Siemens S5-135U/155U - Manual (Page 20)

2-2
System Manual
C79000-G8576-C199-06
2.1 Application
The S5-135U/155U programmable controllers comprise a central controller
(CC) and, if required, one or more expansion units (EUs). You need EUs
when there are insufficient slots in the CC for the modules to be used.
Various interface modules (IMs) are available for communication between
the CC and the EUs and between the EUs. It is therefore possible to install an
EU or EUs in the immediate vicinity of the CC (centralized configuration) or
at some distance (distributed configuration). A combination of both types of
configuration is also possible by connecting additional EUs in a centralized
configuration with a distributed EU (see Figure 2-5).
This is clarified on the following pages.
Centralized and Distributed Configuration of a Programmable Controller