Profibus-DP

INTRODUCTION

PROFIBUS Decentralised Peripherals enables devices from different manufacturers to communicate without special interface adaptations. PROFIBUS-DP is a variant optimised for speed and low connection costs for fast data exchange in the relevant applications. It enables the central control devices, such as SPC (stored program control) / PC, to communicate via a fast, serial connection with specific peripheral devices such as drives, valves or rotary encoders. The data exchange with these peripheral devices is mainly cyclical.

PROFIBUS - schnelle, serielle Verbindung mit dezentralen Feldgeräten

The communication functions required for this are defined by the PROFIBUS-DP basic functions in accordance with the standard EN 50 170.PROFIBUS is a manufacturer-independent, open field-bus standard for diverse applications in manufacturing, process and building automation.

The independence from manufacturers and openness are defined in the standard EN 50 170.The AC 58 is an absolute rotary encoder (encoder, angle encoder). In the version described here, this encoder sends its current position to another bus device via the transmission medium "PROFIBUS-DP" (physical design: twisted and shielded 2-wire cable).

The AC 58 encoder supports all the functions of classes 1 and 2 listed in the encoder profile.

GENERAL INFORMATION

The basic PROFIBUS-DP (DP: decentralised peripherals) functions are only described here in part. Further information is given in the standard for PROFIBUS-DP DIN 19245-3 or the standard EN 50170.

AREA OF APPLICATION

In systems in which the position of a drive or another machine part must be recorded and reported to a control system, this task can be carried out by an AC 58 encoder.

In this way, positioning tasks can be solved, for example, in cases where the feedback on the current drive position is reported by an AC58 encoder to the positioning unit via the PROFIBUS-DP.

BASIC PROFIBUS-DP FUNCTIONS

The central controller (master) cyclically reads the input information from the slaves and writes the output information cyclically to the slaves. For this purpose, the bus cycle time must be shorter than the program cycle time of the central programmable logic controller (PLC), which is about 10 milliseconds in many applications.

In addition to the cyclical transmission of user data, high-performance functions for diagnostics and commissioning are also available with the PROFIBUS-Depth data traffic is monitored by monitoring functions on the master side and slave side.

The following table gives a summary of the basic PROFIBUS-DP (decentralised peripherals) functions:

  • Transmission technology: RS-485 twisted two-wire line
  • Baud rates from 9.6 kbit per second up to 12 Mbit per second

Bus access:

  • Token-passing procedure between the masters and master-slave procedures for slaves
  • Mono-master or multi-master systems are possible
  • Master and slave devices, maximum of 126 connected devices on one bus

Communication:

  • Point-to-point (user data traffic) or multicast (control commands)
  • Cyclic master-slave user data traffic and non-cyclic master-master data transfer

Operating conditions:

  • Operate: cyclical transmission of input and output data
  • Clear: Inputs are read, outputs remain in the safe status
  • Stop: only master-master data transfer is possible

Synchronisation:

  • S Control commands enable synchronisation of the inputs and outputs
  • Sync mode: Outputs are synchronised

Functionality:

  • Cyclical user data transfer between DP master and DP slave(s)
  • Dynamic activation or deactivation of individual DP slaves
  • Checking the configuration of the DP slaves
  • High-performance diagnostic functions, 3 graded diagnostic message levels
  • Synchronisation of the inputs and/or outputs
  • Address assignment for the DP (decentralised peripherals) slaves via the bus
  • Configuration of the DP master (DPM1) via the bus
  • A maximum of 246 bytes of input and output data per DP slave is possible

Protection functions:

  • All messages are transmitted with Hamming distance HD = 4
  • Response monitoring for the DP slaves
  • Access protection for inputs / outputs of the DP slaves
  • Monitoring of user data traffic with an adjustable monitoring timer for the master

Device types:

  • DP master class 2 (DPM2), e.g. programming / configuration devices
  • DP master class 1 (DPM1), e.g. central automation devices such as PLC controller
  • PC DP slave e.g. devices with binary or analog inputs / outputs, drives, valves

BASIC FEATURES / SPEED

For the transmission of 512-bit input and 512-bit output data sent to 32 connected devices, PROFIBUS-DP only needs about 1 millisecond at 12 Mbit per second.

