ibcac() causes the board specified by the board descriptor ud to become active controller by asserting the ATN line. The board must be controller-in-change in order to assert ATN. If synchronous is nonzero, then the board will wait for a data byte on the bus to complete its transfer before asserting ATN. If the synchronous attempt times out, or synchronous is zero, then ATN will be asserted immediately.
It is generally not necessary to call ibcac(). It is provided for advanced users who want direct, low-level access to the GPIB bus.
The value of ibsta is returned.