CS1D adds a number of dual-redundancy options to the well-proven CS1 architecture to ensure around-the-clock operation without minimal downtime. Duplex CPUs, with or without loop control functions, are continuously cross-checked for errors, without the need for special programs to be written by the user. An even simpler way to increase system availability is to use dual power supply units. Hot swapping of CPUs, power supplies and I/O units allow system maintenance with little or no process interruption.