Fieldbus Overview

PROFIBUS is a manufacturer - independent, open fieldbus standard with a wide range of applications in manufacturing and process automation. Manufacturer-independence and openness are guaranteed by the International standards EN 50170 and EN 50254. PROFIBUS allows devices from different manufacturers to communicate without the need for specially adapted interfaces. PROFIBUS is suitable both for fast, time-critical applications and for complex communication tasks.

PROFIBUS offers communication protocols of different functional levels: DP and FMS. According to the application, RS-485, IEC 1158-2 or optical fiber are available as transmission techniques.

PROFIBUS specifies the technical features of a serial fieldbus system with which distributed digital automation devices can be networked together, from the field level up to the cell level. PROFIBUS is a multi-master system, and therefore permits common operation of a number of automation, engineering or visualization systems with their distributed peripheral devices on one bus.

PROFIBUS device types

PROFIBUS distinguishes the following device types:

Master devices determine the data traffic on the bus. A master may transmit messages without having received an external request when it is in possession of the bus access authorization (token). Masters are also referred to as active devices.

Slave devices are peripheral devices such as input/output devices, valves, drives, measuring transducers and the Beckhoff PROFIBUS slaves from the BK3xx0, BC3xx0, IPxxxx-B310, IL230x-B310 and IL230x-C310 series. They do not receive any bus access authorization, so that they are only allowed to acknowledge messages that have been received, or to send messages in response to a request from master. Slaves are referred to as passive devices. They only require a small proportion of the bus protocol, which means that they can be implemented with little effort.

PROFIBUS DP

PROFIBUS DP is designed for efficient data exchange at the field level. The central automation devices such as PLC/PCs or process control systems communicate here over a fast serial link with distributed field devices such as I/O, drives, valves etc.. Data is primarily exchanged with these distributed devices cyclically. The communication functions required for this are specified by the basic DP functions in accordance with EN 50170.

In addition to these basic functions, PROFIBUS DP also offers extended acyclic communication services for such purposes as parameterization and other operations. These are also supported by the Beckhoff PROFIBUS slaves of the IPxxxx-B310, IL230x-B310 and IL230x-C310 series. A central controller (master) cyclically reads the input information from the slaves, and writes the output information cyclically to the slaves. The bus cycle time here should be shorter than the central automation system's program cycle time, which lies around 10 ms in many applications.

A high data throughput is not in itself sufficient for successful use of a bus system. Ease of handling, good diagnostic facilities and secure transmission technology are also of the utmost importance if the user’s demands are to be satisfied. These properties are ideally combined in PROFIBUS DP.

System configuration and device types

PROFIBUS DP allows single master or multi-master systems to be implemented. This permits a high level of flexibility in system configuration. A maximum of 126 devices (master or slaves) can be connected to one bus. A station address between 0 and 99 can be chosen for the Beckhoff PROFIBUS slaves from the IPxxxx-B310, IL230x- B310 and IL230x-C310 series. The specifications for the system configuration contain the number of stations, the assignment of the station addresses to the I/O addresses, data consistency of the I/O data and the format of the diagnostics messages and the bus parameters being used. Every PROFIBUS DP system consists of different device types. Three types of device are distinguished:

Type	Description
DP master class 1 (DPM1) e.g. Beckhoff PC master card FC310x	This involves a central controller that exchanges information cyclically with the distributed stations (slaves) in a specified message cycle. Typical devices include, for instance, programmable logic controllers (PLCs) or PCs.
DP master class 2 (DPM2)	Devices of this type are engineering, project design or operating devices. They are used for commissioning, for servicing and diagnosis in order to configure the connected devices, to evaluate measured values and parameters and to interrogate the status of devices.
DP slave e.g. Beckhoff Bus Coupler IPxxxx-B310	A PROFIBUS DP slave is a peripheral device (I/O, drive, measuring transducer etc.) that reads input information and passes output information on to the peripherals. It is also possible to have devices that only handle either input or output information. The quantity of input and output information is device-dependent, and may not exceed 246 bytes of input data and 246 bytes of output data.

