P-11 — AS-INTERFACE (AS-i)
The yellow cable. 31 slaves. One master. Done.
Estimated 12 minutes.
The simplest industrial bus there is
AS-Interface (AS-i) was designed in 1991 to do one thing: connect a swarm of cheap binary sensors and actuators to a PLC, with absolutely minimum installation cost. No twisted pair, no shielding, no termination, no special tools. Just a flat yellow cable and pierce-it connectors.
Below Profibus, below Profinet
AS-i lives at the very bottom of the automation pyramid — between simple field devices (limit switches, valves, pilot lights) and a higher-level controller. A gateway typically bridges AS-i upward to Profinet, EtherNet/IP, or Modbus TCP.
The iconic yellow ribbon
Two conductors, mechanically keyed. Slaves clip on with insulation-displacement pins.
Power AND data on the same two wires
The yellow cable carries 30 V DC for sensor power AND a Manchester-encoded data signal — superimposed. A standard slave draws up to 200 mA from the bus. High-current actuators get a second cable (the black AUX cable, 24 V DC) for their power.
IDC connectors mean zero stripping
Slave modules have insulation-displacement pins underneath. Lay the yellow cable in the module groove, close the lid, and metal pins pierce through the insulation to make contact. No cutting, no stripping, no crimping. Move a slave 1 metre? Pop it off, clip it on at the new spot.
Why is the cable shaped?
The yellow cable has a distinct asymmetric profile (one side flat, one side keyed). Why?
- It is just brand identity
- Mechanical keying — impossible to install with reversed polarity
- It improves signal speed
One master, 31 slaves
Every AS-i segment has exactly one master and up to 31 standard slaves (addresses 1–31). The master polls every slave once per cycle. With 31 slaves the cycle is around 5 ms — fast enough for almost any discrete-control task.
AS-i v3 doubled it to 62
Spec v3.0 introduced "A/B addressing" — slaves can be addressed as 1A/1B, 2A/2B, etc. doubling capacity to 62 slaves at the cost of slower cycle times and only 3 input bits per A/B device. Standard 31-slave operation is still by far the most common.
4 bits in, 4 bits out
Each AS-i slave exchanges exactly 4 input bits and 4 output bits with the master per cycle. Sounds tiny — but a single I/O module can pack 4 photoeyes or 4 valve coils. Multi-channel sensors typically expose 4 status bits for diagnostics.
Watch the bus exchange
5 slaves on a yellow line. Click any bit to flip it — the master sees this in the next 5 ms cycle.
A 16-bit analog sensor on AS-i
How can a slave deliver a 16-bit analog reading when each cycle only carries 4 data bits?
- It cannot — AS-i is binary only
- It uses an extended profile (CTT2/CTT3) that spreads the value across multiple cycles
- It overrides the master and seizes more bandwidth
Topology freedom
AS-i imposes almost no rules on the cable layout. Line, tree, star, and any combination work — total cable length up to 100 m (extended to 300 m or 500 m with repeaters). The only forbidden topology is a closed ring.
What you can and cannot do
Branch, split, tee — anything except looping back into the master.
Why no ring?
A closed loop on AS-i is not allowed. What is the technical reason?
- It would short-circuit the 30 V supply
- The data signal would arrive at slaves twice with different propagation delays — corrupting reception
- It is just a recommendation, not a hard rule
Addressing — once with the addressing tool
New AS-i slaves arrive with address 0. You set the address with a small handheld addressing tool (or via the master's engineering port) — the slave then keeps that address forever. Two slaves at address 0 on the bus is fine; two at the same non-zero address is a fault.
Slave failed — what now?
A photo eye at address 12 fails. You pull a brand-new spare from the cabinet (still address 0). What do you do?
- Plug it in and the master auto-addresses it
- Address it to 12 with the addressing tool first, then plug it in
- Configure the master to ignore address 0
AS-i Safety: the same cable, certified
AS-i Safety at Work (later AS-Interface Safety, ASIsafe) lets safety-rated input devices (E-stop buttons, light curtains) and safety outputs share the same yellow cable as the standard slaves. A separate "safety monitor" device watches the bus and trips the safe outputs when any safety input opens.
Why mix safety and standard on one wire?
What is the main practical benefit of running safety on the same yellow cable as standard slaves?
- It is faster than dedicated safety wiring
- Massive reduction in cabling — one bus serves both functions, halving installation time
- Safety devices become cheaper to design
Where it sits in a real plant
A typical layout: hundreds of pilot lights, valves, photoeyes and limit switches on yellow AS-i ribbons; each ribbon ends at an AS-i master/gateway; the gateway hands the aggregated I/O up to a Profinet or EtherNet/IP PLC. AS-i hides the cable mess; the upper bus only sees a single integrated device.
Final challenge
A bottling line has 60 photoeyes, 40 solenoid valves, 12 emergency-stop pushbuttons, and 8 light curtains, all distributed across 80 m of conveyor. The PLC is a Siemens S7-1500 with Profinet. What is the cleanest topology?
- Wire every device back to the PLC cabinet with discrete I/O cards
- Run one yellow AS-i ribbon along the conveyor with safety + standard slaves; bring it up to the PLC via an AS-i / Profinet gateway
- Use Profinet down to every device
What you learned
- AS-i is the simplest industrial bus: one master + up to 31 slaves (62 with A/B addressing) on a 2-wire flat yellow cable.
- Power and Manchester-encoded data ride together on the same two wires (30 V DC, ~200 mA per slave).
- IDC piercing connectors mean zero stripping; the cable shape forbids polarity reversal.
- Each slave exchanges 4 in / 4 out bits per ~5 ms cycle. Extended profiles allow 16-bit analog.
- Topology is free (line, tree, star) but rings are forbidden — they corrupt the signal.
- AS-i Safety (ASIsafe) carries SIL 3 / Cat 4 safety on the same cable as standard I/O.
- A gateway bridges AS-i up to Profinet, EtherNet/IP or Modbus TCP.
AS-Interface complete
- You understand the yellow cable, IDC connectors, and shape coding.
- You can size an AS-i segment and choose between standard / extended / safety slaves.
- You know how AS-i fits below Profinet/EtherNet/IP via a gateway.