Communication
3.2 Communication services
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
3-9
3.2.6 Global data communication (MPI only)
Properties
Global data communication is used for cyclic exchange of global data via MPI subnets (for
example, I, Q, M) between SIMATIC S7 CPUs (data exchange without acknowledgement).
One CPU broadcasts its data to all other DP CPUs on the MPI subnet. This function is
integrated in the CPU operating system.
Reduction ratio
The reduction ratio specifies the cyclic intervals for GD communication. You can set the
reduction ratio when you configure global data communication in STEP 7. For example, if
you set a reduction ratio of 7, global data are transferred only with every 7th cycle. This
reduces CPU load.
Send and receive conditions
Conditions which should be satisfied for GD communication:
• For the transmitter of a GD packet:
Reduction ratio
transmitter
x cycle time
transmitter
≥ 60 ms
• For the receiver of a GD packet:
Reduction ratio
receiver
x cycle time
receiver
< reduction ratio
transmitter
x cycle time
transmitter
A GD packet may be lost if you do not adhere to these conditions. The reasons being:
• the performance of the "smallest" CPU in the GD circuit
• asynchronous transmitting / receiving of global data at the stations
When setting in STEP 7: “Transmit after each CPU cycle“, and the CPU has a short scan
cycle time (< 60 ms), the operating system might overwrite a GD packet of the CPU before it
is transmitted. The loss of global data is indicated in the status box of a GD circuit, if you set
this function in your STEP 7 configuration.
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