Process Field Bus
Profibus Technology
PROFIBUS has been interoperability and compatibility allows the high speed smooth cooperative working of process devices on the bus includes many functions which make the handling of field devices even easier for example in case of a device exchange also the new device automatically takes over the role of the predecessor device therefore a device exchange can be carried out easily and without interruption of the system operation. This automatic adaption is also possible with devices of different generations.
PROFIBUS is a digital high speed two wire bus could help us to have the many parameters such as instrument tag, software version, serial number, first process value, 2nd process value, engineering unit, device status, diagnostics, alarm and etc.
If we compare a conventional system with PROFIBUS system the bellow differences will be appeared important.
- 1- Higher precision of process value will be provided.
- 2- Cabling work is and material is lower in PROFIBUS.
- 3- No scaling from mA to measured value is required in PROFIBUS.
- 4- Several measure parameters can be transmitted over the bus.
- 5- In case of failure 22 mA will be transmitted in conventional system but last useable value with bad device status will be transmitted in PROFIBUS.
- 6- Device parameters could be modified from control room in PROFIBUS.
- 7- Purchasing cost is lower in PROFIBUS because of I/O module reduction, marshalling room is not required, one Ex barrier should be used for each bus segment and etc.
- 8- Engineering cost is lower because of easier modification, less electrical diagrams, simple verifications.
- 9- Mean time between failures (MBF) is lower in PROFIBUS so the maintenance cost is lower in this system.
PROFIBUS net works divided to two different sections, PROFIBUS PA with transition speed 31.25 kBit/sec as per IEC 61158-2 and PROFIBUS DP, with higher transition speed up to 12000 kBit/sec.
PA can be used for process in area defined as hazardous and DP used in upper layer so could be connected to control system directly.
The bellow table shows different between PROFIBUS PA and DP:
| Application | PROFIBUS DP Decentralized Periphery |
PROFIBUS PA Process Automation |
|---|---|---|
| Protocol | Common DP protocol • Fast cyclic process data transmission• Cyclic services to access device parameters • Two defined master classes |
|
| Physics | RS485 9600 to 12 Mbit/sec |
MBP 31.25 kBit/sec |