Configuration
4.18 Data points
CP 1243-8 IRC
Operating Instructions, 02/2018, C79000-G8976-C385-03
145
The image memory, the process image of the CP
The image memory is the process image of the communications module. 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 only transferred under the following conditions:
● After a query by the communications partner; see "Transfer after call" in the "Types of
transmission" section below.
● Along with a data frame from the send buffer that needs to be transferred immediately.
The send buffer of the CP is the memory for the individual values of data points that are
configured as an event. The maximum number is divided equally among all configured and
enabled communications partners. You configure the size of the send buffer with the
parameter "Frame memory size", see the section Communication with the CPU (Page 95).
If the connection to a communications partner is interrupted, the individual values of the
events are retained in the buffer. When the connection returns, the buffered values are sent.
The frame memory operates chronologically; in other words, the oldest data is sent first
(FIFO principle).
If a data frame was transferred to the communications partner, the transferred data is
deleted from the send buffer.
If data cannot be transferred for a longer period of time and the send buffer is threatening to
overflow, the protocol-dependent response is as follows:
● Telecontrol Basic, ST7
The forced image mode
If the send buffer reaches a certain fill level, the module changes to the forced image
mode. The fill level for changing to the forced image mode is:
– Telecontrol Basic: 80 % occupation of the send buffer
– ST7: 90 % occupation of the send buffer
New values of data points configured as an event are no longer added to the send buffer
in forced image mode. Rather, they overwrite older existing values in the image memory.
When the connection to the communications partner returns, the module changes back to
the send buffer mode as soon as the fill level of the send buffer has fallen below 50 %.
● DNP3 / IEC
If the fill level of the send buffer reaches 100 %, no more values are saved until the fill
level falls below 100 % again.
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