Question

Describe the information/data exchanges for Fieldbus Systems on device and control level as well as examples of network/communication protocols used.

Answer 1

Field Bus (IEC 61158) is family of protocols for an open, digital bidirectional communication network between process plant and automation system. Field Bus replaces traditional point-to-point connections thereby amount of cabling (e.g. when compared with a 4-20mA analog connection or RS-485 digital connection).

1. Device Level: Typical field bus devices are sensors, controllers and actuators. In a field, there are more instruments (often in thousands) scattered at long distances. So a single pair of wires carries both the device’s power and the digital communications signal. This eliminates the need for a separate power cable, thus keeping the wiring simple and inexpensive. These fieldbus networks were also designed to operate in the often rather harsh, electrically noisy environment found on site. Some of the commonly used Fieldbus protocols are HART (Highway Addressable Remote Transducer) and PROFIBUS (Process Field Bus). HART is a system superimposed on existing 4-20mA system and is compatible with existing devices and allows interoperability. Special and more expensive devices are need for PROFIBUS, while it provides more sophisticated diagnostics and is faster. So based on the level of automation required, designers will make a choice.
2. Control Level: At control level, the distances are short and the speed requirement is high. This tier makes large systems possible by linking together field-level networks from different areas around the site. Intra-area control and supervision becomes possible. The host-level protocol is also the link to business systems, either directly or via historians and other plant information software. The network must be fault tolerant and should be running even in the presence of a fault. This is extremely critical at the control level since the entire site is operated and supervised over this network. Modbus and Ethernet are widely used protocols at control level.