S7-300, CPU 31xC and CPU 31x: Installation 
Operating Instructions, Edition 08/2004, A5E00105492-05 
vii 
Table of contents 
 Preface ......................................................................................................................................................iii 
1  Guide to the S7-300 documentation ....................................................................................................... 1-1 
2 Installation Order .................................................................................................................................... 2-1 
3 S7-300 components................................................................................................................................ 3-1 
3.1  Example of an S7-300 configuration..........................................................................................3-1 
3.2  Overview of the vital modules of an S7-300 .............................................................................. 3-2 
4 Configuring ............................................................................................................................................. 4-1 
4.1 Overview .................................................................................................................................... 4-1 
4.2 Basic engineering principles ...................................................................................................... 4-1 
4.3 Component dimensions ............................................................................................................. 4-4 
4.4 Required clearances .................................................................................................................. 4-6 
4.5  Arrangement of modules on a single rack ................................................................................. 4-7 
4.6  Distribution of modules to several racks ....................................................................................4-8 
4.7  Selection and installation of cabinets....................................................................................... 4-11 
4.8 Example: Selecting a cabinet................................................................................................... 4-14 
4.9 Electrical assembly, protective measures and grounding ....................................................... 4-15 
4.9.1 Grounding concept and overall structure................................................................................. 4-15 
4.9.2  Installing an S7-300 with grounded reference potential .......................................................... 4-16 
4.9.3  Configuring an S7-300 with ungrounded reference potential (not CPU 31xC)........................ 4-17 
4.9.4 Modules with isolated or common potential?........................................................................... 4-19 
4.9.5 Grounding measures ............................................................................................................... 4-21 
4.9.6 Overview: Grounding ............................................................................................................... 4-24 
4.10 Selecting the Load Power Supply............................................................................................ 4-26 
4.11 Planning subnets ..................................................................................................................... 4-28 
4.11.1 Overview .................................................................................................................................. 4-28 
4.11.2  Configuring MPI and PROFIBUS subnets ............................................................................... 4-30 
4.11.2.1 Overview .................................................................................................................................. 4-30 
4.11.2.2  Basic principles of MPI and PROFIBUS subnets .................................................................... 4-30 
4.11.2.3 Multi-Point Interface (MPI) ....................................................................................................... 4-33 
4.11.2.4 PROFIBUS DP interface.......................................................................................................... 4-34 
4.11.2.5 Network components of MPI/DP and cable lengths ................................................................ 4-35 
4.11.2.6  Cable lengths of MPI and PROFIBUS subnets ....................................................................... 4-40 
4.11.3 Configuring PROFINET subnets.............................................................................................. 4-45 
4.11.3.1 Overview .................................................................................................................................. 4-45 
4.11.3.2 PROFINET nodes .................................................................................................................... 4-45 
4.11.3.3 Integration of field bus systems in PROFINET ........................................................................ 4-48 
4.11.3.4 PROFINET IO and PROFINET CBA ....................................................................................... 4-49 
4.11.3.5 PROFINET cable lengths and network expansion .................................................................. 4-54