3.1
3.1Multicomputing
3.1.1 Fundamentals
Multicomputing Mode
Multicomputing mode is the simultaneous operation of several (up to 4) CPUs in a central rack of the S7-400.
The CPUs involved automatically change their modes synchronized with each other; the CPUs start up together and change to STOP together. The user program on each CPU runs independently of the user programs on the other CPUs. This allows control tasks to be performed simultaneously.
Racks suitable for Multicomputing
The following racks are suitable for multicomputing:
• UR1 and UR2
• UR2-H, multicomputing with several CPUs is possible only if the CPUs are in the same subdevice.
• CR3, since the CR3 has only 4 slots, multicomputing is possible only with two CPUs.
Difference Compared with Operation in a Segmented Rack
In the CR2 segmented rack (physically segmented, cannot be set using parameters), only one CPU per segment is permitted. This is, however, not multicomputing. The CPUs in the segmented rack each form an independent subsystem and behave like individual
processors. There is no common logical address space.
Multicomputing is not possible in the segmented rack (see also S7-400 Automation System, Hardware and Installation).
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Special functions of a CPU 41x 3.1 Multicomputing
S7-400 Automation System, CPU Specifications
3-2 Manual, 11/2006, A5E00850746-02
Uses
There are benefits in using multicomputing in the following situations:
• When your user program is too large for one CPU and memory starts running short, distribute your program over several CPUs.
• When a particular part of your plant needs to be processed quickly, separate the relevant program section from the overall program and run this part on a separate "fast" CPU.
• When your plant consists of several parts with a clear demarcation between them so that they can be controlled relatively independently, process plant part 1 on CPU1, plant part 2 on CPU2 etc.
Example
The following figure shows an automation system operating in multicomputing mode. Each CPU can access the modules assigned to it (FM, CP, SM).
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Figure 3-1 Example of multicomputing
3.1.2 Special Features at Multicomputing
Slot Rules
In multicomputing mode, up to four CPUs can be inserted in one central controller (CC) in any order.
Accessibility of the CPUs.
All CPUs can be accessed from the programming device if configured accordingly via the MPI interface, PROFIBUS DP interface or PROFINET PN interface of one CPU.
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Special functions of a CPU 41x 3.1 Multicomputing
Downloading the Configuration in Multicomputing Mode
If you want to use multicomputing, in very rare cases with very large configurations, you may find that the CPUs do not start up after the configuration has been downloaded to the PLC ("PLC > Download to Module" menu command in HW Config).
Remedy: Perform a memory reset for all the CPUs. Then download your system data (and all blocks) to each CPU in succession in Simatic Manager. Start with the CPU with the highest CPU number, always followed by the CPU with the next lowest number. Then switch the CPUs to RUN in the same order.
Behavior during Startup and Operation
During startup, the CPUs involved in multicomputing automatically check whether they can synchronize themselves. Synchronization is possible only in the following situations:
• When all the configured and only the configured CPUs are inserted and ready to operate.
• If correct configuration data was created with STEP 7 and was downloaded to all plugged-in CPUs.
If one of these conditions is not fulfilled, the event is entered in the diagnostic buffer with ID 0x49A4. You will find explanations of the event IDs in the reference help on standard and system functions.
When STOP mode is left, the types of startup COLD RESTART/WARM RESTART/HOT RESTART are compared. If the types of startup are different, the CPUs do not change to RUN.
Address and Interrupt Assignment
In multicomputing, the individual CPUs can access the modules assigned to them during configuration with STEP 7. The address area of a module is always assigned "exclusively" to one CPU.
In particular, this means that every module with interrupt capability is assigned to a CPU.
Interrupts triggered by such a module cannot be received by the other CPUs.
Interrupt Processing
The following applies to interrupt processing:
• Hardware interrupts and diagnostic interrupts are sent to only one CPU.
• If a module fails or is removed/inserted, the interrupt is processed by the CPU to which the module was assigned during parameter assignment with STEP 7.
Exception: A remove/insert interrupt triggered by a CP reaches all CPUs even if the CP was assigned to one CPU during configuration with STEP 7.
• If a rack fails, OB86 is called on every CPU; in other words, it is also called on the CPUs to which no module in the failed rack was assigned.
For more detailed information on OB86, refer to the reference help on organization blocks.
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Special functions of a CPU 41x 3.1 Multicomputing
S7-400 Automation System, CPU Specifications
3-4 Manual, 11/2006, A5E00850746-02
Number of I/Os
The number of I/Os of an automation system in multicomputing mode corresponds to the number of I/Os of the CPU with the most resources. The configuration limits for the specific CPU or specific DP master must not be exceeded in the individual CPUs.
3.1.3 Multicomputing interrupt
Principle
Using the multicomputing interrupt (OB60), you can synchronize the CPUs involved in multicomputing to an event. In contrast to the hardware interrupts that are triggered by signal modules, the multicomputing interrupt can only be output by CPUs. The multicomputing interrupt is triggered by calling SFC35 "MP_ALM".
For more detailed information, refer to the System Software for S7-300/400, System and Standard Functions.
3.1.4 Configuring and programming multicomputing mode
Reference
For information on the procedure for configuring and programming CPUs and modules, refer to the Configuring Hardware and Connections with STEP 7.
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Special functions of a CPU 41x 3.2 System modifications during operation