Siemens S7-318dp cpu andCP342-5 compatability


Thread Starter


Can I use an S7-318dp processor and a cp342-5 communications processor together to expand my maximum DP(profibus) drops to 190?

Hakan Ozevin

You can have two individual profibus networks with your hardware. The theoretical limit is thus 2*124=248 slaves, but it is not advised by
siemens people. However, I think that it is possible depending on size of the input/output bytes from the slaves. But, if you have for
example 32 bytes inputs/32 bytes outputs from each slave, you'd rather configure your system in Step 7 first and see if it is allowed by
Step 7.

david mertens

I agree that it's allways a good idea to check if your configuration is possible by creating it in hardware configuration and compiling the
configuration. However I don't agree that siemens would advise against using a second profibus network, quite to the contrary you will have to use at least 3 different profibus master systems as the CP342-5 only supports a maximum of 64
slaves (The internal DP only 32!!!). Take care also about the fact that a segment of a profibus network should not have more than 32 slaves, this would also require you to use at least 1 RS485 repeater for every DP master system (with the maximum of 64 slaves present). Another problem would be the adressing range, evey DP slave can have up to 240 bytes input and 240 bytes output, with 190 slaves this could give a theoretical needed addressing range of 45600 bytes input and the same amount for output. This largely surpasses the limit of the CPU. Off course if you are using DP slaves with only a few bytes of IO each, this wouldn't be a problem, it would be wise however to count them all up. I suggest therefor you should first have your configuration checked by the siemens hotline before buying the hardware, I think you will find that your
proposed configuration is not sufficient for this project.

Hakan Ozevin

Unfortunately this is not correct. Both CP 342-5 and CPU 318-2 DP allows up to 124 slaves each. Please see the Siemens on-line catalogue. I
also checked this with Step 7, and it was OK.
The limitation here, as I and you told, is the addressing range of the CP and CPU, which is 2160 bytes for inputs and outputs each. This
means that the author can have 190 slaves in total with 16 inputs/16 outputs in average. Some of the slaves may have 32 input/32 output or
even higher.

Hakan Ozevin

I also want to clarify that what Siemens people do not advise is using the full capacity of the network, i.e. 124 slaves in each Profibus
branch. Of course they are not against two separate networks, which is a necessity in the author's case.