6 I/O Redundancy
6.1 Dual Redundancy
A basic dual redundant control has a sensor connected to an I/O module with one I/O pack that communicates data to two
IONET switches (Refer to the following figure). Each IONET switch transmits the data to both controllers, which are online
and running the same application software.
This configuration has the advantage of network and controller redundancy but no redundancy for the sensor or I/O module.
Another option is adding a second sensor and a second I/O module. This increases the fault tolerance and the ability to
perform online repair at least to the module and possibly to the sensor assuming that the sensor can be replaced while the
process is running. Note that online repair is at the I/O pack level. Therefore, replacement of I/O has minimal impact on
monitoring and control of the overall control system. An extension of this is to add a third sensor whose value can be voted in
the application software. This is an example of triple redundant sensors and I/O modules with dual networks and dual
An example of this configuration is in a heat recovery steam generator (HRSG) control that has minimal critical I/O except
for the drum level. Triple redundant sensors can be added just for the drum level control to optimize system availability for
the least cost.
Mark VIe Controls System Redundancy Options