PROFIBUS Manual

Synchronous transmission according to IEC 61158-2 with a specified baud rate of 31.25 kBit/s is used in process automation. It satisfies important requirements of the chemical and petrochemical industries: intrinsic safety and bus supply in two-wire technology. This allows PROFIBUS to be used in areas subject to explosion hazards. The capabilities and limits of PROFIBUS with IEC 61158-2 transmission technology for use in potentially explosive environments are set down in the FISCO model (fieldbus intrinsically safe concept). The FISCO model was developed in Germany by the PTB (National Metrology Institute). Today it is recognized internationally as the basic model for operating field buses in areas subject to explosion hazards. Transmission according to IEC 61158-2 and the FISCO model will follow the principles set out below:

 

•No power is fed to the bus while a bus user is transmitting.  

•There is only one source of supply in each segment: the power supply.  

•Each field device in steady-state operation receives a constant background current.

•Field devices act as passive, current sinks.

•Passive line termination is at both ends of the main bus line.

•Networks are possible in linear, tree and star topologies.

 

Each bus user in a steady state receives a background current of at least 10 mA.  This current serves to deliver energy to the field device in case of bus supply. The transmitting device generates communications signals by modulating the background current +/- 9 mA.

 

A reverse connection on a field device in MBP technology will not have any consequences for the ability of the bus to function, because these devices are normally equipped with automatic polarity detection.

 

The number of bus users that can be connected on one segment is limited to a maximum of 32. However, this is further restricted by the chosen type of ignition protection and any bus supply. With intrinsically safe networks, tight limits are specified for both the maximum supply voltage and the maximum supply current. But even non-intrinsically safe networks have limits on power pack output.

 

Possible power supply performance with MBP:

*)        Data refers to current consumption of 10 mA per device. If a device receives more than 10 mA, the number of devices to be connected will be reduced accordingly.

 

As a rule of thumb for determining the maximum line length, just work out the current requirement of the field devices to be connected, select a power supply from the above table 7, and read off the line length for the chosen cable type from the table below. The necessary current (=P current required) is the sum of device background currents for field devices connected on the segment concerned, plus a reserve of 9 mA per segment for the operating current of the FDE (fault disconnection equipment). FDE stops faulty equipment from permanently blocking the bus.

 

Maximum distances with MBP

 

The shared operation of devices supplied from the bus with externally supplied devices is permitted. Remember that even externally supplied devices draw background current through their bus connection, which should be taken into account accordingly when calculating the maximum available supply current.