes of data to be taken into account. These have to be transmitted simultaneously and without interfering with each other. At the field level, it is necessary to differentiate between I/O data, e.g. setpoints and actual values, parameters. I/O data and process data cover just a few bits, are time-critical, are present in the network in large volumes, and are transmitted cyclically. Parameters are used to set and program "intelligent" devices. As opposed to process data, parameters are of an acyclic nature. This means that the data is only transmitted when needed.

The cycle time of the PLC, i.e. the duration for the processing of one cycle, sets the standard for the cycle time of the fieldbus. If we take into account the performance of today's applications, refreshing all process data within a network must be done within one to five milliseconds.

Deterministic or predictable behavior is vital for open and closed-loop control tasks, as it is otherwise not possible to determine constant and calculable sampling intervals for setpoints and actual values.

Data transmission follows certain rules referred to as the transmission protocol. The protocol transports user data (e.g. the status of a valve) and administrative data (e.g. addressing, command, data backup) to the recipients. The efficiency of a transmission protocol indicates the percentage of user data of the transmitted data as a whole. This value is the quotient of user data to the sum of all transmitted data (user and frame data). There is therefore a lower protocol efficiency when cyclical process data is transmitted, whereas long acyclic parameter blocks have a high efficiency. We distinguish between message-related procedures and summation frame procedures. In the case of the message-related procedure, a full transmission protocol is dispatched for each request. The summation frame procedure groups the data from all sensors and actuators of a network together to form one message. This is then sent to all devices at the same time, which means that the administrative data is only transmitted once. The protocol efficiency therefore increases with the number of devices in the network and for many devices, results in a considerably higher efficiency than the message-oriented procedure. The summation frame procedure guarantees fixed data lengths for the devices and therefore constant transmission times. The deterministic nature of this procedure is the precondition for calculating the time behavior.

When choosing a bus system, the benefits of high protocol efficiency should be taken into account, since high efficiency makes possible a low"gross" transmission of data: The low transmission rate provides greater immunity to electromagnetic influences and the susceptibility to interferences is lower than in bus systems with a higher transmission rate. The extension of the total system with RS 485 transmission also depends on the transmission rate. The faster the system, the smaller the distance between two devices.

In addition to the overall extension, i.e. the range of a system, the flexibility regarding the use of different media also plays a role. Of particular importance here is not only which media can be used, but also what work is involved.

The comprehensive system diagnostics functions mean that repair work can be carried out without special tools or training. It must be possible to replace faulty components without problems and without having to make settings on the device. Speedy pinpointing of the fault and its cause mean a reduction in downtime; statistical evaluation makes preventive measures possible. Since devices from different manufacturers are operated together within an open network, a simple and non-proprietary startup and diagnostics tool in the form of a computer-aided user interface is particularly important.

In addition to technical considerations, product availability is of critical importance. This is the only way to guarantee that a system can be adapted to all requirements and will be able to cope with any future requirements.