KNX TP · Topology · IEC 14543 · 9 min read

KNX TP Topology: Lines, Areas, Backbone and Line Couplers

KNX TP topology is hierarchical and deterministic — understanding the rules prevents the most common commissioning problems: addressing conflicts, signal corruption and current overload. Get the structure right at design stage and you will not chase intermittent bus errors on site.

The three-level KNX hierarchy

KNX TP uses a three-level physical address hierarchy: backbone, area and line. Each level is connected by a coupler device that filters telegrams and provides electrical isolation. The address format Area.Line.Device (for example 1.2.45) maps directly onto this hierarchy.

Level	Coupler	Max segments	Limits	Address field
Backbone	Area coupler (backbone line)	Up to 15 areas	No device limit on backbone itself	X = 0 (backbone)
Area	Line coupler → Area line	Up to 15 lines per area	15 × 64 = 960 devices per area	X = 1–15
Line	Individual devices on TP	64 devices max per line	350m max point-to-point, 640mA PSU	Y.Z = Line.Device

Maximum theoretical size: 15 areas × 15 lines × 64 devices = 14,400 devices on a single KNX TP installation. In practice, large commercial buildings use an IP backbone instead of extending TP, which is faster and more reliable.

Physical address structure

Every KNX device has a unique physical address (PA) in the format X.Y.Z assigned in ETS. The address encodes exactly where the device sits in the topology hierarchy.

Physical address format — X.Y.Z

X   = Area number        (1–15;  0 = backbone)
Y   = Line number        (1–15;  0 = line coupler on that area line)
Z   = Device number      (1–255; 0 = line coupler itself)

Examples:
  0.0.0   → Unaddressed (factory default — bus will not respond)
  1.0.0   → Line coupler on area 1, backbone connection
  1.1.0   → Line coupler connecting area 1 / line 1
  1.1.1   → First device on area 1, line 1
  1.1.45  → Device 45 on area 1, line 1
  2.3.127 → Device 127 on area 2, line 3

Address conflict

Two devices with the same physical address on the same line cause bus communication errors — both devices respond simultaneously, corrupting telegrams. ETS will show a collision warning during download. Always assign addresses before commissioning on site.

Address 0.0.0 — factory default

All KNX devices ship with address 0.0.0 (unaddressed). ETS cannot communicate with an addressed device at 0.0.0 via normal addressing — use the programming button to put the device into programming mode first, then ETS assigns the PA.

Line physical limits

KNX TP lines have four hard limits. Exceeding any one of them causes intermittent or permanent bus communication failure. These are IEC 14543-3 specification limits, not suggestions.

64 devices per line

Including the power supply. Line couplers count as devices on the main line, not the sub-line. ETS enforces this in the topology view.

350m between any two devices

Maximum cable distance measured along the cable between the two furthest devices on the same line segment — not the total cable length.

700m total cable per line

Sum of all main cable + all stub/spur branches on the line. Exceeding this causes signal reflections and timing errors.

640mA current from KNX PSU

Sum of all device current draws on the line must not exceed 640mA (the maximum rated output of a standard KNX 29V power supply). Check each device datasheet in ETS.

Maximum spur length: 30m from the main cable run to any single device on a stub branch. Star topology from a central panel is allowed, but each spur must be under 30m and the total cable budget still applies.

KNX TP cable specification

The standard KNX TP cable is YCYM 2×2×0.8mm — a four-conductor shielded twisted pair in a green outer sheath. Each pair is colour-coded for a specific function and polarity must be maintained throughout.

YCYM 2×2×0.8 conductor assignment

Pair 1 — KNX bus (green/yellow pair):
  Red    → KNX bus + (positive, 29V DC)
  Black  → KNX bus − (negative, 0V)

Pair 2 — spare (red/white pair):
  Can be used for:
    • Auxiliary 24V DC power to devices without own PSU
    • Additional data channel (non-KNX)
    • Spare for cable fault workaround

Shielding:
  Connect shield to earth at ONE end only (panel end)
  Never connect at both ends — creates a ground loop

Never reverse polarity on the KNX bus pair. Some older devices are polarity-insensitive, but many modern KNX TP devices (especially secure-certified ones) require correct +/- orientation. Standardise on red = + from the panel outward.

Line couplers: filtering and isolation

A line coupler (LC) is a KNX TP device that bridges two line segments. It has its own physical address (for example 1.1.0 for the coupler between area 1 backbone and line 1), a group address filter table downloaded from ETS, and provides galvanic isolation between segments.

Line coupler modes

Filter on group address table: only telegrams with group addresses in the coupler's filter table are forwarded. Default and recommended — reduces bus load on each line.
Route all (transparent): all telegrams pass in both directions. Used during commissioning or when filter table is not yet configured. Never leave in production — causes unnecessary bus load.
Block all: no telegrams forwarded. Useful for isolating a faulty line segment during troubleshooting.

