KNX bus voltage: specification and measurement

The KNX TP bus operates at 29V DC (nominal). Voltage is superimposed with a differential signal at 9600 bit/s. The bus voltage is the first diagnostic measurement because it immediately identifies the most serious fault classes: no power, short circuit and overload.

Measurement point: KNX bus terminal block in the panel
  → Between KNX bus + (red) and KNX bus − (black) terminals
  → At the patch point closest to the power supply
  → Then at the furthest device on the line (check voltage drop)

Tool: Digital multimeter, DC voltage range 50V or 100V
  → Acceptable voltage drop along the cable: max 2V
  → If drop > 2V: cable too long, too thin, or too many joints

Never measure bus voltage with the bus carrying live telegrams
using an analogue meter — the signal will deflect the needle
and give a false low reading. Use a true-RMS digital meter.

Bus current budget calculation

Every KNX device draws current from the 29V bus power supply. The sum of all device currents must not exceed 75% of the PSU rated output — the safety margin accounts for inrush current during device startup and bus signal amplitude requirements.

Individual device current draws: simple binary inputs draw 5–8mA; push button interfaces draw 5–12mA; actuators (4-fold, 8-fold) draw 10–20mA; DALI gateways draw 20–30mA; IP routers draw up to 50mA. Always check the device datasheet in ETS6 — the current draw is listed in the device properties window.

Line 1.1 — Floor 1 lighting and blinds (28 devices):
  Device type                     Qty   mA each   Total
  ─────────────────────────────────────────────────────
  MDT BMK2.01 (binary input 2ch)   8  ×   5mA  =  40mA
  MDT AKD-0424.02 (4ch actuator)   4  ×  12mA  =  48mA
  MDT JAL-0410.01 (4ch shutter)    2  ×  20mA  =  40mA
  Gira 509000 (4-button sensor)    6  ×   8mA  =  48mA
  MDT STC-0321.02 (DALI gateway)   2  ×  25mA  =  50mA
  MDT SCN-IP200.02 (IP router)     1  ×  40mA  =  40mA
  ─────────────────────────────────────────────────────
  Total                            23          = 266mA
  Safety margin (+33%)                         = 354mA
  Selected PSU: MDT STC-0640.01 (640mA)       ← comfortable margin

Diagnostic tools: selection guide

Telegram tracing with ETS6 Bus Monitor

The ETS6 Bus Monitor (available in ETS Professional) captures every telegram on the bus including acknowledgement frames, repeat requests and NAK responses. This level of detail is essential for diagnosing application-incompatibility errors and timing issues.

Telegram field structure:
  Time      Source PA   Dest GA     APDU     DPT value   Flags
  ─────────────────────────────────────────────────────────────
  12:04:01  1.1.5       1/1/1       Write    1 (On)      C W -
  12:04:01  1.1.10      1/1/1       Write    1 (On)      C - T   ← actuator confirms
  12:04:03  1.1.5       1/1/2       Read     ?           C - R   ← polling status
  12:04:03  1.1.10      1/1/2       Response 1 (On)      C - T

Key fields:
  Source PA   → Individual address of the sending device
  Dest GA     → Group address the telegram is directed to
  APDU type   → Write (command), Read (poll), Response (reply to poll)
  DPT value   → Decoded data value — verify against expected DPT
  Flags       → C = Communication; W = Write; R = Read; T = Transmit

Repeat telegrams — warning sign:
  If same telegram appears 3 times in sequence → device did not
  acknowledge → possible bus collision or device unresponsive
  → Check individual address, re-download application

Filter by group address (Group Monitor filter bar):
  1/*         → All telegrams to Main group 1
  1/1/*       → All telegrams to Main 1, Middle 1
  1/1/1       → Exactly group address 1/1/1

Filter by source individual address:
  src:1.1.5   → Only telegrams sent from device 1.1.5
  src:1.1.*   → All devices on line 1.1

Combined filter:
  src:1.1.5 1/1/*   → Telegrams from push button 1.1.5 to lighting GAs

Filter by DPT:
  dpt:1.001   → Only boolean switch telegrams (on/off)
  dpt:9.*     → All 2-byte float telegrams (temperature, etc.)

Exclude heartbeat spam:
  !src:0.0.1  → Exclude telegrams from NTP time source

Common fault diagnosis

Hardware bus analyzers: MDT and Lingg & Janke

Software-only monitoring requires a laptop permanently connected to the installation. Hardware bus analyzers operate standalone: they connect directly to the KNX TP bus, capture all telegrams to an SD card or internal memory, and can be left overnight to capture intermittent faults that would never appear during a daytime site visit.

EMC interference identification and mitigation

Electromagnetic interference is the most insidious KNX fault source because it produces intermittent errors that vary with load conditions and time of day. The three most common EMC sources in building installations are leading-edge dimmers, variable frequency drives (VFDs) on HVAC motors, and LED drivers without EMI filtering.

Short circuit isolation: step-by-step

A KNX bus short circuit drops the line voltage to 0V and takes the entire line offline. The PSU enters current-limiting mode (indicated by the overload LED on most MDT and Siemens PSUs). Use the segment disconnection method to locate the fault without an oscilloscope.

Step 1: Confirm short circuit
  → Measure bus voltage at panel terminal: 0V with PSU LED on
  → PSU rated current exceeded: overload indicator lit

Step 2: Disconnect all KNX spur terminals at the panel
  → Remove all KNX TP cables from the distribution terminal block
  → Measure voltage at PSU output: should return to 29V
  → If still 0V: fault is in panel wiring or PSU itself

Step 3: Binary search — reconnect half the spurs
  → Reconnect spurs 1–4 (out of 8 total)
  → Measure bus voltage:
    → Drops to 0V: fault is on spurs 1–4 → reconnect one by one
    → Stays at 29V: fault is on spurs 5–8 → reconnect and test

Step 4: Repeat until fault spur is identified
  → Reconnect cables one by one on the fault spur
  → Voltage drops to 0V when faulty cable is reconnected

Step 5: Trace the faulty cable run
  → Disconnect devices one by one from the faulty cable run
  → Voltage recovers when faulty device or junction box is found
  → Common fault locations: pinched cable at conduit entry,
    reversed polarity at device terminal, damaged cable at bend

Step 6: Line coupler isolation (multi-line installations)
  → If line coupler is in-circuit: set coupler to Block All mode
  → This isolates the sub-line without cutting bus continuity
  → Use ETS6 to put line coupler in block mode remotely
    if the backbone is still live