PLCs S7-300 and S7-200 (Net profibus DP)


Thread Starter


Hi. As I said before, this is my first time working with PLCs from SIEMENS.
The project I'm working with includes a subnet profibus for getting some data from some slaves distributed all over the camp. Three of this slaves were not programmed for us (our team) cause they came as part of some Gas Analyzers, programmed in Germany.
I need to get into my S7-300 master the state of some OUTPUTs and INPUTS (including Analogic Inputs) from this slaves (S7-300 too)
Can some one explain me What functions can I use to get the value of this signals (the analogic ones) and the state of the digital ones (can I read the state of the outputs too?)?
I would thank if you indicate me where can i find some examples about addressing slaves' modules in a profibusDP network.

Hakan Ozevin

There are mainly two versions of Profibus applicable to your case (omitting FMS):
1. DP protocol
2. S7 functions.
With DP protocol you can directly access to I/O's of a station on a subnet, but you cannot access to internal CPU variables (or access is very limited). In this case the addresses are automatically given by the master using STEP 7. You just use these addresses.
With S7 functions, you can SEND and RECEIVE anything between the master and the slave, using mainly the data blocks as the send and receive buffer.
So I think that what you need is the latter one and you can handle it with the so called SFC's (System Function Calls), but I think you have to get S7-300 system manual first and read it carefully. It is available from "": .

As an additional information, provided that the modules support them, simultaneous mixed connections are possible. It means, one master and several slaves can communicate with S7 protocol and the rest of the slaves may use DP protocol in the same subnet. Even FMS between several masters are possible at the same time at the same subnet.

Therefore the existing DP slaves will not prevent you.

I hope this helps.

Hakan Ozevin