Drop connector in Profibus-dp installation


Thread Starter

Fred Zhang

Anybody has the experience of profibus-dp drop connector in MCC/distributed starter design? I am designing the profibus-dp installation for tens of separate starter/VFD cabinets in the electrical room. First, do I need to have drop connector connecting to the individual starter/VFD cabinet or connect the BUS cable directly to the screw terminals in the individual starter/VFD cabinet? Second,who will supply the power to the Profibus-dp communication, the master?

Daniel Chartier

Hello Fred;

Profibus-DP does not like "drop connectors" (they call them "Spur-lines); if you do need them (Weidmuller provides Profibus-DP T-Connectors, for example) the spurs must be kept very short (total length of all spur-limes must not exceed 6 meters at 1,5 MBaud).

The prefered connection on Profibud-DP is through specialized connectors, D-Sub 9-pin, with dedicated connection for the cable shield; it allows for incoming and outgoing cable (daisy-chain connections) and includes a resistance terminator for end-of-line (very important). Some manufacturers do not give acces to a D-Sub connection (anything inside an IP-67 instrument, for example; a D-sub connection would cancel the ingression protection level); they therefore give a screw terminal; if you do bring the cable to the trminals directly, make sure that the cable shield is connected to its terminal (these are generally marked A, B, S).

Power to each Profibus interface (on the PLC, or on each slave) is provided by the host. In your case, the local Profibus card will be powered by each separate motor starter/VFD cabinet. There is no power transmission over the Profibus-DP cable.

For more information, I recommend the manual PIC_PROFIBUS_Cabling_Guide.pdf that you will find on the following link:

http://eeprofs.iust.ac.ir/Shahri/Computer Buses_84/PROFIBUS/PIC PROFIBUS Cabling Guide.pdf

You can also go on www.profibus.com, and look at their Profibus-DP/Technology section.

Hope this helps,
Daniel Chartier
You can do that but at the end of VFD terminals you must use resistor as end of line terminator. or you can use profibus T-connector (siemens or weidmuller, etc) so you save the cable length. I did it in my last project and working well.
The type of connector is dependent on your MCC/VFD vendor, but typically most companies use a sub-d 9-pin connector. Profibus connectors are made by several vendors. Here is a link to the Siemens' connector:


Regarding the power question. Profibus-DP devices are all externally powered. No one device powers the network. When a device transmits, it changes the voltage across the two wires.

Since you have so many devices, don't forget the the maximum number of devices you can have on a segment is 32. If you have more then that, you need a repeater. Also, don't forget to terminate both sides of the bus.

Good luck!