Programmable controller with CPU and communication units and method of controlling same

摘要

A programmable controller has two communication units connected to a same network and a CPU unit. These two communication units are both identified by same unit-identifying data such as node numbers and unit numbers and function such that one of them operates in an active mode and the other operates in a standby mode. Each of these communication units, when in the active mode, operates to carry out data exchanges with the CPU unit and other nodes connected to the network and, when in standby mode, is prevented from carrying out any data exchange with the other nodes connected to the network. When the communication unit in the active mode fails and goes down, the one in the standby mode is switched to the active mode and carries out the data change. This mode-switching operation is carried out in response to a command from the CPU unit although it may be so arranged that the communication units communicate with each other and switch their modes by informing the occurrence of a failure to the other unit.