Overview
ISO/IEC 14908-2:2012 specifies the physical-layer channel for the Control Network Protocol (CNP) over a free‑topology twisted‑pair medium. It defines the complete OSI Layer 1 for networked control systems used in local area control networks and is intended to be used alongside ISO/IEC 14908-1 (protocol stack). The channel supports data transmission at 78,125 kbit/s between multiple nodes and includes detailed requirements for cabling, terminations, link power, node characteristics and repeater behaviour.
Key topics and technical requirements
- Channel & data rate: Free‑topology twisted‑pair channel using Differential Manchester encoding (polarity‑insensitive) at 78,125 kbit/s.
- Node composition: Each node typically includes a transceiver, protocol processor, application processor, power supply and application electronics.
- Topology and distances:
- Supports free, bus, star, loop or mixed topologies.
- Free‑topology: maximum node‑to‑node distance 250 m, maximum total wire length 450 m.
- Bus: maximum bus length 600 m, maximum stub length 3 m.
- Node counts & power:
- Up to 128 nodes on a segment (link‑powered) or 64 locally‑powered nodes, with mixed configurations constrained by (1 × #link) + (2 × #local) ≤ 128.
- Maximum steady‑state power draw per segment 36.5 W.
- Link power:
- Power delivered on the same two conductors as data (link‑powered nodes).
- Link power source = passive coupler + power supply. Differential DC output required 41.0 V – 42.4 V; normally centered to produce +21 V / –21 V relative to earth.
- Link power source must tolerate output shorts and recover/restart the network.
- Termination & repeaters:
- Specified RC termination networks for free vs bus topologies (e.g., R1 values given in the standard).
- Repeaters may interconnect segments; single repeater permitted in a path with delay ≤ 36 µs.
- Standards compliance: Cable shall conform to ISO/IEC 15018 (generic cabling for homes) section referenced.
Applications
ISO/IEC 14908-2 is aimed at low‑speed local area control and automation where reliable, interoperable wiring and link power are required. Typical applications include:
- Building automation (HVAC, lighting, blinds)
- Local area control systems in commercial/residential buildings
- Distributed sensor and actuator networks
- Industrial local control segments where free‑topology wiring and mixed power schemes are useful
Who should use this standard
- System architects and control network designers
- Device manufacturers and transceiver vendors
- Installers, integrators and commissioning engineers
- Specification writers and procurement teams for building automation projects
Related standards
Keywords: ISO/IEC 14908-2:2012, control network protocol, twisted pair communication, CNP, physical layer, link power, Differential Manchester, free‑topology, building automation.