Method for and apparatus for operating a local communications module in

Abstract

A bus structure for interconnecting the modules of an industrial automation controller includes DATA lines and associated control lines that enable interface circuits on each module to send and receive frames of data. An arbitration circuit in each module employs arbitration lines in the bus structure to conduct a two phase competition to determine which module receives the next bus mastership. The arbitration lines include a set of wired-or weight code lines and a single arbitration strobe line. Modules desiring control of the bus structure assert a local weight code onto the weight code lines and determine which module is asserting the numerically greatest local weight code. Arbitration is conducted separately on the bus structure and a winning module is ready to transfer its data frame as soon as the previous winner completes its transfer and relinquishes control of the bus structure. The winning weight code identifies both the source address of the winning module and the priority of the data frame to be sent, and is latched by all modules on the backplane bus as an arbitration vector to provide advance identification of the next module to transmit and the priority of the message.