AS-Interface Communication Protocol

The AS-i (Actuator Sensor Interface) protocol was created in Germany in 1994 by a consortium of factory automation suppliers.

Originally developed to be a low-cost method for addressing discrete sensors in factory automation applications, AS-i has since gained acceptance in process industries due to its high power capability, simplicity of installation and operation, and low cost adder for devices.

Each AS-i segment can network up to 31 devices. This provides for 124 inputs and 124 outputs, giving a maximum capacity of 248 I/O per network on a v2.0 segment.

The AS-i v2.1 specification doubles this to 62 devices per segment, providing 248 inputs and 186 outputs for a total network capacity of 434 I/O points.

Both signal and power are carried on two wires. Up to 8 amps at 30VDC of power are available for field devices such as solenoid valves.

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.

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An AS-Interface network is a collection of network segments. There are very few rules that need to be satisfied when designing an AS-Interface network:

  • There can be no duplicate addresses on a network
  • Each segment must be 100 m or less in total cable length unless a Tuner is used, in which case the segment cable length must not exceed 200 m
  • Each network must power exactly one master
  • Each segment must power exactly one AS-Interface power supply
  • When repeaters are used, a slave can not be more than “two repeater transitions” from the master
  • The shape (i.e. topology) of a segment is arbitrary (unrestricted)

Using these basic rules it should be clear that

  • A linear network, with the master at one end of the network, can be 300 m long
  • A linear network, with the master “in the middle” of the network, can be 500 m long

In some applications longer networks are desirable (e.g. Process Automation applications). This is possible through the installation of Terminator and Tuners.

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