VME provides the flexibility of being a multimaster bus. A VME system may have one, two, or up to 21 masters in a full 21 slot backplane. Each master has the ability to request the bus and carry out a data transfer activity. Modules that do not have master capability are called slaves. Each module that plugs into the backplane can be designed to be a master, a slave, or both.
Masters have the ability to request the bus and put addresses on the bus.
Slaves do not. Because the VMEbus is a shared resource however a mechanism must exist to allow boards to arbitrate for and be granted the bus. This mechanism is called arbitration and is carried out by the VMEbus system controller. The system controller always resides in slot one and provides functionality independent of the module which is plugged into slot one. That is, a master or a slave may reside in slot one along with the system controller functions.