User Manual

PLC concepts made easy
4.4 Memory areas, addressing and data types
Easy Book
58 Manual, 03/2014, A5E02486774-AF
Table 4- 2 Additional events
Event
Description
CPU action
I/O access error Direct I/O read/write error The CPU logs the first occurrence in the diagnostic
buffer and stays in RUN mode.
Max cycle time error CPU exceeds the configured cycle time
twice
The CPU logs the error in the diagnostic buffer and
transitions to STOP mode.
Peripheral access error I/O error during process image update The CPU logs the first occurrence in the diagnostic
buffer and stays in RUN mode.
Programming error program execution error If the block with the error provides error handling, it
updates the error structure; if not, the CPU logs the
error in the diagnostic buffer and stays in RUN
mode.
Interrupt latency
The interrupt event latency (the time from notification of the CPU that an event has occurred
until the CPU begins execution of the first instruction in the OB that services the event) is
approximately 175 µsec, provided that a program cycle OB is the only event service routine
active at the time of the interrupt event.
See also
Going online to monitor the values in the CPU (Page 259)
4.4
Memory areas, addressing and data types
The CPU provides the following memory areas to store the user program, data, and
configuration:
● Load memory is non-volatile storage for the user program, data and configuration. When
a project is downloaded to the CPU, it is first stored in the Load memory area. This area
is located either in a memory card (if present) or in the CPU. This non-volatile memory
area is maintained through a power loss. You can increase the amount of load memory
available for data logs by installing a memory card.
● Work memory is volatile storage for some elements of the user project while executing
the user program. The CPU copies some elements of the project from load memory into
work memory. This volatile area is lost when power is removed, and is restored by the
CPU when power is restored.
● Retentive memory is non-volatile storage for a limited quantity of work memory values.
The retentive memory area is used to store the values of selected user memory locations
during power loss. When a power down or power loss occurs, the CPU restores these
retentive values upon power up.