Process Automation environments, while typically requiring slower procedures, might also be characterized by explosive or hazardous environments. In such applications, we use PROFIBUS PA as opposed to PROFIBUS DP. Similarly, we can use copper wiring or fiber-optic cabling. In the case of the former, instead of RS-485, the physical layer is MBP (Manchester Encoded Bus Powered). It is important to note that even though a different physical layer is employed, PROFIBUS PA is the exact same protocol as PROFIBUS DP. MBP only transmits at one speed: 31.25 kbit/s, which is plenty for process applications. A significant departure, however, is that power and data are transported via the same cable. As such there are rules regarding network topology that must be followed.
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When operating PROFIBUS PA in hazardous areas, there are two concepts used to ensure that a sparking condition does not occur:
- The FISCO (Fieldbus Intrinsically Safe COncept) model makes it easy to plan, install and expand PROFIBUS PA networks. This model is based on the specification that a communication network is intrinsically safe and does not require complex calculations for validating intrinsic safety if the relevant components (field devices, cables, segment couplers, and bus terminations) conform to a set of limit values for voltage/current/output/inductivity/capacitance. Intrinsic safety is guaranteed on a network segment if all components are certified as per FISCO. It is characterized, however, by a considerably low input power into a segment and therefore shorter cable lengths and fewer devices per segment.
- The High-Power Trunk concept relies on the separation in the different zones of explosive and hazardous environments. In less stringent hazardous zones (where only increased safety is required), a trunk cable is laid. It is assumed no ‘hot’ maintenance will be required on the trunk line. Off of this (‘spurs’), field devices are connected that lie in areas where intrinsic safety is required. Proof of intrinsic safety therefore only involves the field barrier and the device(s). This ‘Trunk and Spur’ concept is very popular as it leverages the topology options available with MBP physics.
High Power Trunk to supply Zone 1
For applications which demand high system availability, such as is common in continuous processes, redundant systems are generally used. This can mean:
- Master Redundancy – Two installed controllers, if one fails then another takes over seamlessly
- Media Redundancy – A ring topology is formed, then if one segment is broken the topology is automatically converted to a line configuration.
- Both – Often times both methods are employed; for example, a dual-master ring network.
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