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PROFIBUS Transmission and Communication

3.1 Transmission Technology

In the ISO/OSI reference model, layer 1 defines the method of "physical" data transmission, i.e. electrical and mechanical. This includes the type of encoding and the transmission standard used (RS485). Layer 1 is called the physical layer.

PROFIBUS provides different versions of layer 1 as a transmission technology (see Table 4). All versions are based on international standards and are assigned to PROFIBUS in both IEC 61158 and IEC 61784.

 

Transmission Rate [KBit/s]

Range per Segment [m]

9.6; 19.2; 45.45; 93.75

1200

187.5

1000

500

400

1500

200

3000; 6000; 12000

100

The values refer to cable type A with the following properties:

Impedance 135 to 165 Ω

Capacity 30 pf/m

Loop resistance 110 Ω/km

Wire diameter > 0.64 mm

Core cross-section > 0.34 mm2

 

3.1.1 RS485 Transmission Technology

RS485 transmission technology is simple and cost-effective and primarily used for tasks that require high transmission rates. Shielded, twisted pair copper cable with one conductor pair is used. RS485 transmission technology is easy to use. No expert knowledge is required for installation of the cable. The bus structure allows addition or removal of stations or the step-by-step commissioning of the system without influencing other stations. Subsequent expansions (within defined limits) have no effect on stations already in operation.

One new option is the ability of RS485 to also operate in intrinsically safe areas (RS485-IS, see explanation at the end of this section).

Characteristics of RS485

Various transmission rates can be selected between 9.6 Kbit/s and 12 Mbit/s. One uniform speed is selected for all devices on the bus when commissioning the system. Up to 32 stations can be connected. The maximum permissible line length depends on the transmission rate. These and other properties are summarized in Table 4.

 

Installation instructions for RS485

Topology

All devices are connected in a bus structure (line). Up to 32 stations (masters or slaves) can be connected in a single segment. The beginning and end of each segment is fitted with an active bus terminator (Fig. 6). Both bus terminators have a permanent power supply to ensure error-free operation. The bus terminator is usually switched in the devices or in the connectors. If more than 32 stations are implemented or there is a need to expand the network area, repeaters must be used to link the individual bus segments.

 

Cables and Connectors

Different cable types (type designation A - D) for different applications are available on the market for connecting devices either to each other or to network elements (segment couplers, links and repeaters).

When using RS485 transmission technology, PI recommends the use of cable type A (see data in Table 3).

"PROFIBUS" cables are offered by a wide range of manufacturers; PI particularly recommends the fast-connect system which, when used with a suitable cable and special stripping tool, allows fast, reliable and extremely simple wiring. When connecting the stations, always ensure that the data lines are not reversed. Always use a shielded data line (type A is shielded) to ensure high interference immunity of the system against electromagnetic emissions. The shield should be grounded on both sides where possible and large-area shield clamps should be used for grounding to ensure good conductivity. Furthermore, always ensure that the data line is laid separately and, where possible, away from all power cables. Never use spur lines for transmission rates 1.5 Mbit/s.

Commercially available connectors support direct connection of the incoming and outgoing data cable in the connector. This eliminates the need for spur lines and the bus connector can be connected and disconnected to the bus at any time without interrupting data communications. The type of connector suitable for RS485 transmission technology depends on the degree of protection. A 9-pin D-Sub connector is primarily used for protection rating IP 20. For IP 65/67 there are three common alternatives:

  • M12 circular connector in accordance with IEC 947-5-2
  • Han-Brid connector in accordance with DESINA recommendation
  • Siemens hybrid connector

The hybrid connector system also provides a version for the transmission of data using fiber optics and 24 V working voltage for peripherals over copper cable in a shared hybrid cable.

Problems with data transmission in PROFIBUS networks can usually be attributed to incorrect wiring or installation. These problems can often be solved using bus test devices, which are able to detect many typical wiring errors even before commissioning.

