Siemens CP 1243-8 IRC Operating Instructions Manual page 73

The send buffer (frame memory)
The send buffer of the CP is the memory for the individual values of data points that are
configured as an event. The maximum size of the send butter can be found in the section
Performance data and configuration limits (Page 18).
The configured number of events is divided equally among all configured and enabled
communications partners. For information on the configuration, refer to the parameter
"Frame memory size" in the section Parameter group "Communication with the CPU"
(Page 66).
If the connection to a communications partner is interrupted, the individual values of the
events are stored in the RAM of the CP. When the connection returns, the buffered values
are sent. The frame memory operates chronologically; in other words, the oldest frames are
sent first (FIFO principle).
If a frame was transferred to the communications partner, the transferred values are deleted
from the send buffer.
If frames cannot be transferred for a longer period of time and the send buffer is threatening
to overflow, the response is as follows:
The forced image mode
If the send buffer reaches a fill level of 80%, the CP changes to the forced image mode. New
values of events are no longer added to the send buffer but rather they overwrite older
existing values in the image memory.
When the connection to the communications partner returns, the CP changes back to the
send buffer mode if the fill level of the send buffer has fallen below 50%.
Type of tranmsission (Event classes)
Data points are configured using the "Type of transmission" parameter in the table or in the
"General" tab of the data point configuration as a static value or as an event.
The values of data points configured as an event are entered in the image memory and also
in the send buffer of the CP. They are transferred to the communications partner when the
configured trigger conditions are met ("Trigger") tab.
The following types of transmission are available:
● Transfer after call
The current value of the data point is only entered in the image memory of the CP. New
values of a data point overwrite the last stored value in the image memory.
The current value at the time is transferred only after being called by the communications
partner.
● Every value triggered
The data point is configured as an event.
Each value change is entered in the send buffer in chronological order.
CP 1243-8 IRC
Operating Instructions, 06/2015, C79000-G8976-C385-01
Configuration and operation
4.7 Configuration in STEP 7 Basic
73