Ram Memory Battery Backup Connection Path; Super Capacitor Memory Backup - GE 90-30 PLC Series Installation And Hardware Manual

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RAM Memory Battery Backup Connection Path

CMOS RAM and DRAM memory is a volatile type of memory, which means that it can lose its
contents (ladder program, configuration, etc.) if power is removed. To retain RAM memory
contents under no-power conditions, a long-life lithium battery is provided. This battery is
normally mounted in the rack's Power Supply module. To avoid accidental disconnection of the
memory backup battery, it is beneficial to know the connection path between the battery and the
memory circuits:
For embedded CPUs: The battery connection path to RAM memory is through the Power
Supply's baseplate connector and across the backplane board to the RAM circuits.
For modular CPUs: The battery connection path to RAM memory is through the Power Supply's
baseplate connector, across the backplane board, and through the CPUs baseplate connector to the
RAM circuits inside the CPU module.
Obviously, removing the Power Supply module from the PLC breaks the connection between the
backup battery and the RAM memory circuits for both embedded and modular CPUs. Also, in a
modular CPU system, removing the CPU module would disconnect the backup battery from the
memory circuits. In addition, to avoid the possible problems associated with losing the contents
of RAM memory, we recommend that you maintain an up-to-date backup copy of your program
folder. Instructions for creating program folder backups can be found in the Logicmaster 90,
Series 90-30 Programming Software User's Manual, GFK-0466, and the online help and user's
guides for the Windows-based programming software products.

Super Capacitor Memory Backup

Besides the backup battery, the RAM memory circuits in both Embedded and Modular CPUs are
further protected by a "super capacitor," which can store enough charge to maintain memory for a
short time if the battery is disconnected. The amount of protection time provided by the super
capacitor depends on the following:
6-8 Series 90-30 PLC Installation and Hardware Manual – August 2002
The PLC power supply supplies 5 VDC to the memory circuits, including the super capacitor.
Therefore, when PLC power is turned off, the super capacitor has an initial 5 VDC charge. If
the battery is also disconnected shortly after PLC power is turned off, the super capacitor will
begin discharging from the 5 VDC level until its charge reaches 2 VDC, at which time
memory contents will be lost. When used this way, the super capacitor can maintain memory
contents for a minimum of 1 hour.
The memory backup battery supplies 3 VDC to the memory circuits, including the super
capacitor. Therefore, if PLC power has been turned off for an hour or more and only the
battery is powering the memory circuits, the super capacitor has a 3 VDC charge. Then, if the
battery is disconnected, the super capacitor will begin discharging from the 3VDC level until
its charge reaches 2 VDC, at which time memory contents will be lost. When used this way,
the super capacitor can maintain memory contents for a minimum of 20 minutes.
GFK-0356Q