Today I will cover an article on the Foundation Fieldbus. Foundation Fieldbus network is used to replace extensive 4-20 mA current loop specifically for the hazardous area.

We know that in hazardous area equipment run by controlling power sent to them and in any conventional transmitter is used with a single pair of the cable.

Still in industry SMART or hybrid or analogue type transmitter is widely used. But it was in 1992 an international group, the ISP - interoperable systems project was founded with the intention to create an international uniform standard for the hazardous area.

The Fieldbus network has the same design as Profibus-PA as everyone who works in the plant mostly are familiar with it. But when you look very close and have really good understanding about it you can find great differences between the two networks.

The Fieldbus is an open bus protocol which enables devices of the different manufacturer to be integrated in one single system but the device must be approved by the Foundation Fieldbus.

Below are some of the important feature that the Fieldbus network can handle :

1. Bus-powered field device.

2. Line or tree topology is used.

3. Multi-master communication.

4. Predictable dynamic behaviour.

5. Provide intrinsic safety for hazardous area.

6. Easy to extend a network based on device description.

Fieldbus contain predefined function block. The block implemented in the device provide an information about a task which device can perform.

A task can be like Analog input or output, Digital input or output. To control process automatically PD and PID blocks exist for the task it has to perform.

It provides very flexible and de-centralize process control which largely reduce loads on the central process station.

Fieldbus network use layer communication and consist of three major elements;

1. User application

2. Physical layer

3. Communication stack.

We know that the Fieldbus network comes with H1 and H2 bus system, the H1 bus system is intrinsic safe and slow in response whereas the H2 bus has a fast response.

Below you can see typical Fieldbus network topology.

The connection between controller and field devices is a bus network which could be an H1 or H2 type.

H1 bus specification is based on IEC-61158 standard used for data transfer with data transfer rate of 31.25 kb/sec.

For proper communication between device and controller, each device must require a minimum of 9 volts.

H1 bus allows the field devices to power over the bus.




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