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.