Browse Prior Art Database

Multiplexed Fast Access Token Contention System

IP.com Disclosure Number: IPCOM000035983D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 4 page(s) / 101K

Publishing Venue

IBM

Related People

Balliet, L: AUTHOR [+3]

Abstract

This article describes a single-wire token access bus system especially applicable where low data rates are required. The system reduces looped access delay without requiring additional wires.

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Multiplexed Fast Access Token Contention System

This article describes a single-wire token access bus system especially applicable where low data rates are required. The system reduces looped access delay without requiring additional wires.

Token-passing access control has the advantage of resolving data bus contention without collisions or buffer insertion delay and the associated disadvantages. A token conventionally consists of several

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bits of a byte which indicates a "busy" or "available" status. Traditionally used with loop (ring) topology, each unit on the loop delays its output by 1.5 or more bit times. When there are a large number of units and/or the data rate is low, loop delay becomes significant, thereby adversely affecting system response time. Such would be the case for loop-connected bus networks that interconnect electronic units on automotive vehicles where there can be a large number of units and where electromagnetic interference concern dictates low bit rates.

One of the methods designing a token-like system is to control access by a method used on a multi-wire parallel bus channel, whereby a separate wire is used for access control. The channel control unit initiates a bus "available" signal which is serially passed from one unit to the next. If no unit requires access, the signal ripples back to the channel control unit where it is reinitiated. If a unit requires access, it holds the "available" signal after receiving it, and accesses the shared data bus. Upon completion, the unit reissues the "available" signal and the procedure continues. A separate wire and additional connector pins and circuits are required to implement the design. This disclosure describes a design with similar high speed access performance but which transmits both control and data over a single cable (fiber-optic, twisted-pair, coax, etc.).

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A token-passing bus system is shown in Fig. 1. It interconnects numerous units via a single medium such as a twisted-wire pair or fiber-optic cable. All data and control are passed serially from unit to unit. Signals on the bus appear as Figs. 2 and 3. The two drawings are for cases where transmission delay around the loop is short and long, respectively, relative to the bit time. Each figure shows the input and output of a typical unit on the loop.

Signal levels on each cable are +V, V/2, and 0 where +V, V/2, and 0 correspond to signal level of voltage, current or light. Normal conditions are as follows: +V/2 Idle 1

+V Transition after Idle Token 3

0 After Idle Start...