For a list of suppliers of the many different connectors, cables, repeaters, bus test devices mentioned here, please refer to the PROFIBUS online Product Catalog (www.profibus.com).

 

RS485-IS

There has been great demand among users to support the use of RS485 with its fast transmission rates in intrinsically safe areas.

The PNO has addressed this task and worked out a guideline for the configuration of intrinsically safe RS485 solutions with simple device interchangeability.

Wiring and bus termination for RS485 transmission technology

Fig. 6: Wiring and bus termination for RS485 transmission technology


The specification of the interface details the levels for current and voltage that must be adhered to by all stations in order to ensure safe functioning during operation. An electric circuit permits maximum currents at a specified voltage level. When connecting active sources, the sum of the currents of all stations must not exceed the maximum permissible current.

An innovation of the RS485-IS concept is that, in contrast to the FISCO model that only has one intrinsically

safe source, all stations now represent active sources. The continuing investigations of the testing agency lead us to expect that it will be possible to connect up to 32 stations to the intrinsically safe bus circuit.

 

3.1.2 Transmission in Accordance with MBP

The term MBP stands for transmission technology with the following attributes

  • "Manchester Coding (M)", and
  • "Bus Powered", (BP).

This term replaces the previously common terms for intrinsically safe transmission "Physics in accordancen with IEC 61158-2", "1158-2", etc. The reason for this change is that, in its definitive version, the IEC 61158-2 (physical layer) describes several different connection technologies, including MBP technology, not being therefore unambiguos.

MBP is synchronous transmission with a defined transmission rate of 31.25 Kbit/s and Manchester coding. This transmission technology is frequently used in process automation as it satisfies the key demands of the chemical and petrochemical industries for intrinsic safety and bus power using two-wire technology. The characteristics of this transmission technology are summarized in Table 4. This means that PROFIBUS can also be used in potentially explosive areas and be intrinsically safe.

 

MBP

RS485

RS485-IS

Fiber Optic

Data transmission

Digital, bit-synchronous,

Manchester encoding

Digital, differential

signals according

to RS485, NRZ

Digital, differential

signals according

to RS485, NRZ

Optical, digital, NRZ

Transmission rate

31.25 KBit/s

9.6 to 12,000 KBit/s

9.6 to 1,500 KBit/s

9.6 to 12,000 KBit/s

Data security

Preamble,error-protected,

start/end delimiter

HD=4, Parity bit,

start/end delimiter

HD=4, Parity bit,

start/end delimiter

HD=4, Parity bit,

start/end delimiter

Cable

Shielded, twisted pair

copper

Shielded, twisted pair

copper, cable type A

Shielded, twisted

4-wire, cable type A

Multimode glass fiber,

singlemode glass

fiber, PCF, plastic

Remote feeding

Optional available over

signal wire

Available over

additional wire

Available over

additional wire

Available over

hybrid line

Protection type

Instrinsic safety

(EEx ia/ib)

None

Instrinsic safety

(EEx ib)

None

Topology

Line and tree topology

with termination; also in

combination

Line topology with

termination

Line topology with

termination

Star and ring topology

typical; line topology

possible

Number

of stations

Up to 32 stations per

segment; total sum of

max. 126 per network

Up to 32 stations per

segment without

repeater; up to 126

stations with repeater

Up to 32 stations per

segment; up to 126

stations with repeater

Up to 126 stations per

network

Number

of repeaters

Max. 4 repeater

Max. 9 repeater with

signal refreshing

Max. 9 repeater with

signal refreshing

Unlimited with signal

refreshing (time delay

of signal)

Table 4: Transmission technologies (Physical Layer) at PROFIBUS


Installation Instructions for MBP

Connection technology

The intrinsically safe transmission technology MBP is usually limited to a specific segment (field devices in hazardous areas) of a plant, which are then linked to the RS485 segment (control system and engineering devices in the control room) via a segment coupler or links (Fig. 7).

