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Delay Compensation for CMSA/CD Bit Arbitration Data Bus Access Control

IP.com Disclosure Number: IPCOM000035021D
Original Publication Date: 1989-May-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 4 page(s) / 88K

Publishing Venue

IBM

Related People

Balliet, L: AUTHOR [+3]

Abstract

This article describes a technique which extends the usefulness of carrier sense multiple access/carrier detect (CSMA/CD) bitwise arbitration to loop (ring) topology, higher data rates and greater separation distance for data buses that use CMSA/CD with bitwise arbitration. CSMA/CD with bitwise arbitration is used on automotive data bus networks to permit units to randomly access a common serial data bus and actively arbitrate for control. The technique results in efficient bus utilization but requires that units be physically very close, within a few meters, such as is the case on automotive vehicles. Each message is assigned a priority starting address. When a unit has a message to transmit, it tests to determine if the bus is active, and if not, initiates its transmission.

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Delay Compensation for CMSA/CD Bit Arbitration Data Bus Access Control

This article describes a technique which extends the usefulness of carrier sense multiple access/carrier detect (CSMA/CD) bitwise arbitration to loop (ring) topology, higher data rates and greater separation distance for data buses that use CMSA/CD with bitwise arbitration. CSMA/CD with bitwise arbitration is used on automotive data bus networks to permit units to randomly access a common serial data bus and actively arbitrate for control. The technique results in efficient bus utilization but requires that units be physically very close, within a few meters, such as is the case on automotive vehicles. Each message is assigned a priority starting address. When a unit has a message to transmit, it tests to determine if the bus is active, and if not, initiates its transmission. If simultaneously a second unit transmits the message with the highest priority, it overrides that of the lower priority. The lower priority unit detects the collision and ceases transmitting. Typically, one of the binary transmission levels (usually ground level) is used as the priority override level which is compatible with party line interconnections. At high data rates or when units are connected in series (a ring or loop topology), transmission delay becomes a significant factor.

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The technique disclosed herein uses two principles to achieve performance improvement. First, a change in state establishes priority rather than a logic level. To accomplish this, a code, such as non- return to zero inverted (NRZI), is used. The invention is not limited to the use of NRZI, but does require a state change to define bit content and priority. Fig. 1 illustrates party line and series (loop) connections. Delay is introduced by the distance between units and additionally for the loop case by series circuit delay. An idle state appears on the bus when the bus is not busy. When not in transmit mode, each unit receives active inputs. Messages not applicable are ignored. Loop units pass them through to their output. When ready to transmit, the unit looks at its receiver input and, if busy, the unit waits for an idle signal. If idle, the unit starts transmitting a start bit followed by priority address. Bitwise arbitration takes place if simultaneous t...