1. In a programmable controller which has a processor module for executing a control program, an input/output module to exchange control signals with devices on a machine being controlled and a set of backplane busses which carry signals among the modules in the programmable controller; a communication module comprising: a controller interface circuit for connecting to the set of backplane busses in order to exchange signals with another module; a network interface circuit which sends and receives messages via a serial communication link connected to the network interface circuit; a memory that stores a designation of a starting time, a designation of a length of a message repeat interval, and message information; a group of data, address and control busses internal to the communication module for interconnecting said controller interface circuit, said memory, and said network interface circuit; timing means, connected to said memory, for determining when the starting time occurs and thereafter determining each successive occurrence of expiration of the message repeat interval; and a control mechanism that responds to said timing means by causing said network interface circuit to transmit a timed periodic communication message containing the message information over the serial communication link when the timing means determines either that the starting time has occurred or that said expiration of the message repeat interval has occurred.