ISO1176: Termination of profibus

Hi user6478535,

Thank you for reaching out to us!

Please refer to the following e2e post for the discussion on how the 390/220/390 is calculated.

https://e2e.ti.com/support/interface-group/interface/f/interface-forum/163376/profibus-termination-resistor-power-rating

Regards,

Alfred

Hi user6478535,

To design a passive termination for Profibus, 220ohm resistors are inserted in the first and the last stations. The reason for using a 220 Ω resistor is that the two 220 Ω resistors are connected parallel to each other. The parallel connection of the two resistors makes a total resistance of 110 Ω which matches the loop resistance of a standard Profibus DP cable which is 100 ohm/km.

When designing an active termination to accommodate fail-safe conditions, the 220ohm resistor is inherited from the passive termination. In addition, a terminal voltage of 1.1V is recommended to be maintained between the positive and negative ends to reduce the influence of the large reflected wave at idle time.

Regards,

Alfred

Hi user6478535,

That is the background how the 220ohm was picked based on the loop resistance of the standard Profibus DP cable is 110 Ω/km.

If you look from the characteristic impedance perspective for this active termination, which is 220//(390+390) = 171ohm, which is close to 150ohm.

Albert it is not a perfect match to characteristic impedance, it is good enough.

Regards,

Alfred