Siemens SIMATIC S7-400H System Manual page 120

Using I/Os in S7-400H
10.2 Introduction
Addressing
No matter whether you are using a single-channel, one-sided or switched I/O, you always
access the I/O at the same address.
Limits of I/O configuration
If there are insufficient slots in the central racks, you can add up to 20 expansion units to the
S7-400H configuration.
Module racks with even numbers are always assigned to central unit 0, and racks with odd
numbers are assigned to central unit 1.
For applications with distributed I/O, each of the subsystems supports the connection of up
to 12 DP master systems (two DP master systems on the integrated interfaces of the CPU
and 10 via external DP master systems).
The integrated MPI/DP interface supports the operation of up to 32 slaves. You can connect
up to 125 distributed I/O devices to the integrated DP master interface and to the external
DP master systems.
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System Manual, 09/2007, A5E00267695-03
S7-400H