National Instruments NI-488.2 - Manual (Page 113)

Appendix C Error Codes and Solutions
NI-488.2 User Manual C-12 ni.com
Solutions
Do not call ibonl with ibwait calls still pending in the driver.
Prevent other applications from calling ibonl by locking the interface
with iblck .
EPWR (28)
EPWR results when an interface loses power. This often results when the
system goes to and returns from a standby state.
Solutions
Take all handles offline and reinitialize the application.
Quit the application and restart.
Disable standby and hibernate modes on the PC.