Plant topology and bus powering of field devices using  MBPFig. 7: Plant topology and bus powering of field devices using MBP
transmission technology

Segment couplers are signal converters that modulate the RS485 signals to the MBP signal level and vice versa. They are transparent from the bus protocol standpoint. In contrast, links have their own intrinsic intelligence. They map all the field devices connected to the MBP segment as a single slave in the RS485 segment. There is no limit to the transmission rate in the RS485 segment when using links, so that fast networks can also be implemented using field devices with MBP connection.

Network Topologies with MBP

Tree or line structures (and any combination of the two) are network topologies supported by PROFIBUS with MBP transmission.

In a line structure, stations are connected to the trunk cable using tee adapters. The tree topology is comparable to the classic field installation method. The multi-core master cable is replaced by the two-wire bus master cable, the field distributor retains its function of connecting the field devices and detecting the bus terminator impedance. When using a tree topology, all field devices connected to the fieldbus segment are wired in parallel in the field distributor. In all cases, the maximum permissible spur line lengths must be taken into account when calculating the overall line length. In intrinsically safe applications, a spur line has a max. permissible length of 30 m.

Transmission Medium

A shielded two-wire cable is used as the transmission medium, see Fig. 6. The bus trunk cable has a passive line terminator at each end, which comprises an RC element connected in series with R = 100 Ω and C = 2 μF. The bus terminator is already integrated in the segment coupler or link. When using MBP technology, incorrect connection of a field device (i.e. polarity reversal) has no effect on the functionality of the bus as these devices are usually fitted with an automatic polarity detection function.

No. of Stations, Line Length

The number of stations that can be connected to a segment is limited to 32. However, this number may be further determined by the protection type selected and bus power (if any).

In intrinsically safe networks, both the maximum feed voltage and the maximum feed current are defined within strict limits. But the output of the supply unit is limited even for non-intrinsically safe networks.

As a rule of thumb for determining the max. line length, it suffices to calculate the power requirements of the connected field devices, and to specify a supply unit and the line length for the selected cable type. The required current (=Σ power requirements) is derived from the sum of the basic currents of the field devices connected in the respective segment plus, where applicable, a reserve of 9 mA per segment for the operating current of the FDE (Fault Disconnection Electronics). The FDE prevents faulty devices permanently blocking the bus.

Joint operation of bus-powered and externally fed devices is permissible. Please note that externally fed devices also consume a basic current over the bus terminator, which must be taken into account accordingly when calculating the max. available feed current.

The FISCO model considerably simplifies the planning, installation and expansion of PROFIBUS networks in potentially explosive areas (see chapter 3.1.4).


3.1.3 Fiber Optic Transmission Technology

Some fieldbus application conditions place restrictions on wire-bound transmission technology, such as those in environments with very high electromagnetic interference or when particularly large distances need to be covered. Fiber optic transmission over fiber optic conductors is suitable in such cases. The PROFIBUS guideline (2.022) for fiber optic transmission specifies the technology available for this purpose. When determining these specifications, great care was naturally taken to allow problem-free integration of existing

PROFIBUS devices in a fiber optic network without the need to change the protocol behavior of PROFIBUS (layer 1). This ensures backward compatibility with existing PROFIBUS installations.

The supported fiber optic types are shown in Table 5. The transmission characteristics support not only star and ring topology structures, but also line structures.

In the simplest case, a fiber optic network is implemented using electrical/optical transformers that are connected to the device and the fiber optics over a RS485 interface. This allows you to switch between RS485 and fiber optic transmission within a plant, depending on the circumstances.


3.1.4 The FISCO model

The FISCO model (Fieldbus Intrinsically Safe Concept) considerably simplifies the planning, installation and expansion of PROFIBUS networks in potentially explosive areas.

This model was developed in Germany by the PTB (Physikalisch Technische Bundesanstalt – German Federal Technical Institute) and is now internationally recognized as the basic model for the operation of fieldbuses in potentially explosive areas.

