FOUNDATION Fieldbus

The installation guides for PROFIBUS PA and Foundation fieldbus H1 are similar. Are they addressing the same concept?

Although the installation guidelines of PROFIBUS PA and Foundation fieldbus H1 are similar as they share the same physical layer, the technologies of PROFIBUS and foundation fieldbus are different in several important areas. Foundation fieldbus devices use the decentralised function distribution with following futures:

  • Alarm stamping at source (not at the system)
  • Deterministic communication between the field devices i.e. AI, AO, PID, etc. There is a method to communicate information deterministically, this enable distribution of control into the field. 
  • Device to device communication. There is no master or slave relationship between devices, but there is a communication scheduler with backup facilities.

What is foundation fieldbus HSE?

Foundation fieldbus HSE, being based upon Ethernet and providing the backbone for linking of H1 segments also include the connectivity for complex devices such as analysers, remote IO or other devices which are independently powered.

How many devices can be connected to the FOUNDATION fieldbus H1 segment?

The H1 bus cable is compatible with typical 4-20 mA wire. Devices are connected in one bus segment can accommodate up to 32 devices (externally powered) or 240 if repeaters are used.
IF it is required to power devices from the fieldbus then the number of devices which can be connected is reduced. Experience indicates that for H1 segment without distributed control then between 8 and 10 devices can easily be accommodated.

Is it necessary to connect FOUNDATION fieldbus H1 to HSE segment in a control network?

It would be possible to design a fieldbus solution with H1 segments connected to an H1 card and then directly to the HMI (Human Machine Interface). In practice the use of a high-speed bus (HSE) is often preferred for any large installation.

What does LAS stand for?

FOUNDATION fieldbus uses a combination of a master/slave and token passing as a method of bus access control to permit these functions. A device that has the scheduling function controls the FOUNDATION fieldbus segment. This function is referred to as the Link Active Scheduler (LAS) and can be considered an extension of the FOUNDATION field bus stack. In this case a segment comprises standard devices (a bit like a slave) and at least one LAS device.
FOUNDATION fieldbus H1 can provide roughly 30 Message/second. So for 30 devices the refresh rate is 1 message/second. Do not forget that the LAS could send the same message twice during this one second period to increase the refresh rate for individual parameters.

What is difference between FOUNDATION fieldbus H1 and H2?

The initial H1 implementations do not support redundancy, but the H2 implementations are expected to do so.
The H1 communication rate is fixed 31.25 kbit/s. The H2 communication rate is one of two possible high speed fieldbus communications speeds, can be either 1.0 Mbit/s or 2.5 Mbit/s.

Which standards are applicable to FOUNDATION fieldbus?

  • British Standard DD238
  • Euro Standard EN50170
  • US Standard ISA550

Speaking of PROFIBUS PA/Foundation fieldbus H1 where they can be implanted?

Field devices such as transmitters and positioners maybe require power over the bus or used in hazardous environment. Foundation fieldbus H1/PROFIBUS PA are proper for these applications.
Complex devices such as remote IO analyser are often externally powered. Therefore Foundation fieldbus HSE/PROFIBUS DP can be used for them.

What is DD/CFF file?

Device Description (DD) and Common File Format (CFF) files provide all the information required to configure and integrate a FUNDATION fieldbus device into the system.

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