AS-Interface

What is AS-Interface?

AS-Interface (Actuator Sensor Interface, AS-i or ASi for short) is a standardised fieldbus system (IEC 62026-2) that connects simple sensors and actuators over a single unshielded two-core flat cable carrying both data and power. AS-i serves the lowest level of the automation pyramid: pushbuttons, limit switches, photoelectric sensors, valves and motor starters. An AS-i master links these signals to a PLC, or through a gateway to higher-level networks such as Profibus, ProfiNET or IP. One yellow cable thus replaces hundreds of individual wires running back to a central I/O cabinet.

🧠 How did AS-Interface come about?

AS-i started in 1990 as an initiative of a consortium of eleven companies, mostly sensor manufacturers, including Balluff, Festo, ifm electronic, Pepperl+Fuchs, Sick and Siemens. The aim was to replace expensive, error-prone parallel wiring of binary signals with a bus that electricians without networking knowledge could install. Once development was complete the consortium was dissolved, and in 1991 the independent user association AS-International was founded, which maintains the specification and certifies products.

The first working system was shown at the Hannover Messe in 1994. Standardisation and extensions followed:

The current standard is IEC 62026-2, second edition (2008) with amendment 1 (2019). That edition incorporated A/B addressing for up to 62 slaves and the AS-i safety system. In Europe the adopted version EN 62026-2 applies.

🔧 How does AS-Interface work technically?

AS-i is a single-master system: one master cyclically polls every configured slave and exchanges input and output data with each in turn. There are no collisions, so the cycle time is predictable.

• Yellow flat cable - a two-core profiled cable that carries both communication and power (about 30 V DC, PELV). Its shape makes reverse-polarity connection impossible.
• Insulation piercing - modules clamp onto the cable and contact pins pierce the insulation. If a module is removed, the rubber sheath largely closes up again.
• Black flat cable - a separate auxiliary supply (AUX) for higher-power loads such as valve terminals, so the AS-i network itself is not overloaded.
• Modulation - ASi-3 modulates the serial data onto the supply voltage using Manchester coding and Alternating Pulse Modulation; ASi-5 uses OFDM with carriers between 1 and 2 MHz.
• Free topology - line, star and tree layouts are allowed, with no mandatory terminating resistor.
• Cable length - up to 100 m per segment without accessories. A special bus termination extends this to 300 m; repeaters and extenders extend the network further.
• Addressing - every slave gets an address (1–31, or 1A–31B for A/B slaves) via a handheld addressing unit or via the master.

📊 What are the typical values for ASi-3 and ASi-5?

ASi-5 and ASi-3 can share the same cable, but this requires a new master that supports both generations. Existing ASi-3 modules then simply keep working. Thanks to its larger data capacity, ASi-5 can also pass through IO-Link devices with their full process data and parameters.

🔐 How does safe communication work with AS-i Safety at Work?

Safety at Work adds functional safety to the same yellow cable. Safety slaves, such as an emergency stop, a light curtain or a door switch, transmit a dynamic code sequence that a separate safety monitor or safety master checks every cycle. If the sequence deviates or stops, the monitor switches the associated outputs to a safe state.

The concept achieves SIL 3 to IEC 61508 and PL e to ISO 13849-1. That makes it suitable for most machine safety functions under the Machinery Directive and its successor, the Machinery Regulation, which applies from 20 January 2027. Many manufacturers combine an AS-i safety monitor with a safety PLC or a gateway that forwards the safe signals via PROFIsafe. A separate safety relay per function is then no longer needed.

🔄 How does AS-Interface compare with other fieldbuses?

AS-i and IO-Link are not competitors: IO-Link connects one smart sensor to a master, whereas AS-i bundles many simple devices onto one cable. ASi-5 can in fact carry IO-Link devices over the yellow cable. Compared with DeviceNet and CANopen, AS-i wins mainly on ease of installation; those networks offer more data per node but require more configuration.

🏭 Where is AS-Interface used?

