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Profibus Overview

TURCK 8/1/2018

At-A-Glance

What is it?
PROFIBUS is an industrial network protocol that connects field I/O devices in order to eliminate hard wiring.

What are its basic components?
Master, slaves, communication cable, power supply and power cable (PROFIBUS-DP)

Where is it used?
Machine control applications, process and hazardous area situations.

Who is responsible for it?
PROFIBUS is maintained by the international governing body: PI (PROFIBUS & PROFINET INTERNATIONAL)

Overview

PROFIBUS is an industrial network protocol that connects field I/O devices in order to eliminate hard wiring. The network connection increases device-level diagnostic capabilities, while also providing high-speed communication between devices.

PROFIBUS-DP

PROFIBUS-DP is the version of PROFIBUS that is generally used for factory automation and machine control solutions. It is based on the RS-485 serial data transfer standard. In most cases, the termination and physical media rules for PROFIBUS-DP are the same as those required for RS-485 communication. A PROFIBUS-DP network supports up to 126 nodes and virtually an unlimited variety of I/O. The bus uses a trunkline/dropline topology. Power and communication are provided via separate cables, allowing easy segmentation of the power structure to avoid overloading.

PROFIBUS-DP is capable of running at data rates as high as 12 Mbaud. When used at high data rates, the cable drop length from the trunk to a node is severely limited. For example, when used at 12 Mbaud, nodes must be directly connected to the trunk, with no drop length allowed.

Although PROFIBUS-DP is well suited to machine control applications, it is also useful for process and hazardous area situations (in fact the name PROFIBUS is an abbreviation of Process Field BUS). TURCK’s excom system allows connection from a PROFIBUS-DP network directly to I/O devices in classified areas, resulting in a huge potential savings on barriers and wiring.

PROFIBUS-PA

PROFIBUS-PA is another version of PROFIBUS, designed for hazardous area usage though it may also be used in non-hazardous areas due to a variety of reasons such as topology and device parameterizations. It operates as an extension from the PROFIBUS-DP system. Using the same media specification as FOUNDATION fieldbus (IEC 61158-2), it allows network communication directly in hazardous areas. All devices on a PROFIBUS-PA system are controlled by the PROFIBUS-DP master. The conversion from DP to PA is accomplished by a linking or gateway device, which converts from the high speed RS-485 DP communication to the lower speed (31.25 kbps) IEC 61158-2 communication required for PROFIBUS-PA. The logical communication structure is identical between the systems; only the physical media is different.

A typical PROFIBUS-PA network, connected to a higher-level PROFIBUS-DP system

Profibus Overview

PROFIBUS Structure

A typical PROFIBUS system consists of the following parts:
• Master
• Slaves
• Communication cable
• Power supply
• Power cable (PROFIBUS-DP)

PROFIBUS stations require a network master (also called a scanner) to interface the stations to the host controller. TURCK PROFIBUS-DP stations are designed to be fully compatible with PROFIBUS-DP equipment from other manufacturers.

Communication Rate/Cycle Time

PROFIBUS-DP specifications define multiple transmission speeds ranging from 9.6 kbaud to 12 Mbaud. All nodes on a network must communicate at the same rate. The complete cycle time of a PROFIBUS-DP system is affected by several factors:

• Number of nodes being scanned
• Amount of data produced and consumed by the nodes
• Network communication rate
• Cycle time of the control program
• Number of masters present

All of these factors must be considered when calculating the cycle time of a particular network.

PROFIBUS

Maximum Ratings

The PROFIBUS-DP bus uses a trunkline/dropline topology. The trunk is the main communication cable and requires the appropriate RS-485 active termination at both ends of the network. Active termination requires a bias voltage, typically supplied by the network nodes, to function. Turck offers these terminating resistors as a plug-in eurofast unit or built into their D9 connectors. If terminating at the end of the cable in desired, Turck offers a special Terminating resistor in an IP67 form factor that can be supplied externally with this bias voltage. The length of the trunk depends on the communication rate. Drops or branches off the trunk are allowed, but are greatly limited as the communication rate increases. The list below shows the maximum ratings for a PROFIBUS-DP trunk at different communication rates.

Communication Rate - Max. Segment Length
9.6 kbps - 1200 m
19.2 kbps - 1200 m
93.75 kbps - 1200 m
187.5 kbps - 1000 m
500 kbps - 400 m
1.5 Mbps - 200 m
12 Mbps - 100 m

PROFIBUS-PA has the same physical limitations as FOUNDATION fieldbus, identified in the table below.
 
CABLE NUMBER OF DEVICES MAXIMUM SPUR LENGTH
Trunk 25-32 0 m (0 ft.)
19-24 30 m (98 ft.)
15-18 60 m (197 ft.)
1900 meters 15-18 60 m (197 ft.)
13-14 90 m (295 ft.)
2-12 120 m (394 ft.)
 
Diagnostics

TURCK network stations provide increased diagnostics over using traditional hard-wired I/O systems. TURCK stations also serve as a buffer between I/O devices and the PROFIBUS-DP network by detecting short circuits without disrupting communication. The PROFIBUS-DP system includes a provision for special diagnostic data messages. These messages are triggered when a fault occurs at the station (for example a short circuit on a sensor). When the master asks the station for data, the station responds and includes a flag to indicate that diagnostic data is present. The master then asks for the diagnostic data, which is mapped to a special location in the controller’s memory.
 
Addressing

The valid range of PROFIBUS node addresses is 0 to 125. TURCK station’s addresses are usually set via rotary dials or switches on the node. Changes to the address settings take effect when the station power is cycled or when the station receives a software reset. Care must be taken to prevent the same address from being assigned to more than one node in a system. Bihl+Wiedemann PROFIBUS-DP to AS-i gateways addresses are set in software using the on- unit display.

Cordsets

TURCK offers a complete line of molded PROFIBUS-DP and PROFIBUS-PA cordsets to facilitate network installation, resulting in a faster start-up and fewer wiring errors. The bus and drop cables are specially designed foil-shielded, high-flex cables with very low inductance and capacitance to minimize propagation delay time. PROFIBUS-DP cables consist of a shielded and twisted data pair with a bare drain wire. PROFIBUS-PA cables feature a shielded, twisted pair for data and bus power and a drain wire.

In most cases, connections of the bus cable to the stations are made using 5-pin reverse-key eurofast (M12) connectors for PROFIBUS-DP. A variety of stations are also available that support D9 type connections. Power for most stations is provided through one or two 5-pin minifast (7/8-16UN) connectors.

PROFIBUS-PA connections are typically made with minifast style connectors, though eurofast connections are available as well. TURCK cordsets for the PROFIBUS system are available in standard lengths. Please contact your local sales representative to order custom lengths.

Typical PROFIBUS-DP Cable

Profibus Overview

Typical PROFIBUS-PA Cable

Profibus Overview

GSD Files

GSD files contain detailed information about a PROFIBUS-DP device, including I/O data size and the devices configurable parameters. The information in a GSD file, when used with a PROFIBUS-DP configuration tool, guides a user through the steps necessary to configure a device.