Patent
1997-01-15
1999-06-22
Ray, Gopal C.
395728, 395308, 395856, G06F 1336, G06F 13368
Patent
active
059151013
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The method relates to the coupling of bus systems accompanied by arbitration.
Bus systems such as the "Small Computer System Interface" (SCSI) permit arbitration, that is to say a dynamic priority-controlled bus usage by each connected device. The length of this bus system is limited by the prescribed reaction times. Certainly bus couplers are possible which connect two short bus segments in a noise-proof fashion, for example via an optical connection; the propagation time of the signals limits these connections as well to a length of a few meters, however, although the optical connection would permit distances in the range of kilometers.
SUMMARY OF THE INVENTION
The object of the invention is therefore to provide a bus coupling which couples two bus segments transparently, that is to say by faithfully following the prescribed protocols, and permits reliable arbitration even in the case of long propagation times on the connection.
The invention is based on the observation that arbitrating bus systems permit an interruption of the arbitration sequence. The two coupling units coupling two bus segments transfer the authority to arbitrate between one another when required. If arbitration is attempted on a bus segment whose coupling unit has the authority to arbitrate, the arbitration is carried out normally. If the coupling unit does not have the authority, it requests it from the other coupling unit and simultaneously interrupts the arbitration. The device which has attempted to access the bus repeats this attempt after a short time. If the remote bus segment was inactive, the remote coupling unit immediately relinquishes the authority to arbitrate; the latter is then available upon the next attempt of the device attempting access. If it was active, the segment here is also active via the bus coupling; and the device repeats the attempt to access after the bus has become inactive (free).
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the present invention which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several Figures of which like reference numerals identify like elements, and in which:
FIG. 1 shows the arrangement of two bus segments with associated couplers and three devices,
FIG. 2 shows a diagrammatic circuit for a coupler, and
FIG. 3 shows the sequence in the arbitration according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The structure of a bus coupling is depicted in FIG. 1. Two coupling units KEA and KEB are interconnected via a connection LNK whose design is explained further below. SEGA and SEGB are two bus segments which are connected by the coupling units KEA and KEB. Two devices DEV1 and DEV2 are connected on the segment SEGA; a device DEV3 is connected on the segment SEGB. Further devices are connected in a similar way. Of the bus segments, only the data lines D0 . . . D7 of importance below and the control lines SEL, BSY and I/O are represented. Further control lines are present but not of importance for the invention and are therefore not represented for reasons of clarity.
A connection of two devices on the same segment, for example from DEV1 to DEV2 is performed without the couplers KEA and KEB being required to transmit the user data. A connection between the segments, for example from DEV1 to DEV3, requires the couplers for transmitting the user data. However, the status is transmitted even in the first case; for the sake of simplicity, the transmission of the user data is conventional and without disadvantages. The "Small Computer System Interconnect", SCSI, is used for the following description as the bus system; it is described, for example, in the ANSI Standard X3.1 31-1986 and is taken as known below.
The SCSI bus can operate up to eight different devices, to which one of the eight data lines D0 . . . D7
REFERENCES:
patent: 4845722 (1989-07-01), Kent et al.
patent: 5237695 (1993-08-01), Skokan et al.
patent: 5528765 (1996-06-01), Milligan
patent: 5561669 (1996-10-01), Lenney et al.
patent: 5566345 (1996-10-01), Ostrowski
patent: 5577207 (1996-11-01), Pauget et al.
Kleineberg Michael
Zacharias Ralph
Ray Gopal C.
Siemens Nixdorf Informationsyssteme Aktiengesellschaft
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