In the datasheet some controllers have a software real time clock, others list a real time clock
Infinet and BACnet MS/TP field controllers
Clarification on the operation of the 2 types of controller clock
The current range of controllers have 2 different clock types, all Ethernet level controllers and the larger i2 and b3 controllers have a Real Time Clock (RTC) chip fitted with quartz crystal control for very accurate time keeping, also the controllers onboard battery will permanently keep the clock running.
Some of the lower cost controllers do not have a RTC chip fitted but the controllers software operating system will update the clock every second. If the controller is not running (powered off) the clock will not update, however when a controller is connected back onto the network (Infinet or MS/TP) it will be synchronized back to the bCX or CX level controller time.
So if the controller is to be ever used in a stand alone mode, it is recommended to use one with a Real Time Clock fitted.
For comparison to the clocks used on the Infinity range of controllers see Lessons Learned Article #7911