Bus system wherein a bus-using request signal is issued in advance of a determination that a bus is to be used and is thereafter cancelled if the bus is not used

An apparatus for controlling a bus-using right in a bus system wherein a first master unit, a second master unit, and at least one slave unit are connected to a common bus, said first master unit uses the common bus, and at least said second master unit accesses one slave unit of said at least one slave unit via the common bus to transfer data between said second master unit and said one slave unit at predetermined time intervals (T1) determined by said second master unit, the apparatus comprising:

• first timer means responsive to said second master unit for determining when a predetermined time T2 (T2<T1) has elapsed after said second master unit started one data transfer;
• means responsive to said first timer means for issuing a bus-using request when the time T2 has elapsed after said second master unit started the one data transfer in order to acquire a bus-using right for the common bus for said second master unit in advance of said second master unit starting another data transfer;
• means responsive to the bus-using request for granting the bus-using right for the common bus for said second master unit in advance of said second master unit starting the another data transfer, thereby allowing said second master unit to start the another data transfer immediately when the time T1 has passed after said second master unit started the one data transfer;
• second timer means responsive to the bus-using request for determining when a predetermined time T3 (T2+T3>T1) has elapsed after the bus-using request was issued;
• and
• means responsive to said second timer means and said second master unit for discarding the bus-using right granted in advance when said second master unit has not started the another data transfer when the time T3 has elapsed after the bus-using request was issued.

A bus system with a plurality of units connected to a common bus, wherein at least one unit of the units uses the common bus at predetermined timings, the bus system comprising:

• a bus arbiter coupled to each of said units for carrying out arbitration of a bus-using right of the common bus;
• first timer means for measuring a first time period indicative of a timing at which said one unit will next begin to use the common bus and for producing a first output signal when the first time period has been measured;
• second timer means for measuring a second time period during which said one unit does not use the common bus and for producing a second output signal when the second time period has been measured;
• and
• request control means connected to said one unit, said bus arbiter, said first timer means, and said second timer means for driving said first timer means to start measuring the first time period when said one unit begins to use the common bus, for generating a bus-using request to said bus arbiter in response to the first output signal from said first timer means, for driving said second timer means to start measuring the second time period when the bus-using request is issued, for resetting the second timer means if said one unit begins to use the common bus before the second timer produces the second output signal, and for cancelling the bus-using request if said second timer means produces the second output signal before said one unit begins to use the common bus.