Arbiter (electronics)

The arbiter or arbitration logic (from Latin arbiter, " judge" ) is a functional unit in the form of an electrical, digital circuit or a software routine that access conflicts or collisions triggers access or prioritized. This is necessary for example in the case of bus systems with multiple bus masters - ie units such as a DMA controller may access the data bus active - to decide which bus master gets access.

Generally known under the arbitration, the fairest possible allocation of resources to different users or devices. This process is also the so-called token passing method or in the FDDI technology used. The CSMA / CD method, however, is an example of a procedure which does not guarantee fair allocation of the resource.

In the simplest variant, there is an arbiter of a priority encoder which outputs the number of the input, it makes a request and has the highest priority. The priority encoder is then usually a demultiplexer connected downstream as a selector which controls the corresponding entity. In the VMEbus arbiter is housed in the first slot, which can also fail significantly larger depending on the make, for example with its own processor.

Assign a fixed priority addition to the simple method, several request lines ( Priorized Arbitration), there is still the possibility to rotate the request lines all be treated with the same priority (Single Level Arbitration) or even the priority allocation (Round Robin Arbitration). The latter allows to define a certain kind of fairness: the longer a unit waits, the greater the probability of assignment. The NuBus Macintosh Apple used, for example, this form of arbitration.