Siemens SIMATIC S7-300 CPU Data: CPU 315-T-2 DP User 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:
Reduction ratio
transmitter
x cycle time
transmitter
≥ 60 ms
• The receiver of a GD packet must meet the following requirement:
Reduction factor
receiver
x cycle time
receiver
< reduction factor
transmitter
x cycle time
transmitter
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
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.
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
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.
GD resources of the CPU
Table 5-4 GD resources of the CPU
Parameters Technology-CPU
Number of GD circuits per CPU Max. 8
GD packets transmitted per GD circuit Max. 1
GD packets transmitted by all GD circuits Max. 8
GD packets received per GD circuit Max. 1
GD packets received by all GD circuits Max. 8
Data length per GD packet Max. 22 bytes
Consistency Max. 22 bytes
Min. reduction ratio (default) 1 (8)
S7-300 CPU Data: CPU 315T-2 DP
5-10 Manual, 12/2005, A5E00427933-02

Siemens SIMATIC S7-300 CPU Data: CPU 315-T-2 DP Specifications

General

Brand	Siemens
Model	SIMATIC S7-300 CPU Data: CPU 315-T-2 DP
Category	Control Unit
Language	English