PROFIBUS DP physical layer: cable, segments, termination, addresses

Segment length vs. transmission rate (type A cable)

Values from the Siemens PROFIBUS Network Manual for standard type A cable and terminators to IEC 61158-2; special cables with higher loop resistance reduce them. PI documents quote up to 1200 m at the lowest rates. Spur lines should be avoided at 1.5 Mbit/s and above. Plan a segment with the PROFIBUS segment planner.

Nodes and repeaters

• Max. 32 nodes per segment — repeaters count as nodes.
• Segments are joined with RS-485 repeaters; up to 9 repeaters between any two nodes.
• Max. 127 addresses per network (0 … 126); address 126 is the default for devices without an assigned address and must not stay in a running system; 127 is broadcast.

Termination

A segment is a cable terminated at both ends by its characteristic impedance. PROFIBUS uses active termination: a resistor network powered with 5 V from the device at the end of the segment (390 Ω pull-up, 220 Ω between the lines, 390 Ω pull-down). Consequences:

• The terminator switch is ON only on the first and last connector.
• The device supplying the terminator must stay powered; otherwise use a separately powered active terminator.
• A terminator switched on in the middle cuts the segment: devices behind it disappear.

Cable and shielding

Shielded twisted pair, 150 Ω characteristic impedance (type A). Connect the shield on both ends and at every connector, bond it to the cabinet ground, keep distance from power cables. Use bus connectors with incoming/outgoing terminals so that a device can be unplugged without interrupting the bus.

Station addresses

Set on the device (rotary switches, DIP switches or software). The address must match the configuration and be unique; the PLC can only reach slaves whose GSD and address agree with the project.