If most fieldbuses aim to move rich data, AS-Interface aims to move the simplest data possible as cheaply and reliably as it can. It was designed to replace the bundles of individual wires that connect basic sensors and actuators to a controller, using a single flat cable that carries both power and signals to every device on it. That yellow cable and its distinctive pierce-through connection are its signature. This guide explains what AS-Interface is, how one master polls its slaves in a few milliseconds, how addressing and profiles work, and how an AS-i gateway lifts that bit-level discrete data toward Profinet, Modbus and ultimately a SCADA system.
AS-Interface (AS-i) in one line: AS-Interface, abbreviated AS-i and standing for Actuator Sensor Interface, is the simplest industrial fieldbus, using a single unshielded two-conductor flat cable to carry both power and data to bit-level sensors and actuators. One master polls its slaves in a master-slave scheme, cycling through all of them within a few milliseconds, and each slave has a short address. A profile defines a device's data layout, and an AS-i gateway connects the network to a higher-level fieldbus so its discrete data reaches a controller or SCADA.
The defining feature of AS-Interface is its cable. Instead of the twisted, shielded, terminated wiring most fieldbuses require, AS-i uses a flat two-conductor cable, unshielded and unterminated, with a shaped profile that makes it impossible to connect the wrong way round. Devices attach to it through insulation-displacement connectors that pierce the cable's outer jacket and make contact with the two conductors, so a module can be clamped onto the cable at any point without cutting or stripping it. That mechanical simplicity is a large part of why AS-i installs quickly and cheaply.
The same two conductors carry both the power that runs the field devices and the data they exchange, so most AS-i sensors and simple actuators draw their supply directly from the cable and need no separate feed. A dedicated AS-i power supply drives the network, and larger loads that exceed what the flat cable should carry can be fed from an auxiliary supply, often on a second black flat cable that runs alongside the yellow one. The yellow-and-black pairing is a common sight in AS-i installations for exactly this reason.
AS-i is deliberately positioned at the bottom of the automation hierarchy, below richer fieldbuses, where the devices are simple: proximity switches, photoeyes, pushbuttons, indicator lamps, and solenoid valves. It is not trying to carry high-resolution analogue data or large device parameter sets. Its job is to connect a lot of small on-off points and a few simple values with the least possible wiring and cost, and it is very good at exactly that narrow task.
An AS-i network runs on a strict master-slave scheme with a single master. The master cycles through the addresses on the network in turn, exchanging a small amount of data with each slave, and it completes a full cycle through the whole network within a few milliseconds. This fixed, predictable cycle makes the timing deterministic: the controller knows how long it takes to refresh every input and output on the segment. The exchange with each slave is small, a handful of data bits plus parameter bits, which is why AS-i is described as bit-level rather than byte-level or message-level.
Each slave carries a short address that the master uses to poll it, and the addressing scheme sets the practical limit on how many devices a segment can hold, on the order of a few dozen depending on whether the extended addressing mode is used. Addresses are assigned when a device is commissioned, using an addressing tool or the master, and a common convention leaves address zero as the default for an unaddressed device so a replacement can be dropped in and given the correct address. Keeping a record of which address belongs to which device is part of maintaining an AS-i network.
AS-i uses the idea of a profile to standardise how a given class of device presents itself, defining its data and parameter layout so that masters and tools can recognise and handle it consistently regardless of manufacturer. A profile tells the system what the device's input and output bits mean and what parameters it accepts, which keeps a multi-vendor network interoperable. This is the AS-i counterpart to the device descriptions used by richer networks, kept deliberately lightweight to match the simplicity of the devices it describes.
On its own an AS-i segment is a self-contained island of discrete I/O; to be useful to a control or monitoring system its data has to be lifted into a network the controller understands. That is the job of an AS-i gateway. A gateway acts as the AS-i master on its lower side, polling all the slaves, while on its upper side it connects to a higher-level fieldbus such as Profinet, EtherNet/IP or Modbus. It presents the collected AS-i inputs and outputs to the controller as blocks of I/O, so the discrete signals from the flat cable appear as ordinary tags in the control system.
This gateway pattern is what places AS-i sensibly within a larger architecture. A machine or a section of a line can use AS-i for its dense population of simple on-off devices, keeping that wiring cheap and fast to install, while the gateway hands the resulting data up to a Profinet or Modbus backbone that ties the whole plant together. From above, the AS-i devices are indistinguishable from any other I/O; the simplicity of the flat cable is hidden behind the gateway, and only the data itself travels onward.
For a SCADA or cloud monitoring project this means AS-i data arrives the same way most low-level discrete signals do: already gathered by a gateway and exposed through an IP-capable protocol that an edge collector or PLC can read. The state of a valve, the presence detected by a photoeye, or the position reported by a limit switch on an AS-i segment becomes a tag that can be trended, alarmed and reported like any other. Knowing that AS-i sits below the fieldbus, reached through a gateway, helps an integrator scope the connection correctly rather than expecting to poll the yellow cable directly.
AS-Interface stands for Actuator Sensor Interface, and it is commonly abbreviated AS-i. The name reflects its purpose: connecting simple actuators and sensors to a controller. It is designed to replace the individual wiring of bit-level devices with a single flat cable that carries both power and data, making it the simplest and lowest tier of common industrial fieldbuses.
AS-i uses a two-conductor flat cable, and the same two conductors carry both the network's power and its data signals. A dedicated AS-i power supply drives the cable, and most simple sensors and actuators draw their supply straight from it, so they need no separate feed. Larger loads can be powered from an auxiliary supply, often on a second black flat cable running alongside the yellow one.
Through an AS-i gateway. The gateway is the AS-i master on its lower side, polling all the slaves, and on its upper side it connects to a higher-level fieldbus like Profinet, EtherNet/IP or Modbus. It presents the AS-i inputs and outputs as blocks of I/O to the controller, so the discrete data reaches SCADA as ordinary tags rather than being polled directly off the flat cable.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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