LAN INTERFACE WHICH PERMITS A HOST COMPUTER TO OBTAIN DATA WITHOUT INTERRUPTING A LADDER PROGRAM EXECUTING IN THE INTERFACE

A control system allows for transferring messages between a programmable logic controller and a host computer over a serial data communications network. The programmable logic controller executes a ladder program and has an integral network interface module for permitting direct communicative coupling to the communications network. Messages can be received and sent by the programmable logic controller without interruption of the execution of the ladder program. The programmable logic controller can receive unsolicited messages from the host computer.