The model is based on the specification that a network is intrinsically safe and requires no individual intrinsic safety calculations when the relevant four bus components (field devices, cables, segment couplers and bus terminators) fall within predefined limits with regard to voltage, current, output, inductance and capacity. The corresponding proof can be provided by certification of the components through authorized accreditation agencies, such as PTB (Germany) or UL (USA) and others.

If FISCO-approved devices are used, not only is it possible to operate more devices on a single line, but the devices can be replaced during runtime by devices of other manufacturers or the line can be expanded - all without the need for time-consuming calculations or system certification. So you can simply plug & play - even in hazardous areas! You merely need to ensure adherence to the aforementioned rules (see "Installation instructions for MBP) when selecting supply unit, line length and bus terminator.

Transmission according to MBP and the FISCO model is based on the following principles:

 

Fiber type

Core diameter [μm]

Range

Multimode glass fiber

62.5/125

2-3 km

Singlemode glass fiber

9/125

> 15 km

Plastic fiber

980/1000

< 80 m

HCS® fiber

 

approx. 500 m

 Table 5: Characteristics of optical fibers


  • No power is fed to the bus when a station is sending.
  • Each segment has only one source of power, the supply unit.
  • Each field device consumes a constant basic current of at least 10 mA in steady state.
  • The field devices act as a passive current sink.
  • Passive line termination is implemented at both ends of the bus trunk line.
  • Networks in line, tree and star topology are supported.

With bus power, the basic current of at least 10 mA per device serves to supply power to the field device.

Communication signals are generated by the sending device, which modulates ± 9 mA to the basic current.

Boundary conditions for the application of FISCO

  • Only one power source permitted per segment
  • All stations must be approved in accordance with FISCO
  • The cable length must not exceed 1000 m (ignition protection class i, category a)/ 1900 m (ignition protection class i, category b)
  • The cable must satisfy the following values:

R´= 15 ... 150 Ω/km

L´= 0.4 ... 1mH/km

C´= 80 ... 200 nF/km

  • All combinations of power supply unit and field devices must ensure that the permissible input variables of any of the field devices (Ui, Ii and Pi) must be above the, in case of a fault, maximum possible and approved output variables (U0, I0 and P0; in the US: Vmax, Imax and Pmax) of the relevant supply unit.

User benefits of FISCO

  • Plug & Play supported, even in hazardous areas
  • No system certification
  • Interchangeability of devices or expansion of plant without time-consuming calculations
  • Maximization of the number of connected devices

3.2 Communication Protocol DP

The communications protocol DP (Decentralized Peripherals) has been designed for fast data exchange at field level. This is where central programmable controllers, such as PLCs, PCs or process control systems, communicate with distributed field devices, such as I/O, drives, valves, transducers or analysis devices, over a fast serial connection. Data exchange with the distributed devices is primarily cyclic. The communication functions required for this are specified through the DP basic functions (version DP-V0). Geared towards the special demands of the various areas of application, these basic DP functions have been expanded step-by-step with special functions, so that DP is now available in three versions; DP-V0, DP-V1 and DPV2, whereby each version has its own special key features (see Fig. 8). This breakdown into versions largely reflects the chronological sequence of specification work as a result of the ever-increasing demands of applications. Versions V0 and V1 contain both "characteristics" (binding for implementation) and options, while version V2 only specifies options.

The key contents of the three versions are as follows:

DP-V0 provides the basic functionality of DP, including cyclic data exchange as well as station diagnosis, module diagnosis and channel-specific diagnosis.

DP-V1 contains enhancements geared towards process automation, in particular acyclic data communication for parameter assignment, operation, visualization and alarm handling of intelligent field devices, parallel to cyclic user data communication. This permits online access to stations using engineering tools. In addition, DP-V1 defines alarms. Examples for different types of alarms are status alarm, update alarm and a manufacturer-specific alarm.

