KNX TP bus: no termination resistor, no ring — ever
Installers coming from RS-485 or structured cabling instinctively want to close the loop for redundancy. On KNX TP, that instinct wires a fault straight into the bus.
Why the topology can't be a ring
Trace the trunk, not the loop
Walk the wiring as a trunk with branches feeding off it — a true line/tree. If two ends of the bus ever meet back at a device or DIN-rail terminal block, that's a closed loop, not a redundant path.
Understand why there's nothing to terminate
KNX TP1's twisted pair carries the ~29-30V DC bus supply and the differential data telegram together. Medium access is built around that single shared pair in a line/tree, not a matched transmission line like RS-485 — an end-of-line resistor has no function there.
Check polarity before power-up
A reversed red/black pair is the most common wiring mistake on a new line. Confirm polarity at every device and coupler before you energise the bus, not after devices start behaving oddly.
Verify the topology before ETS download
Walk the installation once more against the as-built plan: no ring, no star-with-loop, spare pair either idle or dedicated to a second independent line. Catching a closed loop here is minutes; catching it after commissioning starts is hours.
What actually happens on a closed loop
One pair, two jobs
The KNX TP1 twisted pair carries bus supply and differential data together. Close a loop and you don't add a redundant path — you create a second current path back to the same supply, and bus arbitration breaks.
Shared power + data pair
Because supply and telegram signal ride the same twisted pair, medium access assumes a single unambiguous path from any device back to the line's supply — not multiple simultaneous paths.
No end-of-line resistor
Unlike some other differential buses, KNX TP has no termination resistor to fit at the line ends — medium access doesn't rely on impedance-matched line ends the way RS-485/Modbus does.
Line/tree only, never a ring
Devices branch off the trunk on stubs; the trunk itself never re-joins itself. Closing a loop, even 'for safety', creates a wiring fault, not a backup path.
Couplers scale the bus properly
Line couplers join multiple lines — each with its own supply — into an area, filtering telegrams between segments; a backbone/area coupler joins areas. That's how you extend reach, not by looping cable.
Separate from 230V, every metre
KNX TP is an SELV/PELV circuit and must run physically separated from mains cabling along the entire route, not just at the panel.
KNX Wiring & Topology Review
Send us the as-built plan, or let our engineers walk the site with you — we check topology, cable spec and polarity before you touch ETS.
- {'t': 'Topology audit', 'd': 'We trace the as-installed trunk and branches on your drawings or on site and flag any closed loop or star-with-loop before commissioning.'}
- {'t': 'Cable & pair check', 'd': 'Confirm certified green KNX bus cable, typically J-Y(St)Y 2x2x0.8mm, with the correct pair carrying power+data and the spare pair used or left correctly.'}
- {'t': 'Polarity verification', 'd': 'Point-by-point check for reversed red/black connections at devices, couplers and DIN-rail terminals before the bus is energised.'}
- {'t': 'Line/area coupler planning', 'd': 'Sizing and placement of line couplers, area couplers and per-line power supplies so the network scales without a topology rebuild.'}
- {'t': 'KNX component supply', 'd': "PSUs, line/area couplers, actuators and accessories from our catalogue, matched to the topology you're actually building."}
Frequently asked questions
Does KNX TP need a termination resistor?
No. KNX TP1's medium access doesn't rely on impedance-matched line ends the way RS-485 or some other differential buses do, so there's no end-of-line resistor to fit — adding one has no function on a KNX line.
Can I wire a KNX bus as a ring for redundancy?
No. A closed ring or star-with-loop creates a second current path back to the same shared power+data pair, which conflicts with normal bus operation and breaks arbitration — it's a wiring fault, not a redundancy feature.
What does correct KNX TP topology look like?
A trunk (the line) with devices branching off on stubs — a line/tree structure. Multiple lines, each with their own supply, connect through line couplers into an area; areas connect through a backbone/area coupler.
What happens if a loop gets wired in by accident?
Symptoms during commissioning: devices that don't respond consistently, intermittent bus errors, or a line that behaves fine with some devices powered and fails once more branches are connected — check topology before you get there, not after ETS reports errors.
Why is there a second pair in KNX bus cable if it's not for termination?
The second pair in certified KNX cable (commonly white/yellow) is a spare — it can carry a second, independent bus line in the same cable jacket, or simply be left unused. It has no termination function.
Get your KNX topology checked before ETS
Send your wiring plan to our engineers — we'll flag loop risks, polarity errors and cable spec issues before commissioning starts.