Fault-tolerant computer system with three symmetric computers

Many computer systems have turned increasingly to control systems, requiring more sophisticated machinery over an ever-widening range. The reliability of the system should be carefully considered in all its aspects. The concept of a control segment is introduced against local malfunctions or failures in order to save the system from going down. This paper also describes the fault tolerant control computer system with dual operation aimed at getting the fail-safe feature of the control output and complex operation features for maintenance and improvement of the system. Three "off-the-shelf" computers are connected symmetrically by three special hardwares and configuration programs not only in hardware, but also software configuration. Degradation techniques in software and hardware are introduced to prevent the system from stoppage. The actual running record shows 99.99 percent operating availability for three years in the case of the command and control system for a railway which is described here.