The MCB is the aggregate of the many SIBs connected to the (possibly several) serial ports of the host computer. If there are multiple serial ports used, they are organized as one logical bus, with the software sorting out which port handles each SIB. The specifications for the MCB can be found in NRAO specification numbers A55001N001 [1] and A55001N002-A [2].
In most cases the MCB is implemented via a 2 pair RS-485 link, configured as a multidrop master-slave network. The host computer is always the master, and the SIBs are always slaves. Only one master is allowed on an MCB. The serial link runs at 57.6K baud. In several cases for the GBT, the MCB is run over a fiber optic link to extend the distance, supress RFI, and provide lightning protection.