Foundation Fieldbus Communication Protocol

Foundation Fieldbus is a digital signal with two way connection. This communication protocol allows to connect different field devices on one single Foundation Fieldbus Network.

The FF communications has three layers:

The Physical Layer

This layer is the FF Network and consists in the physical two wire connection for the power and data. Its function is to connect the devices to the communication stack. The Foundation Fieldbus Network segment can be a H1 with 31.25 Kbits/sec or FF HSE(High Speed Ethernet) with 100Mbits/sec or faster. There are three different topologies of FF networks: Point to point, tree and branch.

Foundation Fieldbus Network Topologies

Communications Stack

The communication stack manage the communication in the Foundation Fieldbus network, for example, it receives input messages, holds a queue of output messages, schedule the transmission of these messages.

User Layer

The user layer is represented by functions blocks that allow the configuration of the devices that can be used for monitoring and control. The resource function block give us information about the transmitter and its capabilities. The transducer block is the interface between the sensor and the other function blocks of the transmitter. The are many different types of function blocks,for example, analog input, analog output, PID, characterizer, selector, integrator, etc.

Lets talk about the Foundation Fieldbus advantages:

The FF communication protocol reduces the wiring costs because you can connect multiple devices in the same FF segment.

The FF communication protocol has the functionality to control devices from the field, that means the control resides in the Foundation Fieldbus devices. These controls can also be implemented on the FF segment, increasing the control speed and the efficiency of the control.

Foundation Fieldbus segment

A FF segment consists of a  a trunk and several spurs connected to the trunk. A segment can support until 16 bus powered devices or 32 individual powered devices. The maximum length of the FF segment trunk is 1900m but you can connect FF repeaters to increase this length.
The length of the spur changes according with the number of the spurs. One spur has a maximum length of 120m, two spurs 90m , three spurts 60m and four spurts have 30m.
The Fieldbus Foundation segment uses a Link Active Scheduler (LAS) to organize  the communication in a segment with multiple devices. The LAS can reside in one of the devices connected on the segment. Also, we can program the segment to have a backup LAS, so in case that the primary LAS fail, then de backup LAS takes over avoiding loss of communication.
This bring another advantage that the Foundation Fieldbus protocol has over the 4-20mA, it is more secure and robust, because the control resides in the devices and the LAS backup keeps communication for the H1 segment, so in case of loss of communication, the devices still controlling.