According to the IEC 61158 specifications, redundancy is possible in H1 bus segments. However, manufacturers have chosen not to implement this capability, on the grounds of cost, bearing in mind that the number of devices on a single segment may be fairly limited in critical process applications (maybe only 4 or 6 – one or two control loops-worth).
If devices were to offer redundant fieldbus connections, there would also be discussion about how far up the protocol stack the redundancy should go within the device; even to the level of duplicating the internal microprocessor and maybe the sensor or actuator. It is very difficult to resolve these questions in a way satisfactory for a wide range of applications. It can be argued that if a particular measurement or actuator is so vital, it is better to simply add a complete duplicate device, and connect it to a different fieldbus segment.
(Note that failures within a fieldbus device should not prevent continued communication by other devices on the segment.)
It was always expected that the high-speed H2 buses would use redundant cabling, since the amount of data carried is much greater, and therefore more important to protect against loss. The HSE (high-speed ethernet) bus now proposed instead of H2 will normally be redundant, just as in commercial networks.