Y-Link as baudrate transformer

Hello Pfaffi;

Bus speed of a Profibus-DP network is selected by the user (in the Profibus configurator, Bus Parameters), downloaded to the Profibus-DP master and then transmitted to each slave at initialization time (start of transmissions). If a slave cannot support the speed requirements of the configuration your Profibus configurator will not allow the initial setup of the network.

This is because all the transmission timings on Profibus are calculated based on TBits (sync time, in terms of bit durations, determined by the overall bus speed), and it is very important that all communication partners on Profibus-DP react to the same sync times, so the network remains deterministic.

For a more detailed presentation on transmission specs over Profibus-DP, I suggest this introduction by Acromag:

Introduction to Profibus-DP:

So you cannot have different sections of the bus running at different speeds, and you will find no gateway that will allow you to do that (except for very specific appications, such as a DP/PA link).

Hope this helps,

Daniel Chartier

dchartier

So you cannot have different sections of the bus running at different speeds, and you will find no gateway that will allow you to do that (except for very specific appications, such as a DP/PA link).

Hello Wago;

I thought that I had expressed the reasons for this clearly, but I may have been not specific enough.

"This is because all the transmission timings on Profibus are calculated based on TBits (sync time, in terms of bit durations, determined by the overall bus speed), and it is very important that all communication partners on Profibus-DP react to the same sync times, so the network remains deterministic."

When you configure a ProfibusDP network, you set all necessary parameters once, before downloading the configuration to the master: number of slaves, number and type of data sent/recieved form each slave, number of repeaters, number of F.O. segments,,,

So the configurator is able to calculate, based on the duration of each transmitted bit (TBit, in microsec) for the selected baudrate, the cycle duration and th time allowed for exchanges for each slave. This allows the master to determine if a slave is replying within its allocated time slot during the communication cycle, and to diagnose transmission and connection errors quickly and effiociently, while maintaining a strict cycle time. These caracteristics allow Profibus-DP to use the term "determinism" when describing the protocol.

If diffesent sections of the bus worked at different speeds, then the TBit calculations would no longer be the same for all participants and the cycle transmission timecould no longer be guaranteed.

In the example of a DP/PA link, PA transmissions (which work at 31,35 kB) can be introduced in a Profibus-DP link at up to 12 MB by segregating the networks, the PA side operating at its speed, while the Profibus_DP side acts as a Profibus slave operating at the nominal DP bus speeds, and resending the same PA values collected from the instruments various times,to the DP-master, while the PA side takes iots own time in reloading the new PA values.

Hope this helps,

Daniel Chartier