DP-V2 contains further enhancements and is geared primarily towards the demands of drive technology. Due to additional functionalities, such as isochronous slave mode and slave-to-slave communication (DXB, Data eXchange Broadcast) etc., the DP-V2 can also be implemented as a drive bus for controlling fast movement sequences in drive axes.

The various versions of DP are specified in detail in the IEC 61158. The following explains the key characteristics.

 

3.2.1 Basic Functions DP-V0

The central controller (master)

  • reads input information from the slaves cyclically and
  • writes output information to the slaves cyclically.

The bus cycle time should be shorter than the program cycle time of the central automation system, which is approx. 10 ms for many applications. However, a faster data throughput alone is not enough for successful implementation of a bus system. Simple handling, good diagnosis capabilities and interference-proof transmission technology are also key factors. DP provides an optimum combination of these characteristics (see summary in table 6).

Functionality of the PROFIBUS DP version with key features

Fig. 8: Functionality of the PROFIBUS DP version with key features


Transmission Speed

DP only requires approx. 1 ms at 12 Mbit/s for the transmission of 512 bits of input and 512 bits of output data distributed over 32 stations.

Fig. 9 shows typical DP transmission times, depending on the number of stations and the transmission rate. When using DP, input and output data are transmitted in a single message cycle. With DP, user data is transmitted using the SRD Services (Send and Receive Data Service) of layer 2.

Diagnosis Functions

The comprehensive diagnosis functions of DP enable fast location of faults. The diagnosis messages are transmitted over the bus and collected at the master. These messages are divided into three levels:

Device-Specific Diagnosis

Messages on the general readiness for service of a station, such as "Overheating", "Undervoltage" or "Interface unclear".

Module-Related Diagnosis

These messages indicate whether a diagnosis is pending within a specific I/O subdomain of a station (for example 8-bit output module).

Channel-Related Diagnosis

These messages indicate the cause of a fault related to an individual input/output bit (channel), such as "Short-circuit at output".

 

System Configuration and Device Types

DP supports implementation of both mono-master and multi-master systems. This affords a high degree of flexibility during system configuration. A maximum of 126 devices (masters or slaves) can be connected to a bus. The specifications for system configuration define the following:

  • number of stations
  • assignment of station addresses to the I/O addresses,
  • data consistences of I/O data,
  • the format of diagnosis messages and
  • the bus parameters used.

 

Device Types

Each DP system is made up of 3 different device types.

DP Master Class 1 (DPM1)

This is a central controller that cyclically exchanges information with the distributed stations (slaves) at a specified message cycle. Typical DPM1 devices are programmable logic controllers (PLCs) or PCs. A DPM1 has active bus access with which it can read measurement data (inputs) of the field devices and write the setpoint values (outputs) of the actuators at fixed times. This continuously repeating cycle is the basis of the automation function.

DP Master Class 2 (DPM2)

Devices of this type are engineering, configuration or operating devices. They are implemented during commissioning and for maintenance and diagnosis in order to configure connected devices, evaluate measured values and parameters and request the device status. A DPM2 does not have to be permanently connected to the bus system. The DPM2 also has active bus access.

Slaves

A slave is a peripheral (I/O devices, drives, HMIs, valves, transducers, analysis devices), which reads in process information and/or uses output information to intervene in the process. There are also devices that solely process input information or output information. As far as communication is concerned, slaves are passive devices, they only respond to direct queries. This behavior is simple and cost-effective to implement (in the case of DP-V0 it is already completely included in the hardware).

In the case of mono-master systems, only one master is active on the bus during operation of the bus system. Figure 10 shows the system configuration of a mono-master system. The PLC is the central control component. The slaves are de-centrally coupled to the PLC over the transmission medium. This system configuration enables the shortest bus cycle times.

In multi-master operation several masters are connected to one bus. They represent either independent

Sub-systems, comprising one DPM1 and its assigned slaves, or additional configuration and diagnosis devices. The input and output images of the slaves can be read by all DP masters, while only one DP master (the DPM1 assigned during configuration) can write-access the outputs.

