Inter and intra priority resolution network for an asynchronous bus system

A data processing system having an asynchronously common system bus to which a plurality of units are coupled to different physical positions along said common system bus for asynchronously transferring requests between said units, each unit including a system bus priority network, said system further including a bus interface unit which couples to one end of said common system bus and to a plurality of local units connected in common to a high speed synchronous local bus, said bus interface unit including a system bus priority network which collectively with said system bus priority network of each of said plurality of units form a distributed bus priority network of said common system bus for granting access to said common system bus on a priority basis defined by each unit's position on said bus relative to said one end of said bus, said bus interface unit further including bus resolver apparatus comprising:

• bus priority resolution circuit means having a plurality of inputs coupled to receive bus request and timing signals from said common system bus defining when said common system bus can be accessed and a plurality of outputs, said resolution circuit means generating signals at said outputs in response to said bus request and timing signals indicating the result of resolving priorities by said distributed bus priority network during a bus resolution cycle of operation between any simultaneous requests made by another one of said plurality of units to use said common system bus;
• request input means having a plurality of request storage means corresponding in number to a number of different types of requests to be processed, said plurality of request storage means being connected to said outputs of said bus resolution circuit means, each request storage means being coupled to said local bus and each of said request storage means generating a signal during a predetermined interval defined by said signals at said outputs of said bus priority resolution circuit means indicating when one of said local units has a one of said number of different types of requests to be transferred to said common system bus;
• logic circuit means coupled to said plurality of request storage means and to said common system bus, said logic circuit means logically combining said signals from each of said plurality of request storage means so as to produce an output request signals on said common system bus for signaling a request to access said common storage bus for processing by said distributed bus priority network;
• and, local priority resolution circuit means for resolving internally the relative priorities of said local units requesting access to said common system bus, said local priority resolution circuit means being directly coupled to said logic circuit means for receiving said output request signal simultaneously applied to said local resolution circuit means and to said common system bus, said local priority resolution circuit means in response to said output request signal generating an output signal within a predetermined minimum amount of time established by said local priority resolution circuit means required for local unit priority resolution during said bus resolution cycle of operation for signalling to said local unit designated by said plurality of request storage means as being the highest priority local unit and having the highest priority of said plurality of units requesting access to said common system bus and that said access to said common system bus has been granted.

A method for processing requests received from a plurality of local units which connect in common to a high speed synchronous local bus, for transfer to a common system bus to which a plurality of units couple to different physical positions along said common system bus for asynchronously transferring requests between said plurality of units on a priority basis controlled by a plurality of like priority networks connected between said units and said common system bus which collectively form a distributed bus priority network, each unit being granted access to said common system bus by said priority network on the basis of said unit's physical position on said bus relative to one end of said bus, said method comprising the steps of:

• (a) positioning a bus interface unit between said one end of said common system bus and said synchronous local bus for receiving said requests from said local units to be transferred to said common system bus;
• (b) receiving bus request and timing signals from said common system bus defining when said common system bus can be accessed by a priority network included in said bus interface unit which is included as part of said distributed bus priority network for granting access on said priority basis to use said common system bus;
• (c) generating output signals in response to said bus request and timing signals by said priority network indicating the result of resolving priorities by said distributed bus priority network during a bus resolution cycle of operation between any simultaneous request made by another one of said plurality of units and said bus interface unit to use said common system bus;
• (d) generating a request signal within a predetermined interval defined by said output signals generated in step (c) in response to each different type of request received from each local unit indicating a desire to use said common system bus;
• (e) logically combining each of the locally generated request signals produced in step (d) during said predetermined interval to produce an output request signal;
• (f) applying said output request signal to said common system bus for resolution by said distributed bus priority network with any other requests applied to said common system bus by said plurality of units during said bus resolution cycle of operation;
• and,
• (g) simultaneously applying said output request signal in parallel with step (f) to a local priority network for resolution of relative priorities of different types of local requests within a minimum amount of time established by said local priority network so as to ensure that said local units are always granted access to said common system bus within said bus resolution cycle of operation.