1 Four-Wire Transmission
For most applications where commercial power is available, the two-wire transmitter is the practical instrument, due to the additional cost of installing and running four cables to one transmitter as compared to two cables.
2 Two-Wire Transmission
3 “Smart” Transmission
Digital signal transmission is faster and more accurate than analog. More information can be transported between the instrument and the control room using the same two cables with bidirectional digital transmission, including configuration and diagnostic information.
Bidirectional communications mean that a value can not only be read from the final device, but can also be written to the device. For example, the calibration constants associated with a particular sensor can now be stored directly in the device and changed as necessary. This often makes it possible to remotely diagnose a field device problem, thus saving a costly trip to the field. The most widespread digital communication is HART® based on Bell 202 FSK standard, there are millions of instruments in the world using this standard, it is simple and well understood.
4 Fieldbus Transmission
Multi-drop communication means that digital communication between several field devices (such as valves, pressure transducers, etc.) and computers, programmable logic controllers (PLC) or remote terminal units (RTU) can take place over the same two cables. . The multiple drop capacity of a field bus may result in the most immediate cost savings benefit for users, since a single pair of cables is shared across multiple devices. While with analog or smart devices, a separate cable is needed between each final device and the control system.
The main advantage of Profibus is that it is well suited also for digital signal transmission. Therefore it is often selected in processes with a high number of digital signals (like batch processes or manufacturing industries). It can reach higher communication speed in the traditional master slave configuration, where a controller acquired data from the transmitter and sets the values of the valve. In case of failure of the main master a reserve master can take over the communication