Browse Prior Art Database

High Speed Arbitration for Multi-Drop, DOT-OR Networks

IP.com Disclosure Number: IPCOM000112150D
Original Publication Date: 1994-Apr-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 4 page(s) / 108K

Publishing Venue

IBM

Related People

Arimilli, RK: AUTHOR [+2]

Abstract

The Micro Channel* Architecture contains a multi-drop, open collector (dot-OR), parallel prioritization arbitration mechanism. This allows the Micro Channel to provide a relatively simple and flexible arbitration mechanism for both the adapters and systems. For parallel prioritization networks, the maximum arbitration time is a function of the width of the arbitration (ARB) bus, and the maximum RC restore time for each arbitration bit. The general equation for the maximum arbitration time (Tarb) for any parallel priortization arbitration network is the following:

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High Speed Arbitration for Multi-Drop, DOT-OR Networks

      The Micro Channel* Architecture contains a multi-drop, open
collector (dot-OR), parallel prioritization arbitration mechanism.
This allows the Micro Channel to provide a relatively simple and
flexible arbitration mechanism for both the adapters and systems.
For parallel prioritization networks, the maximum arbitration time is
a function of the width of the arbitration (ARB) bus, and the maximum
RC restore time for each arbitration bit.  The general equation for
the maximum arbitration time (Tarb) for any parallel priortization
arbitration network is the following:

   Tarb = D + (W-2) x (Dp + Zp)  + S

   D  = max.  time to drive/release an ARB bit (due to GRANT).

   Dp = max.  time to drive/release an ARB bit (due to previous ARB
bit).

   W  = width of ARB bus.

   Zp = max.  RC restore time on each ARB bit for a FULLY loaded
system.

   S  = Skews and setup to the Central Arbitration Control Point
(CACP).

For systems with 8 Micro Channel slots the Tarb is as follows:

  Tarb = 50ns + (4-2) x (50ns + 75ns)  + 15ns

  Tarb = 315ns

   D  = 50ns (T45, T46 of MCA)

   Dp = 50ns (T45A, T47 of MCA)

   W  = 4 ARB bits

   Zp = 75ns (R=470 ohms, C=240pf)

   S  = 15ns

      For most Micro Channel systems, the Tarb is programmable in
increments of 100ns.  Thus, the Tarb is typically configured to be
400ns.  Note that the above general formula applies to any similar
mechanisms.  For example, if the Micro Channel had 6 ARB bits (W =
6), the Tarb would be 565ns.

      This paper describes a method by which the maximum arbitration
time can be significantly reduced by employing the following
techniques:

1.  Separate out all of the Micro Channel GRANT_s and ARBs.  (Do not
    have them "bussed" across all of the connectors).

    o   This minimizes the loading on the ARB lines from 240pf to
        25pf, thus reducing the RC restore time, Zp, from 75ns to
        17ns.

    o   This also allows a structure to selectively skew the ARBs and
        GRANT_s to each specific slot.

2.  During arbitration, use a fast parallel magnitude comparator to
    determine the winner.

    o   This eliminates the Dp term in the Tarb equation.

    o   This allows the (W-2) term in the Tarb equation to always be
        1.

3.  Compare the winning ARB with the ARB's from each slot to
    determine the winning slot, and pro...