Mitsubishi Electronics 700 Series Manual De Usuario

Special relays and special registers are internal relays and data registers, respectively,
whose applications are predetermined by the built-in PLC functions.
They have the following main applications.
(1) Sequence operation check

The special relays and special registers can be used to:
 (a)Check the operating status (RUN/STOP)
 (b)Detect a fault by the self-diagnostic function
 (c)Detect an operation error
 (d)Check the scan time

(2) Timing contact

There are special relays that can be used in a sequence program and differ in
operating status.
 (a)Normally ON/OFF flag
 (b)RUN flag (OFF for 1 scan)
 (c)Initial processing flag (ON for 1 scan)

For the special relays and special registers usable with the built-in PLC function, 
refer to page 10.