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What is PROFIBUS?

 

PROFIBUS is a digital communication system for field devices, the communication is two-way so that devices can receive information from control stations and can also send information to control stations.
PROFIBUS data can be simple - i.e. digital on/off signals, or can be complex - i.e. from/to sophisticated instruments such as on-line process analysers or actuators such as variable-speed drives etc.
To understand how PROFIBUS works, it is important to understand digital communication systems, as well as the physics that allow these communications to operate.

 

Digital Communication Systems

 

Digital communication systems transmit measured information as binary, this means that instrument readings need to be converted to binary either inside the device or via a PROFIBUS IO block. The converted binary string is sent down the cables as high-frequency switching wave-forms. Analogue systems, however, are continuous and do not convert the information, the analogue value is transmitted as a correspondingly varying voltage/current.
The measured information mentioned above is exchanged by grouping bits/bytes of binary together into messages or telegrams that pass between stations. The messages have a fixed/known notation that all the devices know how to decode according to the protocol rule, for example:

 

Created with Sketch. LE LEr SD SDr DA SA FC DSAP SSAP PDU FCS ED PROFIBUS DP Parameterization Telegram 68 hex xx xx xx xx xx xx 3D hex 3E hex xx xx … xx 16 hex 7 Mandatory bytes fore every slave Device and module related parameter Data Parameterization Data Unit (PDU) 244 bytes

The PROFIBUS Family

 

The PROFIBUS protocol family consists of three separate but interoperable standards:
  • PROFIBUS FMS - Fieldbus Message Specification
High-end, applications level communications. The User at cell or controller level. Obsolete and no longer supported.
  • PROFIBUS DP - Decentralised Periphery
Cost-effective, high speed, simple field-level communications. About 90% of current PROFIBUS applications are DP.
  • PROFIBUS PA - Process Automation
Developed specifically for the process industry, very cost-effective two-wire connection carrying both power and data. Particularly cost-effective for hazardous environments (intrinsically safe protection).
 

 

PROFIBUS DP Cable

 

Profibus DP uses RS485 as its physical data transport layer. RS485 uses two wires in a twisted pair configuration where the red wire is the B-Line or "Data line plus" and the A-Line or "Data line minus" is the green wire. It is essential that the colour scheme for A and B be used consistently throughout the network to avoid improper operation.

 

Created with Sketch. Two coloured cores, one Red and one Green Foil shield for very high frequency noise (above 50 MHz) Standard shield for high frequency noise (up to 50 Mhz) PVC outer coating (UV and chemical resistant versions available) Strengthening substrate to maintain cable shape Stranded or solid cores available, stranded cores perform better under vibration or constant bending but cannot be used with fast connect plugs

 

 

 

Digital Systems Physics

 

If a particular voltage or current is placed on the one end of a cable and slowly varied, you would expect a similar voltage or current to appear at the other end of the cable and to vary in a matching fashion. This is true at low frequencies (slowly varying signals). At higher frequencies, the resistance of the cables are no longer the only cable property of interest: the impedance becomes important and it affects the signal transmitted on the cable. If the impedance of the cable suddenly changes at some point along it, there is a reflection of a portion of the original signal back towards the transmitter, which will disrupt further messages travelling down the cable. The type of cable is important, the impedance must match the frequencies the signals vary by on the cable. Remember: Fieldbus cable is not just a wire.
Where ever electric charge is flowing, we will find both an electrical and a magnetic field. These fields can cause charge to flow in nearby conductive objects, this is known as coupling or pickup. If these fields affect nearby communications cable signals, we call the effect interference or noise. Electrostatic interference is reduced as the distance between cables are increased.

 

What are some advantages of Fieldbuses?

 

Significant reduction in the amount of wiring, cable trays, etc. Centralised commissioning of devices (from the control room). More extensive data to and from devices (i.e. diagnostic information, device parameters etc. Greater flexibility in system layout and design. “Ease of future expansion and modification.”

Below is an example of A PROFIBUS network layout

Are you interested in becoming a certified PROFIBUS installer or engineer?

 

Have a look at our certified PROFIBUS Installers course or our certified PROFIBUS engineers course to gain an in-depth understanding of how this protocol works, as well as the ability to install, maintain and troubleshoot any PROFIBUS network.