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.