Operating Without A Memory Backup Battery - GE 90-30 PLC Series Installation And Hardware Manual

6

Operating Without a Memory Backup Battery

Whether it would be to your advantage to use a battery-less scheme depends on your application.
There are various advantages and disadvantages to consider in making your decision.
Possible Advantage
The obvious advantage of operating without a memory backup battery is that you are freed from
the need to maintain the battery. To be able to run without a battery, you need to have a PROM
device - either an EPROM, EEPROM, or Flash PROM - installed in your system. These devices
can store program logic, configuration, and register values without the need for a backup battery,
and you can configure your CPU to read the contents of PROM into RAM memory each time the
PLC is powered up.
Possible Disadvantages
Information is not stored to your PROM device automatically. To store information, you must stop
the PLC, then use a programming device to tell the CPU to write the current PLC (RAM) memory
contents to the PROM device. This requirement may make battery-less operation undesirable for
many users. For example, in many applications, important data is gathered and stored in RAM
register memory, data such as the current level of material in a tank that is being filled, or a running
count of parts produced, etc. This constantly changing data is not being copied automatically to
the PROM device. It only exists in RAM memory. Therefore, if power is lost and there is no
RAM memory backup battery, this data will be lost.
However, one way to preserve data in a battery-less system is to send it over a network to a
computer that can store the data on its hard drive. Also, static data (data that doesn't change)
contained in RAM memory, such as mathematical constants or look-up table type information, can
be stored initially in PROM and automatically written back to RAM each time the PLC powers up.
Another consideration is that if you change your program (or configuration), someone will have to
remember to write the changed information to the PROM device. If that step is forgotten, the
change only exists in RAM memory, and in a battery-less system, it will be lost the next time
power is removed from the PLC.
6-6 Series 90-30 PLC Installation and Hardware Manual – August 2002 GFK-0356Q