PROFIBUS

What does GSD stands for?

The acronym GSD stands for General Station Description. A GSD must be available as a file for each PROFIBUS device. The performance and behaviour on the bus of PROFIBUS DP slave devices can vary. This is due to number of inputs and outputs data to be transferred cyclically, the diagnostics message, as well as bus parameters such as baud rate and time monitoring. Such properties are usually described in the technical manual for the device. For the purpose of the simple plug-and-play configuration, each device manufacturer also provides the description as an “electronic device data sheet” in the form of a GSD file.

GSD is a file provided by local control panel or instrument manufacturer and

  • is used for cyclic communication
  • is integrated into the master
  • has the same task as the EDD

What are different types of PROFIBUS protocols?

The PROFIBUS family is composed of three types of protocols, which is used for different tasks. The three types of protocols are PROFIBUS FMS, DP and PA. PROFIBUS network can be configured as line structure, line with spurs or tree structure.

PROFIBUS FMS
The complex communication protocol PROFIBUS FMS (Field bus Message Specification), was tailored for demanding communication tasks, and had been specified prior to PROFIBUS DP/PA. PROFIBUS FMS is used for non-deterministic communication of data between PROFIBUS Masters.

PROFIBUS PA
PROFIBUS PA (Process automation) is specific application profile. Communication and power supply to transmitter and positioner are handled direct via 2-Link cable. Installation in Zone 1/Div. 1 is possible. For PROFIBUS PA, available communication technologies are MBP (Manchester coded Bus Powered) and the intrinsically safe variations MBP-IS and RS-485-IS in potentially explosive atmosphere.

PROFIBUS PA has a fixed transmission rate of 31.25 Kbit/s. One byte in PROFIBUS PA requires 8 bit.

PROFIBUS DP
PROFIBUS DP (Decentralized periphery) is responsible for communication between the controller level of the automation system (PLC) and the decentralized periphery in the field.

Close circuit communication in accordance with the EIA RS-485 standard is defined as the communication technology for PROFIBUS DP. It uses a shielded two-wire copper cable. In environments subject to heavy interface and to increase the range, glass or plastic fibre optic cables are recommended.

Possible transmission rates for PROFIBUS DP are 9.6 kbit/s to 12 Mbit/s. One 1 byte in PROFIBUS DP requires 11 bit.

How does communication take place in PROFIBUS PA?

The data transfer takes place according to the following basic principles.

  • In each segment there is only one feed in source, for example in the DP/PA linking device.
  • Field devices do not feed any output into the bus.
  • In steady state, current consumption is constant for each field device.
  • Field devices behave like current sinks.
  • Subscribers transmit by changing modulation ±9 mA related to the basic current.

What does restrict the communication rate on PROFIBUS DP side?

It is not possible to mix DP devices with different transfer speeds. This means that the slowest DP component within the master/slave control circuit determines the transfer rate, and all other DP components must be set to this transfer rate.

The transfer rate of field devices whose power is supplied by the bus (PROFIBUS PA) with transfer technology based on MBP (IEC61158-2) is fixed at 31.25 kbit/s. A linking device is therefore required to connect to the controller with PROFIBUS DP. Only one rigid conversion of the transfer rates is permitted when using “passive” segment couplers. The transmission rate is assigned in this case on the DP side at 45.45 or at 93.75 kbit/s. This determines the transmission rate for the reminder of the DP bus at 45.45 or at 93.75 kbit/s. For “active” coupler segments, the PROFIBUS DP transmission rate can be adjusted up to 12 Mbit/s. this is referred to as a “DP/PA linking device”.

What would be the addressing philosophy while using DP/PA segment couplers?

When using a segment coupler (passive), the devices on the PROFIBUS PA segment constitute the address space that is shared with the devise on the PROFIBUS DP. The segment coupler is transparent and it connects the two different physical layers (RS 485 and MBP) with one another. It does not have an address on the PROFIBUS itself.

The DP/PA link (active) is not transparent but a slave on PROFIBUS DP and itself the master in the downstream PROFIBUS PA segments. For this reason a new PROFIBUS network with its own address space from 0 to 125 commences after the link.

How many devices could be connected to a PROFIBUS segment?

Up to 126 DP or PA field devices can be addressed (0-125) by PROFIBUS DP with a connecting circuit of a PLC or a DCS. One additional restriction on the number of subscribers results from the fact that each subscriber on the bus weakens the bus signal. If there are more than 32 subscribers (including the master) a repeater must therefore be used to maintain the availability of the bus signal.

For PROFIBUS PA if you have two-wire field devise in your network, you have to make sure they have enough voltage to operate. Each device should have at least 9 volts. You need to know:

  • The current consumption of each device
  • Its location on the network
  • The location of power supply on the network
  • The resistance of each cable section
  • The power supply voltage

Also consider that only a maximum of six (6) IS-bus-powered devise can be on the same segment.

