Siemen's use of "Station" in Profibus DP


Thread Starter


Siemens talks about a limit of up to 32 stations (masters or slaves) that can be connected per segment on Profibus DP. How does this equate to the limit on I/O modules per segment?

david mertens

A station is a profibus slave, there can be a total of 125 slaves on a profibus network, however you have to use repeaters between segments of up to 32 slaves. Every slave can exchange a maximum of 244 bytes of data (Some DP masters are limited to 122 bytes data, check this !!!). This is the theoretical limit of the number of IO possible, the practical limit depends on the number of IO cards that can be fitted into a DP slave and the number of IO's per card. E.g. an ET200M can hold 8 cards, the maximum analogue card is 8 channels (8 x 8 x 2 bytes = 128 bytes + some extra for diagnostics) and the maximum digital card is 32 channels (8 cards x 4 bytes = 32 bytes).

Hakan Ozevin

Each adressable node is a station.

The limitation of connected I/O modules depends on the master's addressing capacity. For example (as far as I remember) 255 bytes input and 255 bytes output for CPU 315-2DP.

The theoretical maximum number of stations per segment is 124, but in practice 50-60 stations (or even lower) may fill the master's I/O capacity.

Gabriele Corrieri

Hi Spencer,

Siemens has different definition when write 'station'.

More complex I think is based on ET200M, with same modules of S7300, with maximun of 8 modules assorted (maximum could be 256 single I/O points) but if I remember well the very problem isn't the limit of 32 Station, but the full I/O points of all Stations: inside of cpu (or CP) there is a memory that work as Input Process Image, and Output Process Image, than you cannot address more I/O points that theys are mapped into I/O memory of CPU or CP.

Generally every configuration that is compiled without errors by NetPro (Step7) is valid and addressable.

I hope this helps you.

Donald Pittendrigh

Hi All

The calculation method and results differ depending on the slave types however in the case of ET200M, probably the most popular remote I/O station on DP, it is possible to plug 8 PLC cards of 32 DI or 32 DQ into each station or 7936 (31 stations * 8 cards per station * 32 digtals per card) I use 31 stations as one of them is the master and probably has no I/O. This I/O count may not be practically realizable with other station types, but as far as I recall, it represents the maximum I/O count achievable per station i.e. 8 PLC cards * 4 bytes per card which equates to 32 bytes of I/O per station.

Donald Pittendrigh