System Behavior

In order to ensure a high degree of device interchangeability among devices of the same type, the system behavior of DP has also been standardized. This behavior is determined primarily by the operating state of the DPM1. This can be controlled either locally or over the bus from the configuration device. There are three main states:

Stop

No data communication between the DPM1 and the slaves.

Clear

The DPM1 reads the input information of the slaves and keeps the outputs of the slaves in a fail-safe state ("0" output).

Operate

The DPM1 is in the data transfer phase. In cyclic data communication, inputs are read from the slaves and output information written to the slaves.

The DPM1 cyclically sends its status to all its assigned slaves at configurable intervals using a multicast command.

The reaction of the system to a fault during the data transfer phase of the DPM1, for example the failure of a slave, is determined by the "auto clear" configuration parameter.

 

Bus access

  • Token passing procedure between masters and data passing between masters and slaves
  • Mono-master or multi-master system option
  • Master and slave devices, max. 126 stations on one bus

Communication

  • Peer-to-peer (user data communication) or multicast (control commands)
  • Cyclic master-slave user data communication

Operating states

  • Operate

Cyclic transmission of input and output data

  • Clear

Inputs are read, outputs remain in fail-safe state

  • Stop

Diagnosis and parameter assignment, no user data transmission

Synchronization

  • Control commands enable the synchronization of inputs and outputs
  • Sync mode

Outputs are synchronized

  • Freeze mode

Inputs are synchronized

Functionality

  • Cyclic user data transfer between DP master and slave(s)
  • Dynamic activation/deactivation of individual slaves; checking of slave configuration
  • Powerful diagnosis functions, 3 levels of diagnosis messages
  • Synchronization of inputs and/or outputs
  • Optional address assignment for slaves over the bus
  • Maximum 244 bytes of input/output data per slave

Protective functions

 

  • Message transmission at Hamming Distance HD=4
  • Watchdog control of DP slaves detects failure of assigned master
  • Access protection for outputs of slaves
  • Monitoring of user data communication with adjustable monitoring timer in master

Device types

  • DP master class 1 (DPM1) for example central programmable controllers, such as PLCs,

PCs.

  • DP master class 2 (DPM2) for example engineering or diagnosis tools
  • DP slave for example devices with binary or analog inputs/outputs, drives, valves

Table 6: Overview of DP-V0

 

If this parameter is set to True, the DPM1 switches the outputs of all assigned slaves to a fail-safe state the moment a slave is no longer ready for user data transmission. The DPM1 subsequently switches to the clear state.

If this parameter is set to False, the DPM1 remains in the operate state even in the event of a fault and the user can control the reaction of the system.

 

Cyclic Data Communication between the DPM1 and the Slaves

Data communication between the DPM1 and its assigned slaves is automatically handled by the DPM1 in a defined, recurring sequence (see Fig. 11). The user defines the assignment of the slave(s) to the DPM1 when configuring the bus system. The user also defines which slaves are to be included/excluded in the cyclic user data communication.

Bus cycle times of a DP mono-master system Boundary conditions

Fig. 9: Bus cycle times of a DP mono-master system. Boundary conditions:
each slave has 2 bytes of input and output data

Data communication between the DPM1 and the slaves is divided into three phases: parameterization, configuration and data transfer. Before the master includes a DP slave in the data transfer phase, a check is run during the parameterization and configuration phase to ensure that the configured setpoint configuration matches the actual device configuration. During this check, the device type, format and length information and the number of inputs and outputs must also correspond. This provides the user with reliable protection against parameterization errors. In addition to user data transfer, which is automatically executed by the DPM1, the user can also request that new parameterization data are sent to the slaves.

 

Sync and freeze mode

In addition to the station-related user data communication, which is automatically handled by the DPM1, the master can also send control commands to all slaves or a group of slaves simultaneously. These control commands are transmitted as multicast commands and enable sync and freeze modes for event-controlled synchronization of the slaves.