What is the definition of PROFIBUS functions?

There are three types of functions defined for PROFIBUS protocols

Basic function V0      

  • Cyclic data transfer
  • Diagnostics
  • Configuration via GSD file

Extended function V1

  • Acyclic Data transfer
  • Alarm Handling
  • FDT (Field Device Tool)/DTM (Device Type Manager) and EDD device management
  • Function blocks according IEC 61131-3

Extended function V2

  • Device to device communication
  • Time and time stamp
  • Equidistant time cycle
  • Up and down load functions
  • PROFISAFE

What types of masters are defined in PROFIBUS and what is multi-master principle?

There are two type of masters defined in PROFIBUS network.

  • Class 1: Cyclical masters like controllers
  • Class 2: Non-cyclical masters like devices used for setting

A class 1 master can communicate actively only with its configured slaves and is able to communicate in a passive way with a class 2 master.

The class 2 master is the “supervisory” master. He can communicate with other class 1 masters, their slaves and his own slaves for configuration, diagnostic and data/parameter exchange purpose.

When using station addresses in multi-master systems in order to make sure that token rotation is optimised, remember

  • No gap between the master addresses
  • Limit possible master addresses to top
  • No slave addresses between master addresses

A cyclic master can transfer its bus access authorization to another master (token passing procedure). In this manner several masters can retrieve message over a PROFIBUS network from the slaves that are assigned to them. This is known as multi-master principle.

Which standards are applicable to PROFIBUS protocol?

  • British Standard BSEN50170
  • Euro Standard EN50170
  • US Standard None

What does coupler serve for?

“Coupler” is physical interface between trunk and spur or trunk and device.

What is difference between RS232, RS422 and RS485?

The RS 232 signals are represented by voltage levels with respect to ground. This interface is useful for point to point communication at slow speed.

RS 422 is for full duplex data transmission. Two wires are used to transmit data and two wires are used to receive data. The data bits high/low are detected as a voltage difference between the two wires. This is a big difference to RS 232 where the voltage is measured to a common ground wire.

The only difference in hardware of RS 485 and RS 422 is the ability to set the output to high impedance state.

What is trunk, spur and splice in a PROFIBUS network?

A trunk is the longest cable path between any two devices on the network. One you have identified this stretch of cable, all other connections to it are called “spurs”. A “spur” that is less than 1 m in length is considered as a “splice”.

How many PROFIBUS segments can be implemented in the control network?

The maximum of the line of the PROFIBUS can be increased by using repeaters. The number if segments (=number of repeaters in series) depends on the type of repeaters. With standard repeaters (RS 485), up to 3 can be connected in series, while in the case of repeaters with signal refresh, up to 9 can be connected in series.

Pay attention that only a maximum of four repeaters/or active couplers can be used between any two devise on a fieldbus network (IEC/ISA physical layer standard, section 22.22, Rule 3).

What do you recommend for PROFIBUS segment length?

The cable used for PROFIBUS PA/Foundation fieldbus H1 includes two wires.  The power supply to field devices included in this two wires. The recommended segment length depends on

  • Cable type/characteristics/wire gage
  • Network topology
  • Type of devise in network

The copper PROFIBUS cable must be kept less than 200 m. Use fibre optic cables for longer distances. Optical link modules (OLM) can be used for adapting copper cable and fibre optic cable.

Hirschmann OLMs are recommended. Hirschmann switches are DIN connected, very expensive but more secure and can be used for ring network configuration. Limit optic cable length to 15 km.

Sometimes you might need to mix cable types. The maximum length for the two types is determined according to a formula:

 Profibus lentgh

Where

Lx= Length of cable x
Max y= Maximum length for cable y alone.

This idea is easily extended to four cable types. The formula for four types is

Profibus lentgh 2

Speaking of terminators, how do you know where to put them?

To answer this you need to define a “trunk”. A trunk is the longest cable path between any two devices on the network. One you have identified this stretch of cable, all other connections to it are called “spurs”. The termination shall be placed at the ends of trunk. There need to be two (and only two) terminators per bus segment. A terminator should be used when using repeaters for each segment below the repeater.

A 100Ω resistor in series to a 1 µF capacitor makes a termination for PROFIBUS.

What do you recommend for PROFIBUS cable design?

Cable recommendation for PROFIBUS PA

  • Cable design: twisted wire shielded 1×2, 2×2 or 1×4
  • Wire diameter: >0.34 mm2 (AWG22)
  • Impedance: 35 to 165 Ω – 3 up to 20 MHz
  • Capacitance: <30 PF per metre

Back to technical Q&A