Profibus cable termination

Hello guys,

I need your help regarding the proper termination of Profibus cables. I now that in theory in order to minimize the noise and to prevent reflection to disturb the communication, profibus cable must be terminated with termination resistor on each end.
However I have came across one interesting setup in which I have in total four communication processor. On one the connector is terminated (switch in position "ON"), while on all other the connectors are off. I have tried to terminate connector on the other end of the cable, but nothing changes. The communications works without problems.
SO here, I don't have actual problem, but the profibus communication works even when only one connector is terminated.
(CP)-------(CP)--------(CP)--------(CP)
on off off off (here, I thought it should be on)

The total length is abut 100-120 m.

Thanks for any tips.

On the Siemens Profibus connectors there are 2 cable connections, one is actually the "entrance" the other is the "exit", usually indicated by an arrow (pointing inward for the entrance and outward for the exit. In order for proper termination, the connectors at each end of the Profibus segment should be on the "entrance". The termination switch actually disconnects the entrance from the exit when in the ON position.

It's not rare that I've seen improperly terminated Profibus networks work.

Hello sigmadelta,

yes but it is strange to me. The profibus connector have small red swith that can have either ON or Off position (like shown in attachment). ON meas that the end is terminated with the resistor. The actual setup that works is like shown in the attachment.
I think that the only proper way to set up the connection is to have the first CP (left) also switch to "ON" position.

Somehow it works this way. Maybe the cable is not that long.

The fact that your system works when mis-configured does not change the recommendations for terminating Profibus networks, nor does it guarantee that your system will continue to work reliably.

The argument that systems work sometimes when mis-configured just proves that the configuration rules are conservative and intended to encompass a wide variety of installations, hardware, and thresholds.

Follow the Profibus configuration and termination recommendations.

Edit: I argued this for months with Exxon in Qatar, when they refused to install Profibus terminators on a long Profibus DP system. "But it works most of the time ! The problem must be in your software !". I will no longer agree to troubleshoot systems that have obvious installation problems until those problems are fixed.

I have a Simatic Step 7 Profibus network . . .
In learning more about troubleshooting the network I turned a terminator
'On' and had the expected symptoms. Then I turned it back 'Off' and had
worse symptoms. It took me a little while to figure out that that blasted
cheap little switch had not turned back 'Off'.

I was called in to troubleshoot a 150m 4 drop system. Used to be 120m 3 drop and was configured on, off, off. I was told it never failed until the fourth drop was added 30m down the line and was configured on, off, off, off. The new station was communicating 95% of the time. Switched the added node to on and to my knowledge has never missed a beat. Proves you can cheat the system but it will catch up to you sooner or later.

Profibus @187.5 kbps is extremely forgiving.

Thank you guys. I'm going to turn the switch to the "ON" on the first (far left) profibus connector. That way the first and the last connector will have its termination settings switched to ON.
I hope I'll not from a working system make some problem in a future
I discovered this accidentally working on something completely different.

I have had more than one situation where someone troubleshooting decided the switch should be "on" because, well, they wanted the device to be on. Lots of profibus and DH+ connections work fine for years when configured incorrectly right up until the day they don't work anymore...

I always found the terminators to be fairly forgiving... the shield grounding on the other hand...