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Entry type: FAQ Entry ID: 187276, Entry date: 12/08/2014

Which components do you use to terminate PROFIBUS subnets with a terminator?

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With PROFIBUS, each bus segment must be terminated at both ends with a terminator. The line termination consists of a resistor combination. This is integrated in the RS 485 Repeater, in the OLM (channel 2), in the bus terminal, in the active RS 485 terminating element and in some bus connectors. If these components are used at the end of a segment, then the terminator must be activated.

The line termination terminates the bus line with the surge impedance to prevent reflections on the line. At the same time it ensures setting of the signal quiescent state level during the pause between two messages. When a node disconnects from the PROFIBUS, the terminator becomes inactive at that point. Nodes that are not permanently connected to the PROFIBUS should not be connected with this to segment ends.

In order for nodes to be able to connect to and disconnect from the PROFIBUS at will there is the active RS 485 terminating element (article number 6ES7972-0DA00-0AA0). With the active RS 485 terminating element the terminator has a permanent power supply separate from the other IO nodes. Since it is operated independently of the other bus nodes, it can be connected and disconnected without disturbance.

With the RS 485 Repeater and with the OLM (channel 2) the terminator is supplied by the power supply of the device. Interruption of the power supply of one of the two terminators in one segment through switching off the data terminal or the repeater or unplugging the bus connector leads to a bus error.

Furthermore, in high EMC environments, it is recommended to activate the terminators at both ends even with very short segments (like from the OLM to the terminal). In this case you should not use the plug-in connecting cable 830-1.

More information with configuration guidelines for PROFIBUS networks is available in the manual for SIMATIC NET PROFIBUS networks in Entry ID 1971286.

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