A resistor equivalent to the wave impedance at the end of a cable will, in an ideal case, reduce reflections to zero. The resistor absorbs the energy of the signal. PROFIBUS DP/FMS uses an active bus termination with 5V supply according to the following diagram:
Termination for RS485 with cable type A
Please note: For the data signal, the supply should be viewed as a short circuit. The value of the bus termination can therefore be calculated from the parallel circuit of 220 Ohms and the series circuit of the two 390 Ohm resistors. These will produce the upper tolerance for the wave impedance of cable type A.
These bus terminations are
•integrated in an M12 bus termination
•connected in a special active bus termination
•built into devices and switched on as required (no longer recommended; not approved for 12 MBit/s).
For the correct function of a segment, it is essential that both ends should have a termination, but that no other terminations are present. The bus can only work correctly if the terminators have a permanent power supply - even when parts of a system are turned off - otherwise reflections can make communication impossible for stations remaining in the network and the open-circuit voltage level (approx. 1 V) is not maintained.
Communications problems are often caused by incorrect bus terminations.