Abstract: A communication system between a programmable logic controller and a network of a plurality of remote input/output repeater modules uses a synchronous serial communications protocol. The programmable logic controller contains a transmitter module for generating a data packet containing the state of an output device connected to a remote input/output repeater module coupled to the network. A receiver module receives and decodes a returned modified data packet that indicates the state of input devices connected to the same or other remote input/output repeater modules also coupled to the network. The remote input/output repeater modules receive and decode the data packet to determine the state of the output devices that may be connected to it and modifies the data packet to indicate the state of its input device if there is one present. The modified data packet is sent to the next in-line remote input/output repeater module coupled to the network.