Configuring the Siemens AS-Interface F90 4I/4O Slave Module

Overview: AS-Interface and the Siemens F90 4I/4O Module

The Actuator-Sensor Interface (AS-i or AS-Interface) is a single-cable industrial field-level networking standard developed to replace the discrete wiring bundles that traditionally link low-power digital sensors and actuators to a controller. The Siemens AS-Interface F90 4I/4O is a sealed, IP67 slave I/O module that exposes four PNP digital inputs (24 V) and four digital outputs (24 V DC, 1 A) to an AS-i master over the standard yellow AS-i cable.

By itself, the F90 module is not a controller. It contains no ladder logic, no application program, and no internal power supply. It functions strictly as a slave node on an AS-i segment, exchanging a small cyclic I/O image with the master that addresses it. The master, in turn, sits on a higher-level fieldbus (PROFINET, PROFIBUS, EtherNet/IP, Modbus/TCP, or a PLC backplane) and presents the F90's inputs and outputs to the PLC CPU as if they were local rack I/O.

This is the central point most engineers new to AS-i miss: the F90 slave is functionally inert without an AS-i master on the same yellow cable. If you have inherited a cabinet where the master has been removed but the F90 is still wired to the yellow cable, the module will sit idle — the green status LED may be lit, but no I/O will be visible to any controller.

For background on the protocol, see Actuator-Sensor Interface (AS-i): Overview and components - IFM and AS-Interface - Pepperl+Fuchs. For Siemens master and module manuals, refer to Siemens Industry Online Support (SIOS).

What the F90 4I/4O actually does

• Reads four 24 V PNP sensor inputs and transmits a 4-bit nibble to the master on every AS-i cycle.
• Receives a 4-bit nibble from the master and drives four 24 V semiconductor outputs to that pattern.
• Provides a regulated 24 V sensor supply (drawn from the AS-i 30 V rail via onboard DC/DC) to power the connected sensors, up to 200 mA total.
• Reports diagnostic flags (output overload, missing auxiliary 24 V, internal fault) to the master via acyclic AS-i v3 diagnostics.

AS-i Network Architecture and Topology

AS-i is a deterministic, single-master, multi-drop network optimized for binary I/O. The physical layer is a two-conductor unshielded yellow cable that carries both data and DC supply (typically 30 V) to the slaves from a dedicated AS-i power supply with integrated data-decoupling inductors.

A typical AS-i segment topology:

Each slave is addressed with a unique ID from 1A to 31A (standard addressing) or 1A/1B through 31A/31B (extended addressing). The AS-i standard cycle time is approximately 5 ms with up to 31 standard slaves; with extended addressing and 62 slaves, the cycle stretches to roughly 10 ms. The master polls each slave in turn, reads its 4-bit input nibble, writes its 4-bit output nibble, and replicates the consolidated image to the host PLC.

AS-i version comparison

A single AS-i master drives one segment of up to 100 m (longer with repeaters — one repeater extends the bus to 200 m, two to 300 m, with cycle-time penalties). If more devices are needed, multiple masters run independent AS-i segments and converge at the higher-level fieldbus.

F90 4I/4O Technical Specifications

The Siemens F90 family belongs to the AS-Interface SlimLine / flat housing series (catalog prefix 3RK1). The 4I/4O variant combines four inputs and four outputs in a single IP67 housing designed for mounting directly on the machine structure.

Electrical specifications

Mechanical and environmental

Wiring and Pinout

Wiring an F90 module consists of three connections: the AS-i (yellow) cable, the auxiliary 24 V (black) cable, and the field I/O on M12 connectors. Each connection type has its own conventions and pitfalls.

Cable types and color coding

M12 pinout (typical F90 4I/4O)

The M12 A-coded 5-pin connector on the F90 is wired per the module's printed diagram. The following is one common assignment for a 4I/4O F90 variant — always verify against the label on the actual module:

Step-by-step wiring procedure

1. Verify that the AS-i power supply is OFF and locked-out before opening the F90 housing or terminating cable.
2. Insert the yellow AS-i cable into the marked channel on the housing. Tighten the two IDC screws evenly to the torque specified on the housing label (typically ~0.8 Nm). The cable's insulation will be pierced automatically.
3. Repeat with the black AUX 24 V cable if the F90 variant uses a separate AUX input.
4. Connect each PNP sensor to one of the four M12 input channels, observing the sensor supply polarity (pin 1 = +24 V, pin 3 = 0 V per the typical pinout).
5. Connect each load (solenoid, valve, contactor coil, indicator) to one of the four output channels. Outputs source +24 V when active; the load's return path is to the same 0 V reference as the AUX supply.
6. Re-apply AS-i power. The green LED on the F90 should illuminate within 1–2 s; a red or flashing LED indicates a fault.

AS-i Master Configuration

An AS-i master can take several forms:

• PLC-internal AS-i master: Siemens CP 343-2 / CP 343-2P (S7-300), CM AS-i Master (S7-1200), CM 4xAS-i Master (ET 200SP / S7-1500).
• Stand-alone AS-i / fieldbus gateway: gateways to PROFINET, EtherNet/IP, Modbus/TCP, PROFIBUS, or CC-Link from vendors like Pepperl+Fuchs, IFM, Balluff, Sick, or Phoenix Contact.
• Integrated master on a controller family: certain ABB, Schneider, and Beckhoff controllers include AS-i directly.

The master must be told what slaves to expect. Two approaches are supported by every AS-i v2/v3 master:

1. Automatic scan (recommended for commissioning)

1. Power up the AS-i segment with all slaves connected.
2. Open the master's configuration tool (Siemens AS-i Control Tool, TIA Portal AS-i diagnostics, IFM LineMonitor, Pepperl+Fuchs Handheld, etc.).
3. Trigger "Read actual configuration" / "Detect slaves" / "LDS" (List of Detected Slaves).
4. The master reports every slave it sees: address, ID code, I/O code, profile, and any per-slave diagnostics.
5. Save the detected configuration as the active configuration.

