SIMATIC S7?300 I/O Modules for digital and analog inputs/outputs have the following mechanical characteristics:

• Compact design
  The rugged plastic casing contains:
  • Green LEDs for indicating the signal states on the inputs/outputs
  • Red LED to indicate that a diagnosis is being performed (for modules capable of diagnostics)
  • Sockets for front connectors, protected behind front door
  • Labeling area on front door
• Simple assembly
  The modules are mounted one after the other on the mounting rail from the left to the right and connected to neighboring modules with bus connectors. There are no slot rules.
• User-friendly wiring
  The modules are wired with front connectors. When the module is connected for the first time, a coding latches in the connector so that the connector now only fits onto modules of the same type. When a module is replaced, the front connector can be plugged onto the new module of the same type with the complete wiring. The front connectors are available with screw-type or spring-loaded terminals or with the FastConnect connection system.