This circuit provides for the connection of simplex I/O terminals to duplex processor copies. Each processor copy's configuration provides for a terminal control circuit. These copies of the terminal control circuit are cross connected in order to provide access from each processor to each I/O terminal. Normal operation consists of a particular I/O terminal being operated by one processor copy. If the processor copy corresponding to a particular terminal is faulty or removed from service, that terminal is then automatically cross-connected to the active processor copy and receives output from that processor copy. If one processor copy is out of service, both I/O terminals receive output from the active processor copy. If the I/O terminal which is normally connected to the active processor copy becomes out of service and the other processor copy is out of service, the I/O terminal which is normally connected to the other processor copy will be reconfigured to be connected to the active processor. The active processor will then accept input from the I/O terminal of the other copy as well as transmit output to that I/O terminal.

In a method of controlling a multicomputer system which includes a plurality of computers connected to a common transfer bus, each of the plurality of computers decides whether or not the adjacent computer is abnormal, and the computer which has decided that the adjacent computer is abnormal cuts off this adjacent computer from the transfer bus.

All but one of the microcomputers in a multi-computer system are equipped with a rapid-access read-write memory and a fixed content nonvolatile read-only memory. Each of these read-only memories, however, stores a multiplicity of varieties of one or more sets of data, so that, in the case of a computer system for a motor vehicle engine, the same system can be used in any of a large number of vehicle models. One of the microcomputers of the system has a programmable memory which is programmed at the time of installation to designate the portions of the fixed memories that are to be utilized. When the system is turned on, the programmed memory designation number is stored in the read-write memory of all of the computers of the system, with the result that the designated portion of each fixed read-only memory is always referred to in any computer of the system. It is not important that, when the system is turned off, the memory content of the read-write memories is lost.

An optical system including a first microcomputer having a first communication terminal and a second communication terminal, a second microcomputer having a third communication terminal which is connected to the first communication terminal and sends and receives data, and a non-volatile memory having a fourth communication terminal which is connected to the second communication terminal and able to receive data, the non-volatile memory storing contents even after power to the non-volatile memory is turned OFF. The first microcomputer reads first data which is stored in a first region of the non-volatile memory, from the non-volatile memory via the second communication terminal, transfers the first data from the first communication terminal to the second microcomputer immediately after power is supplied, and uses second data, which is stored in a second region of the non-volatile memory and which has been transferred via the second communication terminal, to perform processing.

In a distributed processing system having a plurality of interconnected subsystems of equal level, each subsystem operates to diagnose faults in other subsystems and protects its own subsystem based on the diagnosis of the faults of the other subsystems. The subsystems may be network control processors connected to common signal transmission lines, each of which carries major and minor loop check messages used to detect the faults in the other network control processors and constitutes a bypass route to protect its own unit when the fault occurs. If a transient fault occurs in the systems, an indication of the degree of the transient fault is stored and a possibility that the transient fault will change to a permanent fault is determined based on a time variation of the degree of the fault. If it is determined that the transient fault will change to a permanent fault, it is indicated to a man-machine system. In this manner, a loop transmission system which can be readily prediction-diagnosed for a fault is provided. In a second embodiment, means for calculating a feedback rate of a signal sent out to the transmission line and means for calculating the degree of fault on the transmission line based on the feedback rate calculated by the calculation means are provided.