Omron SYSMAC C Series User Manual page 108

Temporary memory relays
The CPU has eight temporary memory relay points (TRO to 7).
Special auxiliary relays
The CPU has 16 special auxiliary relays, some of which operate or
release according to internal conditions controlled by the hardware
irrespective of the statuses of the I/O devices. Each of these special
auxiliary relays functions as follows:
Relay 1808
This relay operates when a battery failure occurs. An alarm signal
indicating a battery failure can be output to an external device by
programming a circuit incorporating the contact of this relay.
Relay 1809
Normally OFF
Relay 1810
Normally OFF
Relay 1811
Normally OFF
Relay 1812
Normally OFF
Relay 1813
Normally ON.
Relay 1814
Normally OFF
Relay 1815
This relay is turned ON for one scan time at the start of program
execution.
A scan timeis the time required for the PC to execute the user program
once starting from address 0000 to the program's end instruction.
Start of program execution
One scan time
Relay 1900
This relay is used to generate a 0.1s clock. When used in conjunction with
a counter, it functions as a timer that can retain its present value during
power failure.