Configuring
4.2 Basic engineering principles
S7-300, CPU 31xC and CPU 31x: Installation
4-2
Operating Instructions, Edition 08/2004, A5E00105492-05
Caution
Operation of an S7-300 in plants or systems is defined by special set of rules and
regulations, based on the relevant field of application. Observe the safety and accident
prevention regulations for specific applications, for example, the machine protection
directives. This chapter and the appendix
General rules and regulations on S7-300 operation
provide an overview of the most important rules you need to observe when integrating an
S7-300 into a plant or a system.
Central unit (CU) and expansion module (EM)
An S7-300 PLC consists of a central unit (CU) and of one or multiple expansion modules.
The rack containing the CPU is the central unit (CU). Racks equipped with modules and
connected to the CU form the expansion modules (EMs) of the system.
Use of an expansion module (EM)
You can use EMs if the CU runs out of slots for your application.
When using EMs, you might require further power supply modules in addition to the extra
racks and interface modules (IM). When using interface modules you must ensure
compatibility of the partner stations.
Racks
The rack for your S7-300 is a mounting rail. You can use this rail to mount all modules of
your S7-300 system.
