User Manual

Easy to create the device configuration
5.5 Configuring the operation of the CPU and modules
Easy Book
78 Manual, 03/2014, A5E02486774-AF
The CPU initializes these bytes on the transition from STOP mode to STARTUP mode. The
bits of the clock memory change synchronously to the CPU clock throughout the STARTUP
and RUN modes.
CAUTION
Risks with overwriting the system memory or clock memory bits
Overwriting the system memory or clock memory bits can corrupt the data in these
functions and cause your user program to operate incorrectly, which can cause damage to
equipment and injury to personnel.
Because both the clock memory and system memory are unreserved in M memory,
instructions or communications can write to these locations and corrupt the data.
Avoid writing data to these locations to ensure the proper operation of these functions, and
always implement an emergency stop circuit for your process or machine.
System memory configures a byte with bits that turn on (value = 1) for a specific event.
Table 5- 2 System memory
7
6
5
4
3
2
1
0
Reserved
Value 0
Always off
Value 0
Always on
Value 1
Diagnostic status
indicator
• 1: Change
• 0: No change
First scan indicator
• 1: First scan after
startup
• 0: Not first scan
Clock memory configures a byte that cycles the individual bits on and off at fixed intervals.
Each clock bit generates a square wave pulse on the corresponding M memory bit. These
bits can be used as control bits, especially when combined with edge instructions, to trigger
actions in the user code on a cyclic basis.