Siemens SIMATIC S7-400 Hardware And Installation Manual page 25

Rules for Connection
When you connect a central rack to expansion racks, you must observe the
following rules:
You can connect up to 21 ERs of the S7-400 to one CR.
The ERs are assigned numbers to identify them. The rack number must be set
on the coding switch of the receive IM. Any rack number between 1 and 21 may
be assigned. Numbers must not be duplicated.
You may insert up to six send IMs in one CR. However, only two send IMs with
5 V transfer are allowed in one CR.
Each chain connected to the interface of a send IM can comprise up to four
ERs (without 5 V transfer) or one ER (with 5 V transfer).
The exchange of data via the communication bus is limited to 7 racks, meaning
the CR and ER numbers 1 to 6.
The maximum (total) cable lengths specified for the type of connection must not
be exceeded.
Type of Connection
Local connection with 5 V transfer via
IM 460-1 and IM 461-1
Local connection without 5 V transfer via
IM 460-0 and IM 461-0
Remote connection via IM 460-3 and
IM 461-3
S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01
www.gkbpx.com
Installing the S7-400
Maximum (Total) Cable Length
1.5 m
3 m
102.25 m
2-5