wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

INTERBUS 33-7
include the entire data of an application process (e.g., measured values, programs, or events). The process
objects are entered in the object dictionary (OD) as communication objects. The OD is a standardized
public list, in which communication objects are entered with their properties. To ensure that data
are exchanged smoothly in the network, additional items must be standardized, in addition to the OD,
which can be accessed by each device. This includes device features such as the manufacturer name or
defined device functions that are manufacturer independent. These settings are used to achieve a closed
and manufacturer-independent representation of a real device from the point of view of the communication
system. This kind of modeling is known as a VFD.
33.3 Diagnostics
The system diagnostics plays an important role in real-world applications. In increasingly complex
systems, errors must be located quickly using system diagnostics and must be clearly indicated to the
user. For this reason, a good error diagnostics, in addition to detecting errors, must include reliable
error localization. For usual message-oriented fieldbus systems, which are based on a bus structure,
only one message is ever transmitted to a device at any time. An error, which affects the system via
a specific device, can even destroy messages, which are not addressed to the faulty device itself, but
may be directed toward other remote devices. So it is therefore virtually impossible to determine the
exact error location.
INTERBUS uses the CRC algorithm in each device to monitor the transmission paths between two
devices and, in the event of CRC errors, can determine in which segment the error occurred.
In order to maintain data communication in the system, the master must be able to cope at least with
the following errors:
• Cable break
• Failure of a device
• Short circuit on the line
• Diagnostics of temporary interference (e.g., caused by electromagnetic interference)
In all fieldbus systems, in the event of a line interruption, the devices after the interrupt are no longer
reachable. The error localization capability depends on the transmission system used. In linear bus
systems, messages are broadcast to all devices. However, in case of a line break, these messages are lost
because the devices are no longer able to respond. Bus operation cannot be maintained because, due to
the interruption, the condition of the physical bus termination using a termination resistor is no longer
met. This can lead to reflections within the bus configuration. The resulting interference level will make
correct operations not possible. After a certain period, the master will detect the data loss but will not
be able to precisely determine the error location. The wiring diagrams must be consulted, so that the
service or maintenance personnel can determine the probable error location (see Figure 33.7).
Unlike bus systems, the individual devices in the INTERBUS system are networked, so that each
one behaves as a separate bus segment. Following a fatal error, the outgoing interfaces of all devices
are fed back internally via a bypass switch. In the event of a line interrupt between the devices, the
master activates each separate device in turn. To do this, the master opens the outgoing interface,
starting from the first device up until the error location, thus clearly identifying the inaccessible
device. The bus master can then clearly assign the error location. If a device fails, the fieldbus behaves
in the same way as for a line interrupt. However, the functional capability of the remaining stations
differs in bus and ring systems.
Short circuits on the line are a major challenge in a bus system. In the event of a direct or indirect
(e.g., via ground) short circuit on the line, the transmission path is blocked for the entire section.
In bus systems, one transmission line is used for all devices, which means that the master cannot
reach segment parts either. Signal reflections due to impedance mismatching will make communication
impossible. This considerably reduces further error localization. Linear bus systems also support
© 2011 by Taylor and Francis Group, LLC