Interbus Protocol: Features and Applications in Automation Systems

Interbus (stylized as INTERBUS) is an open, digital fieldbus communication protocol designed for real-time data exchange between industrial controllers (PLCs, PACs) and field devices (sensors, actuators, valves) in automation systems. Developed by Phoenix Contact in the 1980s and standardized as IEC 61158 Type 5 and EN 50254, Interbus is a deterministic, serial bus system optimized for high-speed, reliable communication in manufacturing, process control, and industrial automation environments.

Core Architecture of Interbus

Interbus uses a master-slave, cyclic communication model with a linear (bus) topology, consisting of three key layers:

1. Physical Layer

Defines the electrical and mechanical specifications for data transmission:

Media: Supports twisted-pair copper cable (standard), fiber optic cable (for long distances/high noise), and wireless (Wi-Fi/Bluetooth for mobile devices).
Transmission Speed: Up to 500 kbps (copper, 400m) or 2 Mbps (fiber, 10km); deterministic latency (<1ms for 32 nodes).
Topology: Linear bus with optional star couplers for branching; supports up to 254 slave devices per bus segment.
Cabling: Uses a 2-wire (data) + 2-wire (power) cable; connectors are standardized (D-sub 9-pin for masters, M12 for field devices).

2. Data Link Layer

Manages cyclic data transfer between the master and slaves:

Master Device: Typically a PLC or industrial controller that initiates all communication, polls slaves, and synchronizes the bus.
Slave Devices: Field devices (sensors, actuators, I/O modules) that respond to master requests and transmit/receive data.
Cyclic Communication: The master sends a poll telegram to the first slave, which forwards it to the next slave (and so on, in a “token-passing” style). Each slave appends its data to the telegram, which is returned to the master at the end of the cycle. This ensures deterministic, real-time data exchange.
Data Consistency: All slaves update their I/O data simultaneously at the end of each cycle (no partial data transfers), critical for synchronized control.

3. Application Layer

Defines data formats and device-specific parameters:

Process Data Objects (PDOs): Real-time I/O data (e.g., sensor values, actuator commands) – small, fixed-size packets for fast transfer.
Service Data Objects (SDOs): Non-cyclic data (e.g., device configuration, diagnostics, firmware updates) – larger packets for parameterization.
Device Profiles: Standardized profiles for common devices (e.g., digital I/O modules, temperature sensors) ensure interoperability across vendors.

Key Features of Interbus

1. Determinism & Real-Time Performance

Interbus uses a rigid cyclic communication schedule, ensuring fixed latency and jitter-free data transfer – essential for time-critical applications (e.g., assembly line synchronization, robotic control).

2. Diagnostics & Fault Tolerance

Comprehensive Diagnostics: The master monitors every slave device and cable segment, providing detailed error information (e.g., “Slave 12: Short circuit in input channel 3” or “Cable break at segment 5”). Diagnostics are transmitted to the controller/HMI for rapid troubleshooting.
Hot Plugging: Slaves can be added/removed from the bus without interrupting communication (supports maintenance during operation).
Redundancy: Optional dual-ring topology for critical applications (e.g., process control) – if one cable fails, data reroutes through the backup path.

3. Interoperability

As an open standard (IEC 61158), Interbus is supported by over 200 manufacturers (e.g., Siemens, Allen-Bradley, Bosch Rexroth). Devices from different vendors can coexist on the same bus without proprietary adapters.

4. Easy Configuration & Wiring

Single Cable: Combines data and power (24V DC) for field devices, reducing wiring complexity and cost.
Topology Flexibility: Linear, star, or mixed topologies (via couplers) adapt to factory floor layouts.
Configuration Tools: Software (e.g., Phoenix Contact’s Interbus Configurator, Siemens Step 7) automates device mapping, parameterization, and diagnostics.

Interbus Communication Cycle

The Interbus cycle follows a fixed sequence to ensure deterministic data transfer:

Master Initiation: The master sends a poll telegram (empty data frame) to the first slave.
Slave Data Append: Each slave adds its input data (e.g., sensor readings) to the telegram and forwards it to the next slave.
Telegram Return: After reaching the last slave, the telegram is sent back to the master, which extracts all slave input data.
Output Data Distribution: The master inserts output data (e.g., actuator commands) into a response telegram and sends it back through the slaves.
Slave Update: Each slave extracts its output data from the telegram and updates its outputs (e.g., activates a valve).
Cycle Completion: The master confirms the cycle and resets for the next poll (cycle time <1ms for small bus systems).

Interbus vs. Other Fieldbus Protocols

Feature	Interbus	Profibus DP	Modbus RTU	EtherNet/IP
Standard	IEC 61158 Type 5	IEC 61158 Type 3	Modbus Organization	IEC 61158 Type 10
Transmission Speed	Up to 2 Mbps	Up to 12 Mbps	Up to 19.2 kbps (RS-485)	Up to 1 Gbps
Max Nodes	254	126	247	1000+
Determinism	High (cyclic)	High (cyclic)	Low (polled)	Medium (CIP Sync)
Topology	Linear/star	Linear/star	Linear	Star (Ethernet)
Diagnostics	Comprehensive	Basic	Limited	Advanced
Typical Use Case	Factory automation, process control	Automotive, manufacturing	Simple I/O, building automation	Industrial Ethernet, smart factories

Applications of Interbus

Interbus is widely used in industries requiring reliable, real-time communication:

Automotive Manufacturing: Robotic assembly lines, paint shops, and conveyor systems (synchronized control of multiple robots).
Process Control: Chemical plants, oil refineries, and water treatment facilities (monitoring/control of valves, pumps, and sensors).
Packaging Machinery: High-speed packaging lines (synchronized actuation of fill valves, sealers, and labelers).
Material Handling: Conveyor systems, palletizers, and automated storage/retrieval systems (AS/RS) in warehouses.
Machine Tools: CNC machines and metalworking equipment (real-time feedback from position sensors).

Advantages & Limitations

Advantages

Deterministic Communication: Fixed cycle times ensure reliable real-time control.
Robust Diagnostics: Simplifies troubleshooting and reduces downtime.
Open Standard: Vendor-neutral, with broad device compatibility.
Simplified Wiring: Single cable for data/power reduces installation costs.