• Conveyors and intralogistics - position sensors, start/stop buttons and motor starters along hundreds of metres of conveyor, for example in distribution centres and airport baggage systems
• Packaging machines - detecting boxes, bottles or labels and driving pneumatic cylinders
• Process industry - valve position indicators on shut-off valves, so the position of hundreds of valves is known without individual wiring
• Automotive - assembly lines with many decentralised signals in IP67 modules (see IP Code)
• Buildings and infrastructure - simple I/O for pumps, gates and lighting

🧭 How do you design an AS-i network in practice?

1. Inventory the signals - count binary and analogue I/O per machine section and decide whether IO-Link sensors are needed. That determines the choice between ASi-3 and ASi-5.
2. Choose a master or gateway - a master in the PLC rack or a gateway to ProfiNET, EtherNet/IP or Modbus TCP; for safety functions, a gateway with integrated safety.
3. Calculate power and voltage drop - add up the current draw of all slaves, check the voltage at the end of the cable and put heavy loads on the black AUX cable.
4. Watch the cable length - stay below 100 m per segment or plan a bus termination, repeater or extender.
5. Address and document - record address, profile and location for each slave; a replacement module must get the same address (many masters do this automatically when a single slave is missing).
6. Validate safety functions - with Safety at Work, define the target SIL/PL and test every function at commissioning.

Worked example: a sorting line with 40 photoelectric sensors and 20 valves (60 slaves) fits in one ASi-3 network with A/B addressing and a cycle time of about 10 ms. That is ample for conveying, but too slow for fast registration. In that case, choose ASi-5 or wire the fast sensor directly to the PLC.

From an IT/OT perspective, AS-i itself has no authentication or encryption. Protection relies on physical access to the cable and on segmentation of the network the gateway sits on. Treat an Ethernet gateway like any other field device in the Purdue Model: in a protected zone, with the default passwords of its web interface changed.

❓ Frequently asked questions

What does AS-i stand for?

AS-i stands for Actuator Sensor Interface, the abbreviation of the AS-Interface fieldbus system. It is standardised internationally in IEC 62026-2 and maintained by the AS-International association. The name says what AS-i is for: connecting sensors and actuators at the lowest level.

How many devices can be connected to one AS-Interface network?

A classic ASi-3 network supports 31 standard slaves, or 62 slaves with A/B addressing, per master. ASi-5 raises this to 96 devices. More devices require an extra master or a gateway with multiple AS-i channels.

How long can an AS-i cable be?

An AS-Interface segment can be up to 100 m long without accessories. A special bus termination allows 300 m, and repeaters or extenders extend the network further. Keep an eye on the voltage drop along the cable as well.

Can AS-Interface handle safety functions?

Yes, with Safety at Work, AS-Interface can carry safety signals such as emergency stops and light curtains over the same yellow cable. A safety monitor checks a dynamic code from every safety slave each cycle. The system achieves SIL 3 to IEC 61508 and PL e to ISO 13849-1.

What is the difference between ASi-3 and ASi-5?

ASi-3 transmits 4 bits per slave using pulse modulation and supports up to 62 slaves. ASi-5 uses OFDM, supports 96 devices with up to 32 bytes of data each and reaches a cycle time of about 1.2 ms with 24 devices. Both generations can run on the same cable with a master that supports ASi-3 and ASi-5.

Is AS-Interface obsolete?

AS-Interface is not obsolete: the AS-International association still certifies new products, and ASi-5 was presented in 2018 as a new generation. For many binary signals, AS-i remains cheaper and simpler than Ethernet all the way to the field. For data-rich sensors it is increasingly combined with IO-Link.

📌 In summary

AS-Interface is the simplest standardised fieldbus for sensors and actuators: one yellow two-core cable for data and power, polled cyclically by a single master. ASi-3 serves up to 62 binary devices with a cycle time of 5–10 ms, while ASi-5 brings 96 devices with up to 32 bytes of data and IO-Link integration onto the same cable. With Safety at Work (SIL 3 / PL e) and gateways to ProfiNET or EtherNet/IP, AS-i forms an inexpensive and robust base layer beneath the rest of the automation system.