Siemens SIMATIC S7-400H Manual page 181

Availability
Availability is the probability of a system being capable of operation at a specified
point of time. It can be enhanced by means of redundancy – for example, by using
redundant I/O modules or by using multiple sensors at one 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 diagnostic display.
The availability of a system is expressed as a percentage. It is determined by the
mean time between failure (MTBF) and the mean repair time (MTTR). The
availability of a two-channel (1-out-of-2) fault-tolerant system can be calculated
from the following formula:
MTBF
V
MTBF
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
Characteristic Values of Redundant Programmable Logic Controllers
1oo2
100%
MDT
1oo2
A-3