Siemens SIMATIC NET TeleControl S7-1200 CP 1243-1 Operating Instructions Manual page 76

Configuration
4.9 Data points
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
Configuration limits and performance data (Page 15).
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 SNMP (Page 42).
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 as soon as the fill level of the send buffer has fallen below 50%.
Types of transmission / event classes
The following types of transmission are possible:
● Transfer after call
The current value of the data point is entered in the image memory of the CP. New
values of a data point overwrite the last stored value in the image memory.
After being called by the communications partner, the current value at the time is
transferred.
● Triggered (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.
The values of events are saved in the following situations:
– The configured trigger conditions are fulfilled (data point configuration > "Trigger" tab,
– The value of a status bit of the status identifiers of the data point changes see also the
76
see below)
section Status IDs of data points (Page 72).
Example: When the value of a data point configured as an event is updated during
startup of the station by reading the CPU data for the first time, the status "RESTART"
of this data point changes (bit status change 1 → 0). This leads to generation of an
event.
Operating Instructions, 12/2016, C79000-G8976-C365-02
CP 1243-1