The industry began developing fieldbuses for the special requirements of process automation 25 years ago. Foundation Fieldbus H1 and Profibus PA use the 'Manchester Bus-Powered' (MBP) physical layer in accordance with IEC 61158-2 Type 1, which, like the analog 4 to 20 mA current loop, enables remote powering of the devices and allows the development of intrinsically safe devices for hazardous areas.

Although the digital communication protocols are functionally very powerful and offer many advantages in the application, fieldbuses are only used in a small proportion of systems. The reasons for their lack of acceptance are manifold: The control systems only use the process values and not the extensive diagnostic and parameterization options of digital field devices. Concepts based on intelligent field devices, such as 'Control in the Field', are also not supported by the control systems. Therefore, on the one hand, users see hardly any advantages in using fieldbuses, while on the other hand they often suffer from the complexity of commissioning, troubleshooting and device replacement. However, the tide could turn in the future, especially as Industry 4.0 and resulting concepts such as 'Namur Open Architecture' focus on the extensive data that digital field devices provide for diagnostics and asset management. Whatever the case, the fact is that at least three quarters of newly installed transmitters and positioners are still equipped with the 'good old' 4 to 20 mA current loop. This is why the standard versions of the field devices generally have this analogue interface, which is usually supplemented by a HART interface for parameterization. In addition, there are often manufacturer-specific interfaces for parameterization and diagnostics.

Nevertheless, new and sometimes very large systems are also being built using fieldbus technology, forcing device manufacturers to also have a product range for this market segment in their portfolio. For this purpose, the analog current interface and the HART modem must be replaced by a digital fieldbus interface, which requires corresponding development efforts for hardware and software.

A special fieldbus controller is required to generate and receive the Manchester-coded transmission signal and to ensure correct compliance with the bus timings. Furthermore, a so-called Medium Attachment Unit is responsible for signal modulation, which is realized by changing its own current reference, as well as for decoupling the supply energy from the bus. With simple and inexpensive sensors, it is common to connect the fieldbus interface to the microcontroller of the device, but this requires porting the very complex fieldbus protocol software to the device architecture. The simpler way is to develop a fieldbus interface with its own microcontroller, which is optimally matched to the protocol stack, or to obtain the fieldbus interface complete with firmware from one of the specialized providers. This also ensures freedom from feedback between the device application and fieldbus communication, as both components contain time-critical functions.

However, the task is not yet solved with hardware development alone. Foundation Fieldbus and Profibus PA not only specify the communication behavior on the fieldbus, but also many aspects of the application that runs in the field devices. Both fieldbuses use very similar function block models, whereby three categories of blocks can be distinguished:

• Generic function blocks, i.e. implemented identically on each device, implement the input and output functions for the process variables as well as certain control or processing functions such as PID controllers or totalizers. For example, an analog input block that supplies the measured value has a fixed set of parameters - regardless of whether it is a simple temperature sensor or a complex radar level meter.
• Parameters that depend on the measuring principle or control or provide special functions of a field device are mapped via transducer blocks. Each input or output function block has at least one transducer block, which generates the process value for sensors or processes it for actuators. Transducer blocks can also be used, for example, to access diagnostic functions or the local operating interface.
• Furthermore, each field device has exactly one resource block (Foundation Fieldbus) or physical block (Profibus PA), which enables access to specific device data such as manufacturer ID, serial number, version status, etc.

The software task of fieldbus integration is therefore to map the device functions to the function block model so that the device is interoperable and conforms to the specification on the fieldbus side. If a parameter of a function block is read via the fieldbus, the corresponding value must be generated via the device application. Conversely, in the case of write access, the new value must be transferred to the application. Dependencies between the parameters must be taken into account. If, for example, the physical unit of a temperature is changed from Kelvin to Fahrenheit, then only the parameter in the transducer block that represents this physical unit is written from the fieldbus side.

As a result of this change, however, all variables associated with this unit must be converted in the device - for example the process value, the upper and lower limits of the measuring range, alarm limits and much more. As field devices often have hundreds of parameters, a great deal of effort is required to develop an internal device application that establishes the connection between the device function and the protocol software. This also requires intensive collaboration between the stack developer and the device developer if the expertise for communication and device architecture is not concentrated in one person.

The total cost of a fieldbus integration quickly adds up to a six-figure sum, and the duration of such a project is typically between six and twelve months. This expenditure may be justifiable for large manufacturers. For smaller suppliers, however, who only sell small numbers of devices in fieldbus systems, such an approach is not worthwhile.

Integration the easy way

With this in mind, Softing Industrial Automation has developed an integration concept consisting of a hardware module and a development tool that enables very simple upgrading of HART or Modbus devices for Foundation Fieldbus and Profibus PA without any programming effort. A module called commModule MBP with dimensions of 32 mm x 39 mm provides all hardware and software functions for the fieldbus connection. Thanks to single-sided assembly and edge metallization, the ATEX and IECEx-certified and therefore intrinsically safe module can be mounted on a PCB using a pick-and-place machine. For the device manufacturer, the hardware development effort is limited to providing the space and connection points on the PCB.

The supply voltage on the fieldbus is between 9 and 32 V. The module's current consumption can be set by software in the range from 10 mA to 26 mA. The connected field device is supplied with 3.15 V or 6.2 V and up to 16 mA with this energy taken from the fieldbus. This means that a maximum of around 90 mW is available. A programming and debug interface enables firmware and device configuration to be downloaded. Communication with the HART or Modbus device takes place via a serial interface. I/O signals are also available.

Both a Foundation Fieldbus stack and a Profibus PA stack are available for fieldbus communication. The protocol is selected via a hardware line. A HART and a Modbus master stack are available for communication with the field device. The FSK modem for HART and the RS485 interface for Modbus are bypassed in the device, which saves energy and allows a higher serial transmission speed of up to 115.2 kBit/s. As a novelty, there is a configurable application on the module which fills the fieldbus objects with the real data from the field device or transfers data to the field device via HART command or Modbus access.