The slaves begin 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 subsequent user data transmission, the output data are stored at the slave while the output states remain unchanged. The stored output data are not sent to the outputs until the next sync command is received. Sync mode is terminated with the "unsync" command.

In the same way, a freeze command causes the addressed slaves to enter freeze mode. In this mode, the states of the inputs are frozen at their current value. The input data are not updated again until the master sends the next freeze command. Freeze mode is terminated with the "unfreeze" command.

PROFIBUS DP mono-master system

Fig. 10: PROFIBUS DP mono-master system

Protective Mechanisms

For safety reasons, it is necessary to ensure that DP has effective protective functions against incorrect parameterization or failure of transmission equipment. For this purpose the DP master and the slaves are fitted with monitoring mechanisms in the form of time monitors. The monitoring interval is defined during configuration.

 

At the DP Master

The DPM1 uses a Data_Control_Timer to monitor the data communication of the slaves. A separate timer is used for each slave. The time monitor is tripped if no correct user data transfer is executed within the monitoring interval. In this case, the user is notified. If the automatic error handling (Auto_Clear = True) is enabled, the DPM1 exits the operate state, switches the outputs of the assigned slaves to the fail-safe state and shifts to the clear mode.

 

At the Slave

The slave uses the watchdog control to detect errors of the master or transmission. If no data communication with the master occurs within the watchdog control interval, the slave automatically switches its outputs to the fail-safe state.

In addition, access protection is required for the outputs of the slaves operating in multi-master systems. This ensures that only the authorized master has direct access. For all other masters, the slaves provide an image of their inputs and that can be read without access rights.

Cyclic user data transmission in DP

Fig. 11: Cyclic user data transmission in DP


3.2.2 Version DP-V1

Acyclic data communications

The key feature of version DP-V1 is the extended function for acyclic data communication. This forms the requirement for parameterization and calibration of the field devices over the bus during runtime and for the introduction of confirmed alarm messages. Transmission of acyclic data is executed parallel to cyclic data communication, but with lower priority. Figure 13 shows some sample communication sequences. The master class 1 has the token and is able to send messages to or retrieve them from slave 1, then slave 2, etc. in a fixed sequence until it reaches the last slave of the current list (MS0 channel); it then passes on the token to the master class 2. This master can then use the remaining available time ("gap") of the programmed cycle to set up an acyclic connection to any slave (in Figure 13 slave 3) to exchange records

(MS2 channel); at the end of the current cycle time it returns the token to the master class 1. The acyclic exchange of records can last for several scan cycles or their "gaps"; at the end, the master class 2 uses the gap to clear the connection. Similarly, as well as the master class 2, the master class 1 can also execute acyclic data exchange with slaves (MS1 channel).

Additional available services are shown in Table 7.

 

Extended diagnosis

As a further function, the device-specific diagnosis of the DP-V1 have been enhanced and divided into the categories alarms and status messages (see Fig. 12).

Configuration of diagnosis messages in DP-V0 and DP-V1

Fig. 12: Configuration of diagnosis messages in DP-V0 and DP-V1

Cyclic and acyclic communication in DP-V1

Fig. 13: Cyclic and acyclic communication in DP-V1


Isochronous mode

Fig. 14: Isochronous mode


3.2.3 Version DP-V2

Slave-to-Slave Communications (DXB)

This function enables direct and time-saving communication between slaves using broadcast communication without the detour over a master. In this case the slaves act as "publisher", i.e., the slave response does not go through the coordinating master, but directly to other slaves embedded in the sequence, the so-called "subscribers" (see Fig. 15). This enables slaves to directly read data from other slaves and use them as their own input. This opens up the possibility of completely new applications; it also reduces response times on the bus by up to 90 %.

