2.1 PROFIBUS termination

For a good signal transmission it is necessary to terminate PROFIBUS segments by a bus termination. For PROFIBUS RS 485 a bus termination consists of a combination of three resistors. For PROFIBUS MBP (PA) a bus termination consists of a resistor and a capacitor.

Fig.: Bus termination PROFIBUS RS 485

Fig.: Bus termination PROFIBUS MBP (PA)

Bus terminations are available in different forms:

• Bus termination built into in a connector
• Bus termination built into a PROFIBUS-device
• …

Bus terminations that are built into PROFIBUS-devices or into PROFIBUS connectors can often be switched on or off. Because of this it can easily happen that too many bus terminations are used in a PROFIBUS segment. Therefore you must observe that PROFIBUS segments are only terminated at the two segment ends and nowhere else. Any additional terminations can cause corruption of the PROFIBUS signals and network malfunction.

In 9-pin Sub-D-connectors the switch for the bus termination often has a second function of isolating the “outgoing” cable from the connector. It is essential, therefore, that only the “incoming” cable entry is used on connectors located at the ends of a PROFIBUS segment. In such situations only one PROFIBUS cable is connected and the bus termination is switched on. If the incorrect cable entry is inadvertently used at such connectors, neither the PROFIBUS-device nor the termination network will be connected to the segment. Most connectors mark the incoming and outgoing cable entries with arrows.

Fig.:newer switch of 9-pin Sub-D-connectors

please klick the button "Start".