A process plant with 200 automated on/off valves needs to connect each valve's solenoid coil, position feedback sensors, and diagnostic signals back to the DCS — 200 valves × 3 signals × 2 wires per signal = 1,200 individual conductors pulled through cable trays, terminated at marshalling cabinets, landed on I/O cards. The wiring cost alone — roughly $50 to $150 per point, fully burdened with cable, tray, termination, and I/O allocation — approaches $60,000 to $180,000 for the 600 signals. Two industrial communication protocols promise to reduce that to one cable carrying power and data to each valve or cluster of valves: AS-Interface (Actuator-Sensor Interface), in service since 1994 and installed on over 30 million field devices, and IO-Link, standardized under IEC 61131-9 in 2013 and rapidly displacing AS-i in new installations. Both protocols replace multi-conductor home-run wiring with a single cable. Both deliver power and data to field devices. Both provide diagnostic data that hardwired contacts cannot. But they differ fundamentally in topology, data bandwidth, power delivery, and integration architecture. This article compares AS-Interface and IO-Link for process valve automation — so instrument engineers can select the right protocol for brownfield upgrades and greenfield designs.
Topology: Bus vs Point-to-Point
AS-Interface is a bus topology: a single two-conductor flat cable (typically yellow for power and data, with an optional black auxiliary power cable) runs through the field, and devices tap into it via insulation-piercing connectors anywhere along its length. Up to 62 devices (or 31 in extended addressing mode) share one cable segment up to 100 meters long (300 meters with repeaters). The cable carries both 30 V DC power (up to 8 A per segment) and data (167 kbit/s) on the same two conductors, modulated so power and data coexist without interference. The bus topology makes AS-i economical when devices are distributed along a linear path — conveyor systems, packaging lines, tank farms where valves sit along a pipe rack.
IO-Link is a point-to-point topology: each field device connects to an IO-Link master port via its own dedicated 3- or 4-conductor M12 cable, up to 20 meters long. The IO-Link master then connects upstream to the PLC or DCS via a fieldbus (PROFINET, EtherNet/IP, EtherCAT). One IO-Link master serves 4 to 8 ports. The point-to-point topology means each valve gets its own connection back to the master, and a cable fault affects only that valve — not a whole bus segment. IO-Link's topology is more cable-intensive for widely distributed devices but more fault-tolerant for individual device failures.
Data Bandwidth: Diagnostics vs Process Data
AS-Interface delivers 4 bits of input and 4 bits of output data per device per cycle — enough for an on/off valve: one bit for solenoid command (open/close), two bits for limit switch feedback (open closed, closed closed, or in transition), and one bit for a fault alarm (coil short, loss of position). Cycle time for a full 62-device segment is 5 to 10 ms — fast enough for discrete valve automation. AS-i can carry limited analog data (via a profile that packets 16-bit values across multiple cycles), but this is an extension of a protocol designed for discrete I/O and is rarely used in practice.
IO-Link delivers up to 32 bytes of process data per device per cycle — eight times the discrete bandwidth of AS-i — at communication speeds of 38.4 kbit/s (COM1), 230.4 kbit/s (COM2), or 38.4 kbit/s (COM3). This bandwidth supports not just open/close/fault for a valve, but also: valve cycle count, response time monitoring (how many milliseconds from coil energize to limit switch change — a degrading valve opens slower and triggers a maintenance alert before it fails), coil current measurement (detecting a shorted turn before it burns out the coil), ambient temperature at the valve position, and the valve's electronic nameplate (manufacturer, model, serial number, date of manufacture — readable by the DCS without a manual asset register). For process valve automation where diagnostic depth drives maintenance strategy, IO-Link's data bandwidth is the decisive advantage.
Power Delivery
AS-Interface delivers up to 8 A per segment at 30 V DC on the yellow cable, sufficient to power solenoid coils drawing 2 to 8 watts each — a 62-device segment with 4-watt coils draws 248 watts total, within the cable's 240-watt capacity with conservative derating. For valves requiring higher power — large pilot-operated solenoid valves, motorized actuators — the black auxiliary power cable provides a separate 24 V DC supply.
IO-Link delivers up to 200 mA per port at 24 V DC on the standard M12 connection — roughly 4.8 watts, sufficient for a single low-power solenoid pilot valve or position sensor. For higher-power devices, IO-Link specifies a Class B port that provides an additional galvanically isolated 24 V supply on a separate pin — or the solenoid power is supplied separately, and IO-Link provides only communication and diagnostics. For process plants where solenoid valve coils draw 2 to 8 watts, IO-Link Class A (200 mA) is often marginal, and Class B or separate solenoid power wiring is common — partially eroding the wiring simplification that the protocol promises.
Integration: Brownfield vs Greenfield
AS-Interface integrates with any PLC or DCS via a gateway module — the gateway translates the AS-i data into PROFIBUS, PROFINET, EtherNet/IP, DeviceNet, or Modbus TCP for the host controller. The host sees AS-i devices as discrete I/O points. No special AS-i engineering software is required beyond the gateway configuration tool — the PLC program treats each valve as a set of digital input and output tags, exactly as if the valves were hardwired to a digital I/O card.
IO-Link integrates with the PLC or DCS via the IO-Link master's fieldbus interface. But IO-Link's richer data — 32 bytes of process data, device identity, diagnostic parameters — requires the host software to understand IO-Link device descriptions (IODD files, analogous to EDD or GSD files for fieldbus devices). The engineering tool imports the IODD, maps the device's parameters to PLC tags, and configures the data structure. This is standard practice for new installations but adds an integration step absent in AS-i. Some legacy DCS platforms with limited support for IO-Link may require a middleware server (e.g., an OPC UA gateway) to bridge IO-Link diagnostics into the DCS historian and alarm system.
Which protocol for which application?
- Choose AS-Interface when
- The application is discrete on/off valve control (open/close, two limit switches, one fault alarm), the valves are distributed linearly along a conveyor, pipe rack, or tank farm, the DCS or PLC integration is brownfield with limited support for IO-Link IODD management, the valve count per segment is high (30 to 62 devices), and the diagnostic requirement is basic (fault present/absent).
- Choose IO-Link when
- The application requires diagnostic depth (valve cycle count, response time trend, coil current monitoring), the valves are clustered around a local control panel or junction box that can house an IO-Link master, the host control system supports IODD-based device integration, the valve count per area is moderate (4 to 8 per master), and future-proofing for predictive maintenance data matters more than minimizing per-point wiring cost.
In practice, a growing number of process valve terminal manufacturers — Burkert, Festo, Norgren, SMC — now ship IO-Link valve islands that combine up to 24 solenoid valves and their position feedback into a single IO-Link device, eliminating the point-to-point cable limitation while retaining the full diagnostic data bandwidth. These valve islands offer AS-i's bus-topology wiring simplicity with IO-Link's data richness — and they are the architecture to specify for greenfield process valve automation where both wiring efficiency and diagnostic depth matter.
