Real Time Clock (RTC)

Overview

The M251 Logic Controller includes an RTC to provide system date and time information, and to support related functions requiring a real-time clock. To continue keeping time when power is off, a non-rechargeable battery is required (see reference below). A battery LED on the front panel of the controller indicates if the battery is depleted or absent.

This table shows how RTC drift is managed:

RTC Characteristics	Description
RTC drift	Less than 60 seconds per month without any user calibration at 25 °C (77 °F)

Battery

The controller has one battery.

In the event of a power interruption, the backup battery maintains the RTC for the controller.

This table shows the characteristics of the battery:

Characteristics	Description
Use	In the event of a transient power outage, the battery powers the RTC.
Backup life	At least 2 years at 25 °C maximum (77 °F). At higher temperatures, the time is reduced.
Battery monitoring	Yes
Replaceable	Yes
Controller battery type	Lithium carbon monofluoride, type Panasonic BR2032

Installing and Replacing the Battery

While lithium batteries are preferred due to their slow discharge and long life, they can present hazards to personnel, equipment and the environment and must be handled properly.

DANGER
	EXPLOSION, FIRE, OR CHEMICAL BURNS Replace with identical battery type. Follow all the instructions of the battery manufacturer. Remove all replaceable batteries before discarding unit. Recycle or properly dispose of used batteries. Protect battery from any potential short-circuit. Do not recharge, disassemble, heat above 100 °C (212 °F), or incinerate. Use your hands or insulated tools to remove or replace the battery. Maintain proper polarity when inserting and connecting a new battery. Failure to follow these instructions will result in death or serious injury.

DANGER

EXPLOSION, FIRE, OR CHEMICAL BURNS

Replace with identical battery type.
Follow all the instructions of the battery manufacturer.
Remove all replaceable batteries before discarding unit.
Recycle or properly dispose of used batteries.
Protect battery from any potential short-circuit.
Do not recharge, disassemble, heat above 100 °C (212 °F), or incinerate.
Use your hands or insulated tools to remove or replace the battery.
Maintain proper polarity when inserting and connecting a new battery.

Failure to follow these instructions will result in death or serious injury.

To install or replace the battery, follow these steps:

Step	Action
1	Remove power from your controller.
2	Use an insulated screw-driver to pull out the battery holder.
3	Side out the battery holder of the controller.
4	Remove the battery from the battery holder.
5	Insert the new battery into the battery holder in accordance with the polarity markings on the battery.
6	Slide in the battery holder of the controller and verify that the latch clicks into place.
7	Power up your M251 Logic Controller.
8	Set the internal clock. For further details on the internal clock, refer to M251 Logic Controller Programming Guide.

NOTE: Replacement of the battery in the controllers other than with the type specified in this documentation may present a risk of fire or explosion.

WARNING
	IMPROPER BATTERY CAN PROVOKE FIRE OR EXPLOSION Replace battery only with identical type: Panasonic Type BR2032. Failure to follow these instructions can result in death, serious injury, or equipment damage.