Siemens SIMATIC S7-400H System Manual page 439

Availability
Availability is the probability that a system is operable at a given point of time. This can be
enhanced by means of redundancy, for example by using redundant I/O modules or multiple
encoders at the same sampling point. Redundant components are arranged such that
system operability is not affected by the failure of a single component. Here, again, an
important element of availability is a detailed diagnostics display.
The availability of a system is expressed as a percentage. It is defined by the mean time
between failure (MTBF) and the mean time to repair MTTR (MDT). The availability of a two-
channel (1-out-of-2) fault-tolerant system can be calculated using the following formula:
Figure A-4
S7-400H
System Manual, 03/2012, A5E00267695-11
Availability
Characteristic values of redundant automation systems
A.1 Basic concepts
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