Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 161)

Special Programming Techniques
Chapter 11
11Ć14
The values in the data table at start-up will depend on whether or not the
memory was retained by back-up battery. If a battery was used, the data table
will contain the values which existed when power was removed. If no battery
was used, the values programmed into EPROM will be transferred into the data
table at power-up.
Start-up conditions can be summarized for manual start-up (using one of the
suggested programming techniques) and for automatic start-up (where neither
technique is used) as follows:
a. From initial start-up conditions: remove the battery from the system
b. From in-process conditions at time of power loss: maintain battery
back-up.
CAUTION: The battery should not be replaced during AC
power loss. Your application program would be lost. Replace
the memory back-up battery only with power applied to the
system.
An EPROM to RAM transfer will not take place upon power-up when the
battery is operating and the memory content matches EPROM content. A
transfer will take place if any alteration of memory content occurred while the
battery was being changed. If a transfer occurred (memory was altered), the
data table will contain the values programmed into the EPROM. If the transfer
did not occur (memory was not altered), the data table will contain the values
which existed at the time the system power was removed. There is not way of
determining whether the transfer will occur if the battery is replaced during a
power loss.
Cascading is defined as a program technique that extends the ranges of timers
and/or counter instructions beyond the maximum values that may be
accumulated. Figure 11.8 illustrates a 24-hour clock program. Again we
emphasize not to enter these instructions using your on-line production
equipment. This clock program is not accurate. Do not use it as a real time
clock device.
Cascading