Specified line couplers

MDT SCN-LC0160.01: DIN rail, 2TE, supports 1,000 group addresses in filter table, ETS6 direct
Hager TXS571D: DIN rail, 4TE, line coupler + area coupler configurable, 256 filter entries
Siemens 5WG1 140-1AB02: DIN rail, 2TE, galvanic isolation, 1,000 entries, KNX certified

Line coupler vs line repeater

Line coupler and line repeater look similar on a wiring diagram but have fundamentally different functions. Choosing the wrong one is a common design error.

Feature	Line Coupler	Line Repeater
Physical address	Yes — e.g. 1.1.0	No — transparent to topology
Group address filter	Yes — ETS filter table	No — all telegrams pass both ways
Topology separation	Yes — independent segments	No — same logical line
Bus isolation	Yes — galvanic isolation	Partial — electrical buffering only
Extends line length	Yes (+350m new segment)	Yes (+350m same segment)
Fault isolation	Yes — fault on sub-line is isolated	No — fault propagates both ways
ETS programming access	Yes — individual address	Not required
When to use	Topology separation between floors/zones	Physical line extension only, no topology change

Use a line repeater only when you need to extend the cable run beyond 350m on the same logical line. Use a line coupler whenever you want topology separation, fault isolation, or filtered telegram forwarding between zones. A repeater does not create a new line — the device count still applies to the combined segment (max 64 total).

IP area coupler

A KNXnet/IP router functioning as an area coupler replaces the TP backbone with standard Ethernet. Each IP router has a TP side (connecting to a TP line) and an IP side (connecting to the LAN backbone). Logically, it behaves identically to a TP area coupler — it has a physical address and a group address filter table — but the backbone transport is IP multicast instead of twisted pair.

MDT KNX IP Router SCN-IP200.02

• Acts as both line coupler (TP side) and IP backbone participant
• Physical address: 1.0.0 (area coupler) or 1.1.0 (line coupler) depending on configuration
• Group address filter table: 1,000 entries — sufficient for large commercial projects
• 4 simultaneous ETS6 tunneling connections for engineering access
• KNXNET/IP Secure (AES-128 routing) supported with ETS6 Secure project
• DIN rail 2TE — fits in standard KNX panel alongside MCBs

Power supply sizing

Every KNX TP line requires at least one 29V DC power supply. Current draw is the sum of all connected devices. Always add a 20% safety margin and never mix two power supplies on the same line segment without a choke or galvanic separator.

Power supply calculation method

Step 1: List all devices on the line with current draw from ETS datasheet
Step 2: Sum all current draws (mA)
Step 3: Add 20% margin
Step 4: Select PSU rated at or above the result

Example — 28-device office line:
  12 × MDT SCN-BMK2.01 (binary input)  = 12 × 5mA  =  60mA
   8 × MDT AKD-0424.02 (4-fold actuator)= 8 × 12mA  =  96mA
   4 × Gira 509000 (push-button sensor)  = 4 × 8mA   =  32mA
   4 × MDT STC-0321.02 (DALI gateway)   = 4 × 25mA  = 100mA
                                          Sum         = 288mA
                                          +20%        = 346mA
   → Select MDT STC-0640.01 (640mA) — comfortable margin

PSU rating	Typical device count	Common model	Use case
160mA	~8 devices	MDT STC-0160.01	Small residential line, motion sensors only
320mA	~16 devices	MDT STC-0320.01	Medium residential line, mixed devices
640mA	32+ devices (full line)	MDT STC-0640.01 (most common)	Commercial lines, DALI gateways, actuators

Common topology mistakes

More than 64 devices on one line without line coupler

Fix: Insert a line coupler at device 64. The sub-line becomes a separate logical line (e.g. 1.2.x → 1.3.x). Add a second power supply on the new sub-line.

Cable run exceeding 350m point-to-point

Fix: Insert a line repeater at the midpoint to regenerate the signal. Total cable budget per segment is still 700m, and device count on the combined segment is shared.

Power supply at the end of the line

Fix: Place the power supply in the middle of the cable run (or as close as possible). This halves the effective cable distance to the furthest device and distributes voltage drop symmetrically.

Two power supplies on the same segment without choke

Fix: Either add a KNX choke (e.g. MDT STC-00CH.01) between the two PSUs to isolate their DC outputs, or use a galvanic separator. Two PSUs without isolation fighting each other cause current loops and voltage instability.

Line coupler left in 'route all' mode in production

Fix: Configure the group address filter table in ETS and set the coupler to 'filter' mode. Route-all mode means every telegram on every line is flooded to every other line — wastes bus bandwidth and masks real problems.

Spur branches longer than 30m

Fix: Re-route cable so the stub is under 30m. If the device location cannot change, run a new segment from the nearest suitable point on the main cable — not from the device itself.

Related guides

KNX Networks

KNXnet/IP Routing: IP Interfaces vs IP Routers