A machine which is Dual Modular Redundant has duplicated elements which work in parallel to provide one form of redundancy. A typical example is a complex computer system which has duplicated nodes, so that should one node fail, another is ready to carry on its work. For instance: the Submarine Command System SMCS used on submarines of the Royal Navy employs duplicated central computing nodes, interconnected by a duplicated LAN.
A lockstep fault-tolerant machine uses replicated elements operating in parallel. At any time, all the replications of each element should be in the same state. The same inputs are provided to each replication, and the same outputs are expected. The outputs of the replications are compared using a voting circuit. A machine with two replications of each element is termed dual modular redundant (DMR). The voting circuit can then only detect a mismatch and recovery relies on other methods. Examples include 1ESS switch. A machine with three replications of each element is termed triple modular redundant (TMR). The voting circuit can determine which replication is in error when a two-to-one vote is observed. In this case, the voting circuit can output the correct result, and discard the erroneous version. After this, the internal state of the erroneous replication is assumed to be different from that of the other two, and the voting circuit can switch to a DMR mode. This model can be applied to any larger number of replications.