Take a typical machine, industrial plant or laboratory installation with a variety of switches, proximity detectors, indicators, air actuators and a host of other actuators and sensors positioned not just in or on panels but spread all round the system, all requiring their own individual wiring.
Now imagine how much simpler, quicker and less error prone installation and maintenance would be if all that wiring could be replaced with just one two-core cable which simply goes around the area and all those devices just clip onto it. Further, imagine being able to expand the system in the future by simply clipping new actuators or sensors onto the existing cable rather than having to add new cabling, expansion ports in the central system and so on. You’ve just imagined the basic concept of a fieldbus.
Take the concept further, suppose that the system is so designed that the cable does not need terminators and the technical specification is such that you can run all the safety elements (for example, emergency stops, guard switches, light curtains) on the SAME cable at the SAME time as all the other non-safety actuators and sensors! Expand the range of actuators and sensors to cover analogue as well and you have the Actuator and Sensor interface system known as AS-i.
AS-i has now been around for many years, with millions of devices in the field: it is a mature, well-proven technology used in a very wide variety of industry sectors. It started out as a simple intelligent cabling system to replace and simplify the often large amount of point-to-point wiring needed in connecting binary (on/off) devices such as switches and lights. the system has then been expanded to include such things as safety devices and analogue measurements. Nevertheless, it has remained true to its roots and, although new versions are being brought out regularly, any new version of AS-i is always guaranteed to be fully backwards compatible with any older versions. So users are never forced to throw away perfectly good older devices.