A single resource in the form of a data bus (BS) is shared among a number of users in the form of Modules (M). Control of the bus is restricted to not more than one module at a time. Each module is assigned a different n-digit binary identity number, all the identity numbers containing the same number m of ones, and associated with BS there are n lines AB which perform the wired-OR function. When a request for the control of BS is made, the module or modules determines whether the resulting number carried by AB contains exactly m, or more than m, ones. If it contains more than m ones, indicating two or more requestors, arbitration between them takes place in a known manner to determine which of them shall control BS first. But if the number on AB contains exactly m ones there can be only one requestor and so arbitration is disposed-with.
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