2. Manual configuration

1. Determine what slaves should be present from the electrical drawing.
2. For each address, enter the expected profile (e.g., S-0.0 for a basic 4I/4O) and I/O code.
3. Save the configuration.
4. Compare actual vs. configured; resolve mismatches before putting the segment in run mode.

Once configured, the master maps each slave's 4-bit input nibble and 4-bit output nibble to the host PLC's process image. For a Siemens S7-1200 with a CM AS-i Master, the I/O appears in the master's input/output image and is accessed like any local I/O in the TIA Portal project.

Slave Addressing Procedure

Every F90 module ships from the factory with address 0. A master cannot poll address 0 (it is reserved for new devices). Each slave must be assigned a unique address from 1A to 31A (standard) or 1B to 31B (extended).

Addressing options

• Handheld addressing unit (e.g., Siemens 3RK1904-2AB02, Pepperl+Fuchs VBP-HH1): plugs into the slave's M12 addressing port, sets the address, disconnects. The most reliable method in the field.
• Master-driven addressing: many AS-i masters support automatic addressing when they detect a slave at address 0. The master walks the new slave through an address assignment.
• Configuration tool: the same tools used for scanning can sometimes assign addresses remotely over the AS-i cable.

Procedure with a handheld unit

1. Power the slave via the AS-i cable (or via the handheld's auxiliary power).
2. Connect the addressing cable to the slave's addressing interface (typically the same M12 connector used for I/O).
3. Read the current address — should read 0 for a new slave.
4. Enter the desired address (e.g., 5A).
5. Write. The new address is stored in non-volatile memory in the slave.
6. Disconnect the handheld and verify the slave responds to the master.

Address collision handling

If two slaves on the same segment share an address, the master will see only one (the lower serial number typically wins). Re-address the misplaced slave offline using a handheld unit; do not attempt to assign an address while another slave holds it.

Diagnostics and Fault Detection

AS-i v3 supports per-slave acyclic diagnostics that the master reads on demand. Common diagnostic flags reported by the F90 and other AS-i v3 slaves:

F90 LED behavior

Master-side diagnostics procedure

1. Open the master's configuration / diagnostic tool.
2. Trigger "Read AS-i diagnostics" or equivalent.
3. Identify slaves flagged as "not detected," "configuration error," or "periphery fault."
4. For periphery faults, read the per-channel diagnostic if supported (AS-i v3).
5. Cross-reference with the electrical drawing and physical inspection.

AS-i vs. Conventional Wiring vs. Higher-Level Fieldbus

AS-i occupies a specific niche in industrial networking: low-cost, fast, binary I/O aggregation at the machine level. The F90 4I/4O is positioned for that niche.

AS-i makes economic sense when you have many binary devices (typically 10 or more) within a 100 m radius that would otherwise require long home-run cables back to the panel. It is less appropriate for analog signals, fast motion, or distributed I/O with mixed discrete and analog I/O in the same drop.

Field Application Notes

In the field, the F90 4I/4O is typically used in:

• Conveyor systems: photoeyes, inductive sensors, diverter solenoids, stop requests.
• Packaging machines: limit switches, valve banks, low-power motor contactors, index confirmation.
• Material handling: small roller drives, sortation actuators, turntable position feedback.
• Automotive body-in-white: weld cell detection, fixture clamps, safety gate monitoring (paired with a safety AS-i monitor for Cat. 3/4 stops).

Power budget planning

AS-i power supplies are typically rated 2.8 A or 8 A per segment. With each F90 drawing up to 200 mA from AS-i (plus sensor supply), plan segment loading carefully:

• 31 F90 slaves × 200 mA = 6.2 A → requires an 8 A AS-i power supply.
• If sensor supply dominates, count up the sensor current and add the F90 quiescent draw.

Grounding and shielding

• AS-i yellow cable is unshielded; no shield grounding required.
• The F90 housing should be bonded to functional earth via its mounting screws or FE pin.
• If a shielded AS-i variant is used (rare), ground the shield at one end only.

EMC routing

• Route the yellow AS-i cable at least 200 mm away from VFD output cables, or use a grounded metal partition.
• Avoid parallel runs longer than 50 m with VFD or servo motor cables.

Output protection

Outputs are short-circuit and overload protected. For added serviceability, fuse each output externally at 1 A or per the load rating. Fuses simplify troubleshooting — a blown fuse points directly at the load, not at the F90.

Maintenance and replacement

1. Power down the AS-i segment or isolate the slave's AUX 24 V.
2. Remove the M12 connectors and unscrew the housing.
3. Replace with a new F90 of the same variant.
4. Use a handheld unit to address the new slave to the same address as the old one (e.g., 7A → 7A).
5. Power up and verify the master reports the slave as "present" with no configuration mismatch.

Troubleshooting Matrix

Safety Considerations

The F90 4I/4O is a standard (non-safety) AS-i slave. It is not rated for use in safety functions up to PL d / SIL 2 or higher without additional measures. If a Cat. 3 / Cat. 4 stop or safe position monitoring is required, use a safety-rated AS-i slave (e.g., from Sick, Pilz, Phoenix Contact, or Siemens) and pair it with an AS-i safety monitor on the same segment.

Safety-related signals must not be wired to the F90 4I/4O standard inputs and outputs. Use a dedicated AS-i safety I/O module on the same yellow cable, addressed to a different slave number, and monitored by the safety monitor.

Frequently Asked Questions