Expectations on fieldbus systems
The first thought when dealing with the topic of fieldbus technology is that only a single bus cable is required for the entire system, which offers enormous potential for saving money on cabinets, cables, and wires. This should also go hand in hand with massive reductions in documentation and commissioning work. Another interesting factor for system operators is that many of today's field devices have a high level of intelligence (and decentralized intelligence as well). This can only be taken advantage of through the use of a fieldbus system (digital communication). With the fieldbus, all available information from field devices can be used completely.

Fundamental advantages
One advantage that is always present when using field devices together with a fieldbus is that measurement values are transferred in digital form. If a maximum resolution of 0-65535 digits is achieved by using 4-20 mA signals (at 16-bit resolution), then fieldbus devices can display the measured value as a 32-bit floating point value. The value range is therefore between 10-38 and 10+38. A restriction on the physical measurement range that was previously necessary to achieve the highest possible resolution is no longer required. This allows the limits on sensors to be to their fullest. In addition to the measured value, several other pieces are available in the field device if needed. TAG, software version, serial number, possibly several process values (e.g. temperature and resistance values for a temperature transmitter), unit, status, and diagnostic data are available in each fieldbus device.

FISCO model
The FISCO model (Fieldbus Intrinsically Safe Concept) makes it possible for the user and operator to quickly and easily design PROFIBUS PA and FOUNDATION Fieldbus H1 systems for use in areas with explosions. If listed requirements are fulfilled afterward, a separate system certificate for a PA / H1 segment is not necessary. The devices being used must be certified according to FISCO (EC-type conformity certificate). In addition, U, I, and P must correspond to EN 50 020, the cable parameters (R, L, C) must be adhered to (type A cable), the lines have to be terminated according to guidelines, and the total length of the line system (incl. stubs) cannot exceed 1000 m.

Profiles / Blocks
Profiles and blocks are specified to guarantee the necessary interoperability (fieldbus devices from different manufacturers must run on one bus without errors), interchangeability (replacing PROFIBUS devices with the same type, but with other manufacturers as well), and availability of basic functions (to ensure the system is operated the same way throughout).

Device representations
A field device is represented via various layers, through resource blocks, transducer blocks, and function blocks. Resource block for maintenance information The resource block contains data that is specific for the device hardware and software (manufacturer, device type, software version, hardware version, diagnostic information, etc.). The resource block mode checks all of the device's other function block modes.

Transducer block as the interface for operation
The transducer block isolates the function blocks from the device-specific function that handles the sensors and actuators. The transducer block specifies access to the device using an interface and defines function blocks for it. There are different transducer blocks for the fill state, flow, pressure, analysis, temperature, and valves. This provides methods for simple device settings like linearization, basic configurations, diagnostics, and security settings.

Function block for application software
Function blocks are the most important part of the Foundation fieldbus specifications and are the key for implementing field-based control. The device manufacturer can decide which function blocks are available for the device (e.g. analog inputs, analog outputs, discrete inputs, discrete outputs, PID controllers, signal adjustments, etc). A FB AI can e.g. perform a simulation, scale a value, forward replacement values, and provide limit value monitoring for LL, L, H, and HH.

Function blocks
The structure of a function block contains inputs, outputs, and parameters (standard block parameters and block parameters). It is therefore possible to carry out a uniform and simple block-oriented configuration of functions. In addition, it can be clearly defined as to which information and functions need to be communicated. The distribution and execution of function blocks in field devices and their run sequence is specified by their assignment and arrangement. Transducer blocks and function blocks always return values and states. Multi-variable transmitters have n TB/FB blocks per field device. This data is transferred using cyclic (measurement value), acyclic, or spontaneous services (device diagnostics).

HART via PROFIBUS DP
Since the installed base for field devices consists over 80% of HART field devices, it's clear that they have to be taken into account when considering fieldbus technology. The solution with regard to integration involves HART-capable I/O modules that communicate digitally with a field device, with the data being transferred through PROFIBUS DP using a "tunneling" method (virtual I/O channels). This also makes cyclic access to multi-variable HART field devices, status, and diagnostic data possible. Naturally, transfer speed is relatively modest due to HART's modulation process (FSK). FDT/DTM technology is the foundation for manageable integration. HART devices frequently must be used since the measurement value with a conventional 4-20 mA signal is available very quickly in comparison with fieldbus devices where this process generally takes longer.

PROFIBUS / FOUNDATION fieldbus comparison
PROFIBUS clearly scores higher with regard to its installed base, simple master/slave principle, and established FDT/DTM technology. Thanks to the Link Active Scheduler, FOUNDATION fieldbus technology includes a redundant design, direct device-to-device communication, and the option of time stamping on the device. The powerful function blocks in the field devices make possible new approaches regarding the segmentation of automation. With HSE, FOUNDATION fieldbus technology is heading towards ETHERNET; PROFIBUS is going the same way with Profinet. How long it takes until I/O modules are available on a wider basis will provide a hint of the future for both fieldbus solutions.

