Configuring Hardware and Communication Connections STEP 7.pdf

Configuring Global Data Communication
13.2 Determining the Communication Capabilities from the
GD Resources
Tip
You can determine how high the performance of an S7 CPU is with regard to GD
communication using the following technical specifications ("GD resources"):
• The number of GD circles (in which the CPU can participate)
• The maximum number of net data per GD packet
• The maximum number of receive GD packets per GD circle
• The length of the consistent data per packet
The remaining documented GD resources are identical for all S7 CPUs and are not
therefore relevant for selecting a CPU.
The technical data listed above provide indirect information about how much data
the CPUs that are connected together via an MPI subnet or S7-400 backplane bus
can exchange cyclically. How send data are "bundled up" to form GD packets and
how many GD circles are used is illustrated in the following sections.
If you only want to transfer small amounts of data (only a few bytes) between a few
CPUs, simply enter the address areas in the GD table and compile the table.
STEP 7 packs the data and allocates the resources automatically. You can see the
total resources (GD circles and GD packets) used up after compilation in the first
column ("GD Identifier") of the GD table.
The following sections outline the principle according to which GD packets and GD
circles are used up.
Configuring Hardware and Communication Connections with STEP 7
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