Process Image, Type Of Transmission, Event Classes, Triggers - Siemens S7-1200 Operating Instructions Manual

4.8

Process image, type of transmission, event classes, triggers

The image memory, the process image of the CP
The image memory is the process image of the CP. All the current values of the configured
data points are stored in the image memory. New values of a data point overwrite the last
stored value in the image memory.
The values are sent after querying the communications partner, see "Transfer after call" in
the section "Types of transmission" below.
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 17).
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 Communication with the CPU (Page 74).
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%.
CP 1243-7 LTE
Operating Instructions, 01/2015, C79000-G8976-C381-01
4.8 Process image, type of transmission, event classes, triggers
Configuration and operation
51