The following figure shows the typical transmission time for PROFIBUS-DP depending on the number of connected devices and the transmission speed. The high speed is due in particular to the fact that the transmission of the input and output data is carried out in one message cycle by using the Layer 2 SRD service (Send and Receive Data Service).

Diagnostic functions:

The extensive diagnostic functions of PROFIBUS-DP enable fast fault location. The diagnostic messages are transmitted via the bus and collated by the master. They are divided into three levels:

Location-specific diagnosis

messages on the general operational readiness of a connected device, such as an excessive temperature or under-voltage.

Module-specific diagnosis

These messages show that a diagnosis is required in a certain Input / Output section (e.g. an 8-bit output module) of a connected device.

Channel-specific diagnostics:

The cause of the error is specified here, identifying a single input / output bit (channel), such as a short circuit on output 7.

Bus cycle time of a PROFIBUS-DP mono master system, limit conditions: each slave has 2 bytes of input data and 2 bytes of output data; the minimum slave interval time is 200 microseconds; TSDR (Time Series Data Repository) = 11 bit times.

SYSTEM CONFIGURATION AND DEVICE TYPES

With PROFIBUS-DP, mono-master or multi-master systems can be set up. This enables a high degree of flexibility in the system configuration. A maximum of 126 devices (masters or slaves) can be connected to one bus. The system configuration specifications include the number of devices, the assignment of the device address to the Input / Output addresses, data consistency of the Input / Output data, format of the diagnostic messages and the bus parameters used.

Every PROFIBUS-DP system consists of different types of devices. Three different types of devices are classified:

DP (decentralised peripherals) Master class 1 (DPM1):

This is a central controller that exchanges information with the decentralised devices (DP slaves) in a defined message cycle. Typical devices are, for example, programmable logic controllers (PLC), PC or VME (Virtual Machine Environment) systems.

DP Master class 2 (DPM2):

Devices of this type are programming, project planning or operating devices. These are used during commissioning to create the configuration of the DP system or for system operation when the system is in use.

DP-Slave

A DP slave is a peripheral device (Input / Output, drives, HMI interface, valves) which reads input information and transmits output information to the peripheral devices. Devices are also possible which only provide input data or only output data. The amount of input and output data depends on the device and may not exceed 246 bytes of input data and 246 bytes of output data.

PROFIBUS-DP mono-master system

With mono-master systems, only one master is active on the bus when the bus system is in operation. The figure above shows the system configuration of a mono-master system.

The PLC controller is the central control component. The DP slaves are connected as peripherals to the PLC controller via the transmission medium.

The shortest bus cycle time is achieved with this system configuration. In multi-master operation, there are several masters on one bus.

They either form sub-systems independent from each other, each consisting of a DPM1 master and its dependent DP slaves, or additional project planning and diagnostic devices (see the figure below).

The input and output images of the DP slaves can be read by all the DP masters. The outputs can only be written by a DP master (the DPM1 assigned during configuration).

Multi-master systems achieve an average bus cycle time.

PROFIBUS-DP Multi-Master System

SYSTEM CONDITIONS

In order to achieve extensive device interchangeability, the system behavior of PROFIBUSDP has also been standardized. It is essentially determined by the operating status of the DPM1. This can be controlled either locally or via the bus from the configuration device. A distinction is made between the following three main states:

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stop

There is no data traffic between the DPM1 and the DP slaves.

clear

The DPM1 reads the input information of the DP slaves and keeps the outputs of the DP slaves in a safe state.

operate

The DPM1 is in the data transfer phase In cyclic data traffic, the inputs are read by the DP slaves and the output information is transferred to the DP slaves.

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The DPM1 sends its local status cyclically to all assigned DP slaves in a configurable time interval with a multicast command.

The system reaction after the occurrence of an error in the data transfer phase of the DPM1, such as the failure of a DP slave, is determined by the operating parameter "Auto- Clear".

