Peculiarities - Siemens SIMATIC S7-400 Reference Manual

Structure of a CPU 41x
1.9.1

Peculiarities

Slot Rules
In multicomputing operation, up to four CPUs can be inserted at the same time in a
central controller (CC) in any order.
If you use CPUs that can only handle module start addresses that are divisible by
4 (usually CPUs before 10/98), you must keep to this rule for all the configured
CPUs when you assign addresses! The rule applies should you also use CPUs
that allow the bytewise assignment of module start addresses in single-computing
operation.
Bus Connection
The CPUs are connected to one another via the communication bus (K bus). In
other words, if configured appropriately, all the CPUs can be reached by the
programming device via an MPI interface.
Behavior at Startup and During Operation
At startup, the CPUs involved in multicomputing operation automatically check
whether they can synchronize with each other. Synchronization is only possible if:
• All the configured CPUs (but only those) are inserted and not defective.
• Correct configuration data (SDBs) have been created and loaded for all the
inserted CPUs.
If one of these prerequisites is not met, the event is entered in the diagnostic buffer
with ID 0x49A4. You can find explanations of the event IDs in the reference
information for standard and system functions.
When STOP mode is exited, a comparison of the types of startup (COLD
RESTART/REBOOT (WARM RESTART/RESTART) is carried out. If their startup
type differs, the CPUs do not go into RUN mode.
Assignment of Addresses and Interrupts
In multicomputing operation, the individual CPUs can each access the modules
that were allocated to them during configuration with STEP 7. The address area of
a module is always assigned exclusively to a CPU.
Each interrupt-capable module is assigned to a CPU. Interrupts originating from
such a module cannot be received by the other CPUs.
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Automation System S7-400 CPU Specifications
A5E00165965-01