Foundation Fieldbus is one of the most popular digital communication protocols based on which smart sensors are built. Distributed Data Transfer (DTT) is the main principle according to which communication tasks in Foundation Fieldbus protocol are carried out. Like all communication protocols adopting fieldbus technology, Foundation Fieldbus allows for the connection of up to 32 field devices to a single twisted pair of wires in a single segment, provided that the field bus should be terminated from both sides in order to avoid jitter and reflections which might lead to communication failure. In Foundation Fieldbus protocol, the field devices can administrate some communication tasks regardless of the authority of the host controller. The communication process in Foundation Fieldbus protocol is supervised by the LAS (Link Active Scheduler) which can be any of the field devices connected to the fieldbus. Communication tasks in Foundation Fieldbus are divided into two categories, cyclic and acyclic tasks. Acyclic communication tasks take place at the time breaks between time intervals dedicated to cyclic communication. The type of data processed in cyclic communication is mainly related to process control and measurement variables, while the type of data processed in acyclic communication tasks is related to diagnostic and parametric information. 

Each Foundation Fieldbus segment should include a segment power supply the capacity of which is dependent on the Foundation Fieldbus model adopted by the segment. Other than regular power supplies, FF segment power supply ensure impedance matching all along the bus, in addition to superimposing the Manchester coded signal rendered by the host on the supply voltage (9-32 VDC) provided to the field devices included in the segment. The model (Non-intrinsically safe/intrinsically safe) adopted by an FF segment is basically determined by the safety requirements imposed by the location at which the field devices included in the segment will be installed. For safe-area applications, non-intrinsically safe model will be adopted while for explosive hazardous areas, an intrinsically safe model will be adopted. Foundation Fieldbus allows for up to 5 intrinsically safe models:

1-Entity model.

2- FISCO (Fieldbus Intrinsically Safe Concept) model.

3- FNICO (Fieldbus Non-Incendive Concept) model.

4- HPTC (High-Power Trunk Concept) model.

5- DART (Dynamic Arc Recognition and Termination) model.

Entity model allows for a segment power supply capacity of 80 mA, which makes it possible to connect 2-3 field devices per segment. According to Entity model, cable characteristics (Resistance, inductance and capacitance) are taken into account during ignition curves estimation, that's why, more restrictive curves (inductive curves) are considered in intrinsically safe calculations. Other than Entity model and based on experimental results, FISCO model neglects cable characteristics during intrinsically safe calculations which allows for segment power supply capacities of 320 mA and 180 mA for IIB and IIC applications respectively. FNICO model is an extension for FISCO model, however FISCO model is dedicated to Zone1/Division 1 applications, while FNICO model is dedicated to Zone2/Division 2 applications. The main idea of any intrinsically safe system is to limit the energy that might lead to ignition in explosive hazardous areas. Both of Entity and FISCO models are undertaking such a mission by limiting the energy level all along the segment (cables, connection units and output ports). Other than Entity and FISCO models, HPTC model limits the energy in explosive hazardous areas only at connection interfaces (segment protectors or field barriers) to which field devices are connected. In other words, HPTC model distributes the energy consumed by the segment all along the segment so that minimal levels of energy can be ensured at field devices installed at explosive hazardous areas through using dedicated connection interfaces to provide reduced level of energy for such devices. There are two types of connection interfaces adopted by HPTC model, segment protectors and field barriers. Segment protectors are used for Zone2/Division2 applications, while field barriers are used for Zone1/Division 1 applications. Two major differences distinguish between segment protectors and field barriers. Firstly, segment protectors maintain constant voltage at a specific field device connected to a specific segment protector regardless of the voltage drop on the spur, while field barriers maintain constant output voltage all along the segment at the output port dedicated to a specific field device regardless of the total voltage drop on the H1 bus main trunk cable. Secondly, the current consumed by segment protector is simply the sum of the operating current of the segment protector (4-5 mA) in addition to the sum of the currents of all field devices connected to the segment protector and the assumed short circuit current, however for field barriers the overall current consumed by the barrier is determined by five variants indicating the mechanism by which HPTC model performs the role of limiting the energy in specific locations all along the segment (locations where field devices are connected to the bus through field barriers). In case it was assumed that a single FF segment adopting HPTC model included multiple field barriers to which a number of field devices are connected, the current consumed by each of these field barriers will be dependent on the following variants:

1- The overall length of the H1 bus trunk cable. 

2- The lengths of the cable sections connecting between the field barriers. 

3- The current consumed by the field barriers which precede and follow that specific field barrier 

4- The overall current consumed by the field devices connected to that specific field barrier. 

5- The total number of field  barriers included in the segment. 

The results obtained from simulation of Foundation Fieldbus HPTC test segments constructed by Emerson Segment Design Tool were analyzed using MATLAB to derive the mathematical equation describing the polynomial relation between the current consumed by the field barrier and the overall length of  H1 bus main trunk cable (excluding the lengths of the spurs) for specific total current consumption by the field devices connected to the barrier. Due to such a polynomial relation, It was noticed that the current consumed by the field barrier tends to increase when increasing the length of the total H1 bus trunk cable. The rate of such a current increase is proportional to the total consumed by the field devices connected to the barrier. Moreover and due to the technique adopted by HPTC model to distribute the segment energy all along the segment, the overall noticed current consumption in an FF HPTC segment using field barriers, is the highest in comparison with the overall current consumption if other FF models were adopted (Non-intrinsically safe, Entity, FISCO, FNICO and DART). DART model is the latest intrinsically safe model adopted by Foundation Fieldbus protocol. The basic idea of DART model is to extinguish the spark at an early stage of its formation based on the calculated current rate of change with respect to time. This process is administrated by dedicated segment power supply the capacity of which is 360 mA. The segment power supply limits the energy of the bus only at the first 5-10 microseconds of spark ignition so that it can be early extinguished.