Siemens S7-300 Global data communication MPI only, Reduction ratio, Send and receive conditions

Communication 3.2 Communication services

3.2.6Global 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 ratiotransmitter x cycle timetransmitter ≥ 60 ms

•For the receiver of a GD packet: Reduction ratioreceiver x cycle timereceiver

< reduction ratiotransmitter x cycle timetransmitter

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.

CPU 31xC and CPU 31x, Technical data	3-9
Manual, Edition 08/2004, A5E00105475-05