Bus-control mechanism

• US 4,661,905 A
• Filed: 09/22/1983
• Issued: 04/28/1987
• Est. Priority Date: 09/22/1983
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: Digital Equipment Corporation (HP Inc.)
• Inventors: Schumann, Reinhard, Jenkins, Steven R., Bomba, Frank C., Binder, Paul
• Primary Examiner(s): WILLIAMS JR, ARCHIE E
• Application Number:
  US06/534,781
• Time in Patent Office: 1,314 Days
• Field of Search: 364/200 MS File, 364/900 MS File
• US Class Current: 710/113
• CPC Class Codes: G06F 13/374 using a self-select method with individual priority code comparator

Abstract

Control of a communications path interconnecting separate devices in a digital computer system is provided by only two signals generated and received locally within each device on the path and asserted on a systems-wide basis on two separate lines of the path. The generation and utilization of the control signals is independent of the physical location of the devices on the path, and position-dependent delays in transmission of the signals from one device to another are avoided.

Claims

1. For use in a data processing system that includes a plurality of bus devices and a common communications path for conducting signals asserted thereon, the path including the NO ARB line, a BUSY line, a plurality of command lines, at least one timing line for carrying timing signals defining timing cycles, and a plurality of arbitration lines, each of which is associated with a different one of the bus devices, a subject bus device, adapted for connection to the communications path to operate in cycles in accordance with the timing signals on the timing line and operable to assume a control-requiring state, in which it requires control of the communications path, the subject bus device comprising:

A. monitoring means for, when the subject bus device is in its control-requiring state, monitoring the NO ARB line to determine whether a signal has been asserted on the NO ARB line;

B. arbitration means, responsive to the determination of the monitoring means, for refraining from placing signals on the arbitration lines when a signal has been asserted on the NO ARB during the previous cycle and, when the subject bus device is in its control-requiring state, arbitrating for control of the communications path during a given cycle whenever the NO ARB line was unasserted during the previous cycle, regardless of the signals on any of the other lines of the communications path, by (a) asserting a signal on an arbitration line associated with the subject bus device, (b) monitoring the other arbitration lines to determine whether a signal has been asserted on any arbitration line associated with a bus device whose priority, in accordance with a predetermined priority protocol, is higher than the priority of the subject bus device, and (c) if the arbitration means determines that the subject bus device has the highest priority, causing the subject bus device to assume a pending-master state, in which it is the next device to take control of the communications path;

C. pendency means for, when the subject bus device is in its pending-master state, asserting the NO ARB line and monitoring the BUSY line to determine whether the BUSY line is unasserted; and

D. current-master means, responsive to the pendency means, for taking control of the communications path during a given cycle whenever the pendency means determines that the BUSY line was unasserted during the previous cycle, regardless of the signals on the other lines of the communications path, by releasing assertion of the NO ARB line and placing on the command lines command signals designating a transaction to be performed during a plurality of successive cycles.

2. A method for according to a subject bus device control of a common communications path in a data processing system that includes a plurality of bus devices, the common communcations path including a NO ARB line, a BUSY line, a plurality of command lines, at least one timing line for carrying timing signals defining timing cycles, and a plurality of arbitration lines, each of which is associated with a different bus device, the subject bus device being connected to the communications path and operable to assume a control-requiring state, in which it requires control of the communications path, and a pending-master state, in which it is the next device to take control of the communications path, the method comprising the steps of:

A. monitoring the NO ARB line to determine whether a signal has been asserted on it;

B. regardless of the signals on any of the other lines of the communications path, arbitrating for control of the communications path during a given cycle, whenever the NO ARB line was unasserted during the previous cycle, by (a) asserting the NO ARB line and an arbitration line associated with the subject bus device, (b) monitoring the other arbitration lines to determine whether a signal has been asserted on any arbitration line associated with a bus device whose priority, in accordance with a predetermined priority protocol, is higher than the priority of the subject bus device, and (c) causing the subject bus device to assume a pending-master state if it has the highest priority;

C. asserting the NO ARB line when the subject bus device is in the pending-master state;

D. when the subject bus device is in the pending-master state, monitoring the BUSY line to determine whether the BUSY line is unasserted; and

E. regardless of the signals on the other lines of the communications path, causing the subject bus device to take control of the communications path during a given cycle, whenever the subject device was in its pending-master state and the BUSY line was unasserted during the previous cycle, by releasing assertion of the NO ARB line and placing on the command lines command signals designating a transaction to be performed during a plurality of successive cycles.