 

Isochronous Mode

This function enables clock synchronous control in masters and slaves, irrespective of the bus load. The function enables highly precise positioning processes with clock deviations of less than one microsecond. All participating device cycles are synchronized to the bus master cycle through a "global control" broadcast message. A special sign of life (consecutive number) allows monitoring of the synchronization. Fig. 14 shows the available times for data exchange (DX, green), access of a master class 2 (yellow) and reserve (white). The red arrow identifies the route from the actual data acquisition (TI) over control (Rx) through to the setpoint data output (TO), which usually extends over two bus cycles.

Clock Control

This function (a real-time master sends time stamps to all slaves over the new connectionless MS3 services, designed for this purpose) synchronizes all stations to a system time with a deviation of less than one millisecond. This allows the precise tracking of events. This is particularly useful for the acquisition of timing functions in networks with numerous masters. This facilitates the diagnosis of faults as well as the chronological planning of events.

 

Upload and Download (Load Region)

This function allows the loading of any size of data area in a field device with few commands. This enables, for example, programs to be updated or devices replaced without the need for manual loading processes.

Function Invocation

Function Invocation services allow to control (start, stop, return, restart) of programs or call of functions (for example acquisition of measured values) in a DP slave.

 

3.2.4 Addressing with Slot and Index

When addressing data, PROFIBUS assumes that the physical structure of the slaves is modular or can be structured internally in logical functional units, so-called modules. This model is also used in the basic DP functions for cyclic data communication, where each module has a constant number of input/ output bytes that are transmitted in a fixed position in the user data telegram. The addressing procedure is based on identifiers, which characterize a module type as input, output or a combination of both. All identifiers combined produce the configuration of a slave, which is also checked by the DPM1 when the system starts up.

The acyclic data communication is also based on this model. All data blocks enabled for read/write access are also regarded as assigned to the modules and can be addressed using slot number and index. The slot number addresses the module and the index addresses the data blocks assigned to a module. Each data block can be up to 244 bytes (see Fig. 16). In the case of modular devices, the slot number is assigned to the modules. The modules begin at 1 and are numbered in ascending contiguous sequence. The slot number 0 is for the device itself.

Compact devices are regarded as a unit of virtual modules. These can also be addressed with slot number and index.

Through the length specification in the read/write request it is also possible to read/write parts of a data block. When access to the data block was successful, the slave sends a positive read/write response or may otherwise be able to classify the problem by means of its negative response.

Slave-slave data exchange

Fig. 15: Slave-slave data exchange

 

Services for Acyclic Data Communication between the DPM1 and Slaves

Read

The master reads a data block from the slave

Write

The master writes a data block to the slave

Alarm

An alarm is transmitted from the slave to the master, which explicitly acknowledges receipt. The slave can only send a new alarm message after it has received this acknowledgment; this prevents any alarms being overwritten.

Alarm_Acknowledge

The master acknowledges receipt of an alarm to the slave

Status

A status message is transmitted from the slave to the master. There is no acknowledgment.

Data transmission is connection-oriented over a MS1 connection. This is set up by the DPM1 and is closely linked to the connection for cyclic data communication. It can be used by the master that has parameterized and configured the respective slave.

 

Services for Acyclic Data Communication between the DPM2 and Slaves

Initiate

Abort

Setup and termination of a connection for acyclic data communication between the DPM2 and the slave

Read

The master reads a data block from the slave

Write

The master writes a data block to the slave

Data_ Transport

The master can write application-specific data (specified in profiles) acyclically to the slave and, if required, read data from the slave in the same cycle.

Data transmission is connection-oriented over a MS2 connection. This is set up before the start of the acyclic data communication by the DPM2 using the Initiate service. The connection is then available for Read, Write and Data_Transport services. The connection is terminated correspondingly. A slave can maintain several active MS2 connections simultaneously. However, the number of connections is limited by the resources available in the respective slave.

 Table 7: Services for acyclic data communication

 

Addressing with slot and index

Fig. 16: Addressing with slot and index

Go back to the header article PROFIBUS Technology and Application


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