Dear Experts,

Regarding Profibus-dp,

I want to know why the Bitrate should be limited by cable distance.

What factors limit baud rate ? voltage-drop ?.

And one thing I checked in my project,

27 profibus nodes are connected with Profibus Master in 1 segment,

The baud rate is 12Mbps.and Check with the Profibus analyzer, there is a voltage drop on the rear end.In this condition, a communication error occurs occasionally. I think the voltage drop would have affected the profibus communication.

But, No error occurred after speed change.(12M -> 500k).

Then If the speed is low, is it less affected by the voltage drop?

 Hello JoDooMan;

Have a look at this site, entitled "How Far and How Fast Can You Go with RS-485?":

https://www.maximintegrated.com/en/app-notes/index.mvp/id/3884

Profibus-DP ios built upon a RS-485 physical layer, so it is to be expected that the limitatioons of RS-485 affect it also.

The main factors limiting data rate on RS-485 (therefore Profibus-DP) are listed in the attachment below.

Among other factors, the impedance of the line is particularly important. The line capacitance increases with frequency (or speed), and shortens the length the signal can travel with recoverable attenuation: the faster the bus speed, the shorter the length of the segment. Conversively, the lower the bus speed, the more "forgiving" the network becomes, the easier it is for the signal to cross the same distance, as you have observed yourself.

Hope this helps,

Daniel Chartier

Hello JoDooMan

the common length definition for PROFIBUS is:

the slower rate you use the longer have the packages time to get through the whole bus to the receiver. The Signal Strengh is one topic but not the only. 

Bye Murof