Siemens SIMATIC S7-400 Configuration And Use Manual page 91

Interfacing Rules
Interface modules are divided into send IMs and receive IMs A send IM sends
signals to the connected expansion device, the corresponding receive IM receives
and forwards the signals. The following rules apply to connecting interface
modules:
• The send IM must always be located in the central controller.
• You can connect up to four receive IM to one send IM.
• Up to six send IMs can be installed in the central controller.
Maximum of 22 Racks
Interfacing with send and receive IMs, you can connect up to 21 expansion devices
to a central controller.
Automation System S7-400 Configuration and Use
A5E00442711-02
Configuration Variants of the S7-400
6-11