Global Data Communication - Siemens SIMATIC S7-300 Manual

Communication with the S7-300
5.3 Communication services on the MPI/DP interface (X1)
5.3.6

Global data communication

Global data communication
Global data communication is used for cyclic exchange of global data (for example, I, Q, M)
between SIMATIC S7 CPUs (data exchange without acknowledgement). One CPU
broadcasts its data to all other CPUs on the MPI subnet. This function is integrated in the
CPU operating system.
Send and receive conditions
Conditions which should be satisfied for GD communication:
• The transmitter of a GD packet must meet the following requirement:
• The receiver of a GD packet must meet the following requirement:
A GD packet may be lost if there requirements are not met. 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
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
Reduction factor
The reduction ratio specifies the cyclic intervals for GD communication. You can set the
reduction ratio when you configure global data communication in
you set a reduction ratio of 7, global data are transferred only with every 7th cycle. This
reduces CPU load.
GD resources of the CPU
Table 5-4
Parameters
Number of GD circuits per CPU
GD packets transmitted per GD circuit
GD packets transmitted by all GD circuits
GD packets received per GD circuit
GD packets received by all GD circuits
Data length per GD packet
Consistency
Min. reduction ratio (default)
5-10
Reduction ratio
transmitter
Reduction factor x cycle time
receiver
< reduction factor
transmitter
STEP 7
: "Transmit after each CPU cycle", and the CPU has a short scan
STEP 7
GD resources of the CPU
x cycle time ≥ 60 ms
transmitter
receiver
x cycle time
transmitter
configuration.
STEP 7
. For example, if
Technology-CPU
Max. 8
Max. 1
Max. 8
Max. 1
Max. 8
Max. 22 bytes
Max. 22 bytes
1 (8)
S7-300 CPU Data: CPU 315T-2 DP
Manual, 12/2005, A5E00427933-02