Communications interface modules can be optionally fitted in the S7-400 CPUs in order to match the latter to the respective application.

The CPUs 414-3, 416-3 have one vacant slot for this purpose, and the CPU 417-4 has two slots. By inserting such interface modules, additional DP lines can be produced as master or slave whose functionality corresponds to that of the integral interface. Up to 125 nodes can be controlled via a DP interface.

More than 400 PROFIBUS slaves can be connected to a CPU with 4 interfaces; an S7-400 station can therefore control more than 1600 PROFIBUS slaves.