1. An industrial control system for managing a plurality of input/output devices comprising a primary data processor having a memory and operable to receive signals from said input/output devices, said primary data processor including a central processing unit operable to carry out an applications program and apply signals to said input/output devices in accordance with determinations made in said applications program to control said input/output devices, said primary processor maintaining in said memory of said primary processor a record of status data including a status data component representing a plurality of parameters of the current operation of said applications program, said central processing unit in carrying out said applications program operating on and in response to said status data in said current record maintained in the memory of said primary processor, a back-up data processor having a memory and having an active mode of operation and a back-up mode of operation, said back-up processor being operable in said active mode of operation to receive signals from said input/output devices, carry out said applications program and apply signals to said input/output devices in accordance with the determinations made in the applications program carried out by said back-up processor to control said input/output devices, said back-up processor maintaining a record of status data in the memory of said back-up processor corresponding to the record of status data maintained in the memory of said primary processor, said back-up processor, when carrying out said applications program, operating on and in response to the status data in the record thereof maintained in the memory of said back-up processor, said primary processor and said back-up processor operating in said back-up mode comprising means to periodically transfer a copy of the status data in the memory of said primary processor to a buffer memory section of the memory of said back-up processor, said means to transfer status data from the memory of said primary processor to a buffer memory section of the memory of said back-up processor including a dual ported memory connected between said primary processor and said back-up processor, said primary processor operating to periodically transfer a copy of the status data from the record of status data maintained by said primary processor in the memory of said primary processor to said dual ported memory, said back-up processor, when operating in said back-up mode, operating to periodically transfer a copy of the status data in said dual ported memory to the buffer memory section of the memory of said back-up processor, said central processing unit generating for each copy of status data transferred to said back-up processor information from which the validity of the status data can be determined, said information being included with the corresponding copy of status data transmitted to said back-up processor, said back-up processor including means operable when said back-up processor is in said back-up mode to determine from the entire content of said status data component and the corresponding information in each periodic copy of status data received in said buffer memory section whether said status data component is valid and, if the copy of said status data component is determined to be valid, to update the record of status data maintained in the memory of said back-up processor in accordance with the copy of the status data in said buffer memory section, means to monitor the condition of said primary processor independently of said status data and to generate a fail over signal when said primary processor fails, and means to switch said back-up processor from said back-up mode of operation to said active mode of operation in response to said fail over signal.