Siemens Automation System S7-400 - Manual (Page 54)

Addressing the S7-400
3-2
Automation System S7-400 Hardware and Installation
A5E00850741-01
3.1 Geographical and Logical Addresses
Addresses
In order to control a process, you must address the channels (inputs and outputs)
of the signal modules from the user program. You must establish a unique
assignment between the (geographical) location of a channel and an address in the
user program.
Geographical Addresses
The geographical address of a particular channel is permanently assigned. It is
governed by the physical location of the input or output. In particular, this depends
on the following basic conditions:
• In which rack (0 to 21) is the signal module fitted?
• At which slot (1 to 18 or 1 to 9) in this rack is the signal module inserted?
• Which channel (0 to 31) of this signal module is addressed?
Section 3.2 describes the method of establishing the geographical address of a
channel.
Logical Addresses
The logical address of a module and, therefore, of a channel is freely selectable. It
is used in the program to address (read or write to) a particular input or output. The
physical location of the relevant module need not be known during programming.
You establish the assignment between logical and geographical address with
STEP 7.
The Two Stages of Addressing
You assign the channel address in two stages:
• Determine the geographical address of the channel from its location in the
entire configuration.
• Assign a logical address to the geographical address under STEP 7. This
logical address is used for addressing the channel in the user program.
Note
If your S7-400 comprises only a CR without ER, you can also use default
addressing.