AS-Interface (Actuator Sensor Interface, AS-i) is an industrial networking solution (physical layer, data access method and protocol) used in PLC, DCS and PC-based automation systems. It is designed for connecting simple field I/O devices (e.g. binary ON/OFF devices such as actuators, sensors, rotary encoders, analog inputs and outputs, push buttons, and valve position sensors) in discrete manufacturing and process applications using a single 2-conductor cable.
AS-Interface is an ‘open’ technology supported by a multitude of automation equipment vendors. According to AS-International Association there are currently (2013) over 24 Million AS-Interface field devices installed globally, growing at about 2 million per year.
AS-Interface is a networking alternative to the hard wiring of field devices. It can be used as a partner network for higher level fieldbus networks such as Profibus, DeviceNet, Interbus and Industrial Ethernet, for whom it offers a low-cost remote I/O solution. It is used in automation applications, including conveyor control, packaging machines (e.g. Schubert’s), process control valves, bottling plants, electrical distribution systems, airport baggage carousels, elevators, bottling lines and food production lines (e.g. 2SFG).
AS-Interface provides a basis for Functional Safety in machinery safety/emergency stop applications. Safety devices communicating over AS-Interface follow all the normal AS-Interface data rules. The required level of data verification is provided by dynamic changes in the data. This technology is called Safety at Work and allows safety devices and standard, non-safe devices to be connected to the same network. Using appropriate safe input hardware (e.g. light curtains, e-stop buttons, and door interlock switches), AS-Interface can provide safety support up to SIL (Safety Integrity Level) 3 according to EN 62061, CAT 4 according to EN954-1 as well as Performance Level e (PL e) according to EN ISO 13849-1.
The AS-Interface specification is managed by AS-International, a member funded non-profit organization located in Gelnhausen/Germany. Several international daughter organizations exist around the world.
AS - Interface
AS-Interface is a system that requires four basic components:
- Exactly one network master, in most cases in the form of a Gateway to a higher level industrial network or a PLC backplane card,
- A number of network slaves, in most cases input and output modules,
- Exactly one power supply used to power the network slaves and enabling communication with the network master, and
- The wiring infrastructure, in most case accomplished using the yellow flat cable.
Using these components an AS-Interface segment can be constructed.
The underlying communication procedure is a Master-Slave method, by which the master initiates data exchange with a slave and requires the slave to respond within its defined maximum time, making AS-Interface a deterministic networking solution. Conformance testing by an independent certification lab assures that certified products from all manufacturers will communicate on a given network. Suppliers wishing to have their products conformance tested must contact AS-International.
AS-Interface data exchanges are based on a Master-Call, where the data frame consists of a 5-bit device addresses, 4-bit data packets (e.g. digital output information) and framing bits. The total length of the Master-Call is 14 bit. The resulting Slave-Response is 7 bit long, containing 4 bit of user information (e.g. the values of slaves inputs).
Voltage levels on the network range between 29,5 … 31,6 V DC and data protection, in addition to the framing bits, is accomplished via Manchester-II coding, a highly symmetrical, floating layout with Alternating Pulse Modulation. The networks bit time is 6 µs. Segment length is limited to 100 meters.
Multiple segments can be combined to form longer networks. In this case it is necessary to assure that every slave is only two repeaters ‘away’ from the network master. This is required due to the time delay introduced when a message packet transitions the repeater. Under certain conditions a so-called Terminator and/or Tuner can be used to extend the allowable segment length to 200 m or 300 m, respectively.