In single master systems only one master is active on the bus in the operating phase of the bus system. The PLC controller is the central control element. The distributed slaves are coupled to the PLC controller via the transmission medium. The shortest bus cycle time is achieved with this system configuration.

In a multi-master mode there is more than one master on the bus. They either form sub-systems that are independent of one another, each consisting of one DPM1 and the associated slaves, or additional project design and diagnostic devices. All the DP masters can read the input and output images of the slaves. Writing the outputs is only possible for one DP master (the one assigned as DPM1 during the project design). Multi-master systems achieve a medium bus cycle time. In time-critical applications, the increase in bus cycle time should be observed by adding a diagnostic tool.

Basic device files (GSD)
In PROFIBUS DP, the performance characteristics of devices are documented by the manufacturers and made available to users in the form of a device data sheet and of a basic device file. The structure, content and coding of these basic device files (GSD) is standardized. They make it easy to plan a project with any PROFIBUS DP slaves using project planning devices from a various manufacturers. The PROFIBUS User Organization (PROFIBUS Nutzer Organization - PNO) archives this information for all manufacturers, and will provide information about the GSD from any manufacturer on request. The GSD files are read by a PROFIBUS master configuration software, and appropriate adjustments are transferred to the PROFIBUS master. Please see the appropriate software manual from the master manufacturer for a description.

The Beckhoff GSD files may be obtained from the internet under www.beckhoff.com.

Diagnostic functions

The extensive diagnostic functions of PROFIBUS DP allow rapid fault localization. Diagnosis of the Beckhoff Bus Coupler is not activated in the default setting of the type file or the GSD file. The diagnostic messages are transmitted over the bus and collated by the master.

They are divided into three levels:

Diagnosis type	Description
Related to the station	Messages relating to the general readiness of a device for operation such as over-temperature or under-voltage
Related to the module	These messages indicate that diagnostic signals are pending within a specific I/O sub range of the device (e.g. an 8 bit output module)
Related to the channel	Here the cause of an error is related to a single input/output bit (channel), such as a short circuit on output 2

The Beckhoff PROFIBUS slaves from the IPxxxx-B310, IL230x-B310 and IL230x-C310 series support the PROFIBUS DP diagnostic functions. Assessment of the diagnostic data by means of the controller depends on the support for the PROFIBUS master. Please refer to the device manuals for the master interfaces for details of how to handle the diagnosis.

Sync and Freeze Mode

In addition to the user data traffic related to the device, which is automatically dealt with by DPM1, a DP master has the option of sending control commands to one DP slave, to a group of them or to all of them at the same time. These control commands are transmitted as multicasts. These control commands can be used to specify the sync and freeze operating modes, in order to synchronize the DP slave. They permit event-controlled synchronization of the DP slaves.

The DP slaves start sync mode when they receive a sync control command from the assigned DP master. In this operating mode, the outputs of all the addressed DP slaves are frozen at their current values. In the following user data transmissions, the DP slaves store the output data, but the output states themselves nevertheless remain unchanged. Only when the next sync control command is received from the master the stored output data is switched through to the outputs. Sync operation is ended with an unsync control command.

A freeze control command similarly causes the addressed DP slaves to enter freeze mode. In this operating mode the states of the inputs are frozen at their current value. The input data is only updated again when the DP master has sent the next freeze control command to the devices concerned. Freeze operation is ended with an unfreeze command.

System behavior

The system behavior is also standardized in PROFIBUS DP, so that devices can to a large extent be interchanged. It is largely determined by the operating condition of the DPM1. This can either be controlled locally, or over the bus by the project design device.

The following three principal conditions are distinguished:

Operation mode	Description
Stop	There is no data traffic between the DPM1 and the DP slaves. The Bus Coupler only addresses the Bus Terminals once after the power has been switched on (none of the I/O LEDs are lit).
Clear	The DPM1 reads the input information from the DP slaves, and maintains the outputs of the DP slaves in a safe state (depending on the reaction to fieldbus errors, the green I/O LED is lit and the outputs are set).
Operate	The DPM1 is in a data transfer phase. In the course of cyclic data traffic the inputs of the DP slaves are read and the output information is transmitted to the DP slaves (the green I/O LED is lit).

