Siemens Automation System S7-400 - Manual (Page 18)

Installing the S7-400
2-2
Automation System S7-400 Hardware and Installation
A5E00850741-01
2.1 S7-400 Installation
Introduction
An S7-400 programmable controller consists of a central rack (CR) and one or
more expansion racks (ERs), as required. You can add ERs to compensate for lack
of slots for your application or operate signal modules at remote locations (e.g. in
the immediate vicinity of your process).
When using ERs, you need interface modules (IMs) as well as the additional racks,
and additional power supply modules if necessary. When using interface modules,
you must always use the appropriate partners: you insert a send IM in the CR, and
the matching receive IM in each connected ER (see Reference Manual,
Chapter 6).
Central Rack (CR) and Expansion Rack (ER)
The rack containing the CPU is known as the central rack (CR). The racks
containing modules in the system and connected to the CR are the expansion
racks (ERs).
Shown in Figure 2-1 is a rack with 18 slots configured as a CR.
PS CPU SMs
Figure 2-1 Rack Fitted with Modules in the S7-400 System