Importance

Loading Importance Indicators...

Abstract

A method is provided for communicating among a plurality of programmable logic controllers (PLC's) coupled together on a common bus. A master PLC and a plurality of slave PLC's are coupled to the bus. An attach signal is transmitted by the master PLC to address and select the particular slave PLC to which the master PLC desires to transmit a message. The master PLC then transmits a mailbox signal including data and message length information regarding a next message if not all data from the master PLC and intended for the slave PLC could fit in the mailbox message. Additional next messages, each including data and next message length information, are sent by the master PLC to the selected slave PLC as necessary until all of the data intended for the selected slave PLC is transmitted by the master PLC. This communication method permits the data in the mailbox message and the data in any next messages to be transferred directly from the bus to a memory within the slave PLC via a direct memory access (DMA) controller in the slave PLC without the intervention of a CPU in the slave PLC. Since non-selected slave PLC's receive next message length information, such non-selected slave PLC's are substantially free to perform other activities for those periods of time they know will be consumed by next messages.

Loading the Abstract Image...

First Claim