Omron CPM1 Programming Manual page 333

Section 7-15
Flags ER: The port specifier and control data are incompatible.
(For example: P=010 and C=000)
The address specified for D or D+1 exceeds the data area boundary.
There is an error in the operand settings.
Indirectly addressed DM word is non-existent. (Content of *DM word is
not BCD, or the DM area boundary has been exceeded.)
PRV(62) is executed in an interrupt subroutine while a pulse I/O or high-
speed counter instruction (INI(61), PRV(62), CTBL(63), SPED(64),
PULS(65), ACC(----), PWM(----), or SYNC(----)) is being executed in the
main program.