The DPM1 sends its local status at a configurable time interval using a multicast command cyclically to all the DP slaves that have been assigned to it. The reaction that the system has to the occurrence of an error during the DPM1's data transfer phase, such as the failure of a DP slave, is specified in the Auto-Clear operating parameter. If this parameter is set to True, then the DPM1 switches the outputs of all the associated DP slaves into a safe state as soon as one DP slave is no longer ready for the transfer of user data. The DPM1 then switches into the Clear state. If the parameter is False then the DPM1 remains in the operating state even after a fault, and the user can himself specify the system's reaction.

Data traffic between the DPM1 and the DP slaves

The data traffic between the DPM1 and the DP slaves that have been assigned to it is automatically executed by the DPM1 in a specified, continuously repeated sequence. The user specifies the assignment of a DP slave to the DPM1 when the bus system's project is being planned. Those DP slaves that are included in or excluded from the cyclic user data traffic are also defined.

The data traffic between the DPM1 and the DP slaves is divided into the parameterization, configuration and data transfer phases.

Before a DP slave is included in the data transfer phase, the DPM1 checks, in the parameterization and configuration phase, whether the theoretical configuration that has been planned agrees with the actual configuration of devices. The check requires the device type, the format and length information, as well as the number of inputs and outputs, to be in agreement. The user is thus provided with reliable protection against errors in parameterization. In addition to the transfer of user data, which is automatically carried out by the DPM1, it is possible to send new parameterization data to the DP slaves at the user's request.

Protection mechanisms

In the context of distributed peripherals it is necessary, for reasons of safety and reliability, for the system to be given extremely effective functions to protect against incorrect parameterization or the failure of the transmission mechanisms. PROFIBUS DP uses monitoring mechanisms in the DP Master and in the DP Slaves. They are implemented in the form of time monitors. The monitoring interval is specified in when the DP system project is planned.

Protection mechanisms	Description
At the DP Master	The DPM1 monitors the slave's transfer of user data with the Data_Control_Timer. An individual monitoring timer is used for each assigned slave. The time monitor triggers if a proper transfer of user data does not take place within the monitoring interval. In this case the user is informed. If automatic error reaction is enabled (Auto_Clear = True) then the DPM1 leaves the Operate state, switches the outputs of the assigned slaves into a safe state, and then goes into the Clear operating mode.
At the DP Slave	The slave uses communication monitoring in order to detect errors of the master or in the transmission segment. If data is not transferred with the assigned master within the communication monitoring interval the slave switches the outputs into the safe state itself. The slave inputs and outputs further require access protection in multi-master systems, to ensure that direct access is only made from the authorized master. The slaves will make an image of the inputs and outputs available to other masters, and this can be read by any other master even if it does not have access authorization.

Protection mechanisms

Description

At the DP Master

The DPM1 monitors the slave's transfer of user data with the Data_Control_Timer. An individual monitoring timer is used for each assigned slave. The time monitor triggers if a proper transfer of user data does not take place within the monitoring interval. In this case the user is informed. If automatic error reaction is enabled (Auto_Clear = True) then the DPM1 leaves the Operate state, switches the outputs of the assigned slaves into a safe state, and then goes into the Clear operating mode.

At the DP Slave

The slave uses communication monitoring in order to detect errors of the master or in the transmission segment. If data is not transferred with the assigned master within the communication monitoring interval the slave switches the outputs into the safe state itself. The slave inputs and outputs further require access protection in multi-master systems, to ensure that direct access is only made from the authorized master. The slaves will make an image of the inputs and outputs available to other masters, and this can be read by any other master even if it does not have access authorization.

Ident number

Every DP slave and every DPM1 must have an individual identification number. This is required so that a DP master can identify the types of the connected devices without any significant protocol overhead. The master compares the identification numbers of the connected DP devices with the identification numbers in the project planning data specified by DPM2. The transfer of user data only starts if the correct device types are connected to the bus at the correct station addresses. This provides protection from project planning errors. Manufacturer-specific identification numbers are issued by the PROFIBUS User Organization (PNO). The PNO administers the identification numbers along with the basic device data (GSD).