The present invention relates to control systems of the type having a plurality of remotely located process control units connected together through a communications link and, more particularly, to a control system in which one or more redundant control units each serve as a back-up for a plurality of the remotely located process control units.
Many system type industrial installations, for example, those related to industrial process-type manufacturing and electrical power generation, employ a large number of physically distributed controlled-devices and associated sensors for effecting coordinated operation of the overall system. In the past, coordinated control of the various devices has been achieved by manual operation and various types of semi-automatic and automatic control systems including electromagnetic relay systems, hardwired solid-state logic systems, and various types of computer control systems. The computer systems have included central systems in which the various sensors and controlled devices are connected to a central computer; distributed control systems in which a remotely located computer is connected to each of the controlled devices and to one another, and hybrid combinations of the central and distributed systems. The successful functioning of the control system is vital to any industrial process, and, accordingly, distributed systems have generally been preferred over central systems because the failure of one of the remotely located control computers generally does not cause a system wide failure as in the case of the failure of the central computer in the central system. In copending application Ser. No. 115,161, filed Jan. 14, 1980, invented by Michael E. Cope and assigned to the assignee of this application, there is disclosed a distributed control system. The preferred embodiment of the present invention is employed in a distributed control system as disclosed in this copending application.
In the control system disclosed in application Ser. No. 115,161, U.S. Pat. No. 4,304,001, a plurality of remote process control units R.sub.n (remotes) are connected to various controlled devices and sensors and communicating with one another through a communications link, which transmits data serially. Each remote is assigned a unique succession number or position in a predetermined succession order with each remote unit assuming supervisory communication control of the communications link on a revolving or master for the moment basis in accordance with the remote's relative position in the succession order. Information transfer including process data and command control information is accomplished between a source remote R.sub.s and a destination remote R.sub.d by successively transmitting information blocks over the communications link with the destination remote R.sub.d testing the validity of the blocks and, if valid, responding with an acknowledgement signal (ACK), and, if invalid, a non-acknowledgement signal (NAK) is sent by the destination remote R.sub.d. The source remote R.sub.s will retransmit the information blocks in response to a non-acknowledgement signal from a destination remote.
In accordance with the present invention, one or more of the remotes connected to the communications link is a redundant remote. Each redundant remote is designed to monitor a plurality of other remotes which are referred to as primary remotes and each of which may perform active control operations. The redundant remote detects whether or not any one of the primary remotes has failed and if it has failed, it then will take over operation of the inputs and outputs of that remote by sending instructions or commands over the communications link to the failed remote .