In some application of high requirement of system reliability, the hot backup system (redundancy) shall be considered in design of DCS; it means that some key modules in the system or network in the design have one or more backups. When part of the work is has problems now, the system can automatically switch to the backup by the special software or hardware, thus ensuring the uninterrupted working of the system. Usually the redundant modes designed include: CPU redundancy, network redundancy and power supply redundancy. In extreme cases, some systems will consider redundancy in the whole system, which includes the I/O redundancy.