Cheatsheet
Distances and limits of the KNX TP1 bus
All critical lengths on a single sheet
3 minFree accesstopologiecabledistancesegment
Memorisable in 3 minutes. Keep at hand during the Basic exam (open book) and to check your wiring diagrams at the design stage.
⚡ The 4 figures to know by heart
| Limit | Value | Mnemonic |
|---|---|---|
| Max distance device ↔ power supply (PSU) | 350 m | "350 = one kilometre across 3 return paths" |
| Min distance between 2 PSUs on the same line | 200 m | "200 minimum so they don't interfere" |
| Max distance between any 2 devices | 700 m | "700 = 2 × 350, one device either side of the PSU" |
| Max cumulative length of a segment | 1 000 m | "1 km per segment, period" |
🧮 Structural limits
| Element | Maximum | Notes |
|---|---|---|
| Devices per segment (without repeater) | 64 | Native TP1 limit |
| Segments per line (with repeaters) | 4 | I.e. 4 × 64 = 256 devices/line max |
| Lines per area | 15 | Linked via Line Couplers |
| Areas per installation | 15 | Linked to the backbone via Area Couplers |
| Backbone | 1 | Area "0" (reserved) |
| Total max devices without repeaters | 14 400 | 15 × 15 × 64 |
| Total max devices with repeaters | 57 600 | 15 × 15 × 256 |
🔌 Standard bus cable
| Characteristic | Value |
|---|---|
| Type | J-Y(St)Y 2 × 2 × 0.8 mm² |
| Conventional colour | Green (but not mandatory by standard) |
| Polarisation | Yes (red = +, black = −) |
| Voltage | 30 V DC SELV |
| Isolation from 230 V | 4 kV (separator or insulated sheath required) |
| Coexistence with SELV cables | Allowed (telephone, alarm, other buses) |
| Bus termination | None required (≠ RS-485) |
| Permitted topology | Line, star, tree — closed loop forbidden |
| Min bending radius | ~8 × outer diameter |
⚠️ Classic pitfalls in exams / on site
Pitfall 1 — Confusing 350 / 700 m
- 350 m = device ↔ PSU (the most quoted)
- 700 m = between any two devices (often confused with the previous one, classic T/F exam item)
Pitfall 2 — "Segment" vs "Line"
- Segment = physical portion with max 1 PSU + ≤ 64 devices
- Line = logical entity (ID in the ETS topology), can contain 1 to 4 segments
- The individual address
Z.L.Didentifies the line, not the segment
Pitfall 3 — Dedicated PSU
- A bus PSU must be dedicated (with choke to decouple signal/power)
- NEVER use a bus PSU to power an external 24 V actuator
- NEVER use a generic 30 V PSU without a choke (telegrams won't pass)
Pitfall 4 — Polarity
- The bus IS polarised: swapping red/black prevents communication
- Some modern devices have protection but it is not standard
💡 Key takeaway
350 / 200 / 700 / 1000 — four figures, memorise them in order "device ↔ PSU, two PSUs min, two devices max, one segment". Everything else follows.
Sources:
- KNX Specifications V3.0.0 (public)
- Hager BAC071, Porto KNX Basic Course
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