Hi,
On a GPIB bus, you must have only one controller in charge (CIC). And the ibdev(...) function can only be used with a CIC.
This error could be caused by the fact that you use the ibdev(...) function with a slave device, or by the fact that you have more than one CIC on the GPIB bus.
Please check this two points. It shoudl solve your problem.
Regards,