PROFIBUS
Current PROFIBUS specifications include the following:
PROFIBUS DP (DP = Decentralized Peripherals) for high-speed data exchange, e.g. with FUs, remote I/Os, analysis devices, etc. (V0, V1, and V2 specifications exist).PROFIBUS PA (PA = Process Automation) was developed for the demands of process automation (sensors, actuators) and covers EEx requirements. Fieldbus supply and communication is handled with 2-line technology. Various device profiles (Profile 3.0) exist for PA devices (e.g. pressure, temperature, etc.).

Master/Slave principle
PROFIBUS differentiates between master and slave stations. The slaves are passive and polled by the master. On the master, tasks are divided into cyclic tasks (master class 1) and non-cyclic tasks (master class 2). A token controls access to multi-master mode. Effective data transfer is achieved by the master sending output data to the slave, which then responds by sending its input data directly thereafter. GSD file The device master file (for PROFIBUS DP / PA) is provided by the manufacturer of the field device and contains all of the data important for communication. A GSD import procedure passes this information on to the controller.

PROFIBUS topology
Limited by RS-485 technology, up to 32 stations can be connected on a PROFIBUS DP segment. Up to 4 segments can be cascaded. However, there are repeaters on the market today that permit much higher levels of cascading. Despite this, the address range is still always in the range 0-125 (for master and slave), whereby each master and slave must have its own clear address.

Segment length for PROFIBUS DP
PROFIBUS DP uses RS-485 (with copper cables), which allows it to cover a distance of 1200 m or 100 m depending on the transfer rate (9.6 kBit/s - 12Mbit/s). 400 m is possible if using the recommended uncritical transfer speed of 500 kBit/s. If repeaters and fiber optic ports are used, then distances up to 15 km or more are possible. The bus is designed in a line structure since branch lines should be avoided (plugs for connections with cable input and output).

PROFIBUS PA – non-Ex
PROFIBUS PA segments can comprise up to 32 PA field devices, with 24V / 400 mA being provided for the segment. In this case, the segment length can be up to 1900 m. The bus for PA is designed as a line or tree structure, with the permitted length of branch lines dependent on the components being used. PROFIBUS PA –EExi PROFIBUS PA/EExi segments can comprise up to 6-8 PA field devices, with 12.6V / 100 mA being provided for the segment. In this case, the segment length can be up to 1000 m. What's important here is the starting current process of the field devices or the current needs of the individual field devices. This usually brings the theoretical value of 10 field devices down to around 6-8.

PROFIBUS bus cycle time
A basic rule is that the slowest station on the bus determines the transfer rate. When using PROFIBUS DP slaves with 12 Mbit/s transfer speeds, less than 1 ms is needed on the bus in order to transfer the maximum possible 244 bytes per slave. For PROFIBUS PA, the transfer rate is specified at 31.25 kBit/s, with cyclic data exchange (4 bytes for the measurement value and 1 byte for the status) typically needing 10-20 ms per slave. In practice, this means that PA segments with 20 field devices are polled every 200-400 ms.

Design notes

DP connections
The starting point is the PROFIBUS DP master in the controller based on RS-485. This allows any number of DP slaves to be operated. Intrinsically safe and Hart-capable inputs and outputs can also be built up in this way. Using suitable repeaters also allows these devices to be used in ex-zone 1. The transfer rate can fall anywhere between 9.6 kBit/s and 12Mbit/s. The limit is 1.5 Mbit/s when using the intrinsic design. As with ETHERNET TCP/IP, media converters (electrical/optical interface converters for PROFIBUS DP) can be used to set up a fiber optic ring. If there is ever an interruption, then this provides an alternative path. These repeaters also make it possible for segments that are more or less scattered across larger distances to be grouped together. These repeaters with FO ports achieve complete electrical isolation for the segments (lightning protection for bus lines spanning buildings).

PA connections
These are based on the fact that the PA segment can only be operated with a transfer rate of 31.25 kBit/s. PA can only be connected to DP with a segment connector (DP/PA connector). First-generation segment connectors (SC1) triple the transfer rate (from 31.25 to 93.75 kBit/s), isolate DP and PA from each other electrically, and supply the (intrinsic) PA segment with current). This allows non-ex and ex-segments to be connected to non-ex DP segments. Second-generation segment connectors (SC2) are designed as transparent gateways and can be operated at up to 12Mbit/s. Profibus DP / Profibus PA / HART DP/PA links are not a satisfactory basis for a good fieldbus solution since they are not uniform. Through the use of accessories available on the market, transparency on PA segments can be achieved in case there is a short circuit on a branch line. In addition, the <10 device limit can be avoided when applying EExi solutions.