Mitsubishi Electric MELSEC iQ-F FX5 Programming Manual page 638

Operation error
Error code Description
(SD0/SD8067)
1811H The number of times the IST instruction is simultaneously driven exceeds 1.
2820H The device numbers specified in (d1) and (d2) show the following relationship:
(d1)  (d2)
Eight points are not secured from the device specified in (s).
An unavailable device is set in (s).
An unavailable device is set in (d1).
An unavailable device is set in (d2).
8 APPLICATION INSTRUCTION
636
8.19 Initial State