Notes:
The PLC's program is stored in the EEPROM area and does not require MX-RTC to maintain the program.
if your purpose is only to periodically store some data in non-volatile memory area, there are 1700 words of data EEPROM available for your program use and MX-RTC is not always necessary.
MX-RTC ($39)
The built-in real-time clock (RTC) on every standard M-series PLC only works when power is supplied to the PLC and is reset when power is turned OFF. For applications that require the RTC to continue running after power off you can purchase a MX-RTC module and fit it in between the 28 pin socket and the RAM chip. The PLC senses the presence of the MX-RTC and will use it automatically.
If you turn on DIP switch #1 with the presence of MX-RTC, then all the Data Memory and I/O status will be preserved when the power to the PLC is turned off. Hence, you have the option of preserving internal data and I/O status of the PLC when the power failure occurs and resuming operation once the power returns.
