Siemens SIMATIC S7-300 Series Reference Manual page 34

CPUs
Details
... on the communication topic are found in the STEP 7 Online Help and in the
manual Communication with SIMATIC.
... on communications SFCs/SFBs are found in the STEP 7 Online Help and in the
Standard and System functions reference manual.
Global Data Communication with S7-300 CPUs
Below you will find important features of global data communication in the S7-300.
Send/Receive Conditions
For the communication via GD circuits, you should observe the following
conditions:
S Required for the GD packet transmitter is:
Reduction
S Required for the GD packet receiver is:
Reduction
time Transmitter
Non-observance of these conditions can lead to the loss of a GD packet. The
reasons for this are:
S The performance capability of the smallest CPU in the GD circuit
S Sending and receiving of global data is carried out asynchronously by the
sender and receiver.
Loss of global data is displayed in the status field of a GD circuit if you have
configured this with STEP 7.
Note
Note when communicating via global data: sent global data is not acknowledged
by the receiving partner!
The sender therefore receives no information on whether a receiver and which
receiver has received the sent global data.
Send Cycles for Global Data
In STEP 7 (as of Version 3.0), the following situation can arise if you set "Send
after every CPU cycle" with a short CPU cycle time (< 60 ms): the operating
system overwrites GD packets the CPU has not yet transmitted. Tip: Loss of
global data is displayed in the status field of a GD circuit if you have configured this
with STEP 7.
1-18
Cycle
ratio Transmitter time Transmitter
Cycle
ratio Receiver timer eceiver
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
w 60 ms (CPU 318-2: w10 ms
t Reduction
ratio Transmitter
A5E00111190-01
Cycle