If this parameter is set to True, the DPM1 switches the outputs of all associated DP slaves to the safe state as soon as a DP slave is no longer ready for user data transmission.

Afterwards the DPM1 changes to the clear state.

If this parameter = False, then the DPM1 remains in the operational state even in the event of an error, and the user can determine the system response himself.

CYCLICAL DATA TRANSFER BETWEEN DPM1 and the DP-SLAVES

The data traffic between the DPM1 and the DP slaves assigned to it is automatically handled by the DPM1 in a fixed, recurring sequence. When configuring the bus system, the user defines the assignment of a DP slave to the DPM1. Furthermore, it is defined which DP slaves are to be included in or excluded from cyclic user data traffic.

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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 parameterisation and configuration phase whether the configured setpoint configuration corresponds to the actual device configuration.

The DPM1 is then used to transfer the data to the DP slave.

For this check, the device type, format and length information as well as the number of inputs and outputs must match. This provides the user with reliable protection against parameterization errors. In addition to the user data transfer, which is carried out automatically by the DPM1, it is possible to send new parameterisation data to the DP slaves at the user's request.

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user data transmission with PROFIBUS-DP

DATA TRANSFER BETWEEN DPM1 AND PROJECTING DEVICES

In addition to the functions between DP master and DP slaves, master-master communication functions are available, see table. They enable the configuration and diagnostic devices to configure the system via the bus.

In addition to the upload and download functions, the master-master functions offer the possibility of dynamically switching the user data transfer between the DPM1 and individual DP slaves on or off and of changing the operating status of the DPM1.

function
Get_Master_Diag
meaning Read the diagnostic data of the DPM1 or the collective diagnostic of the DP slaves. DPM1 strong>br /> P DPM2 strong>br /> O
download / upload group
(Start_Seq, Down/ Upload,
End_Seq)
Act_Para_Brct
Load or read the complete configuration data of a DPM1 and the corresponding DP slaves.
Activation of the bus parameters simultaneously for all addressed DPM1 devices

O


O

O


O

Act_Param Activate parameters or change the operating status of the addressed DPM1 device. O O

P: mandatory, O: option
Function overview for the master-master Functions with PROFIBUS-DP

SYNC MODE

In addition to the subscriber-related user data traffic, which is automatically handled by the DPM1, the master can send control commands to one, a group or to all slaves simultaneously. These control commands are transmitted as multicast. They enable the Sync and Freeze operating modes for event-controlled synchronisation of the DP slaves. The slaves start the sync mode when they receive a sync command from the assigned master.

The outputs of all addressed slaves are then frozen in their current state. During the following user data transmissions, the output data is stored at the Slaves, but the output states remain unchanged. The stored output data are only switched through to the outputs after the next Sync command has been received. With Unsync the Sync operation is terminated.

PROTECTIVE MECHANISMS

For safety reasons it is necessary to provide PROFIBUS-DP with effective protective functions against incorrect parameterization or failure of the transmission equipment. For this purpose, monitoring mechanisms are implemented at the DP Master and the DP Slaves in the form of time monitoring. The monitoring interval is defined during project planning.

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The DPM1 monitors the data traffic of the slaves with the Data_Control_Timer. A separate timer is used for each slave. The time monitoring responds if no proper user data transfer takes place within a monitoring interval. In this case the user is informed. If the automatic error response (Auto_Clear = True) has been enabled, the DPM1 leaves the operate state, switches the outputs of the associated slaves to the safe state and goes into the clear state.

At the DP slave

The slave carries out response monitoring to detect master or transmission errors. If there is no data traffic with the master within the response monitoring interval, the slave automatically switches the outputs to the safe state.

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Additional access protection is required for the inputs and outputs of the DP slaves when operating in multi-master systems. This ensures that direct access is only granted by the authorized master. For all other masters, the slaves provide an image of the inputs and outputs that can also be read without access authorization.

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COMMUNICATION INTERFACE

The communication interface corresponds to the PROFIBUS-DP encoder profile class 2.

includes the functions of class 1.