hard KNX Basic exam questions

Hard level questions

1. Q01

   On how many bits is the Group Address encoded inside a KNX TP1 telegram?

   • A.8 bits
   • B.16 bits
   • C.24 bits
   • D.32 bits

   2. TP Topology· Kodierung der Gruppenadresse· Hard

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   Correct answer

   B — 16 bits

   Learning tip

   The Group Address is encoded on 16 bits in the TP1 telegram, regardless of the display structure chosen in ETS (Free, 2-level, 3-level). The display structures merely split the same 16 bits differently for human readability: Free uses 16 free bits, 2-level uses Main (5 bits) / Sub (11 bits), 3-level uses Main (5 bits) / Middle (3 bits) / Sub (8 bits). The wire-level telegram is identical in every case. A and C are wrong because 8 bits would be far too few and 24 bits do not appear in the TP1 frame. D is wrong because 32 bits is the Individual+Group total in some Extended-frame variants, not the Group Address alone.

   Source: KNX Specifications V3.0.0 (public) — Telegram Format, Group Address field

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2. Q02

   The Group Address space in KNX TP1 allows up to 65 536 different Group Addresses (2^16).

   TrueFalse

   2. TP Topology· Adressraum Gruppenadressen· Hard

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   Correct answer

   True

   Learning tip

   A Group Address is encoded on 16 bits inside the telegram, giving 65 536 possible values. In practice, the address 0/0/0 is reserved as a broadcast, so 65 535 addresses remain available for application use. Whether the project uses the 2-level (main/sub) or 3-level (main/middle/sub) representation does not change the underlying 16-bit address space — it only changes how the bits are visually grouped in ETS.

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3. Q03

   During a download, ETS returns the error "Address conflict". What is the most likely cause?

   • A.The target device's individual address is already in use by another device on the bus
   • B.The target device has no power supply
   • C.The line coupler's filter table is blocking the download
   • D.The bus cable polarity is reversed

   8. Diagnostics· Adresskonflikt· Hard

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   Correct answer

   A — The target device's individual address is already in use by another device on the bus

   Learning tip

   An "Address conflict" during a download means that two devices respond to the same individual address. This typically happens when a device already installed has the same address as the new one being programmed, or when a device has been duplicated in the project. Choice B (no power supply) would yield a timeout error, not a conflict. Choice C is incorrect because filter tables apply to Group Addresses, not to point-to-point downloads. Choice D is intrinsically wrong because the KNX TP bus cable is non-polarised, so polarity cannot be "reversed".

   Source: ETS troubleshooting documentation

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4. Q04

   The FDSK (Factory Default Setup Key) of a Secure device is a factory key, printed on the device as a QR code, that is used when the device is first integrated into ETS.

   TrueFalse

   9. KNX Secure· FDSK· Hard

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   Correct answer

   True

   Learning tip

   The FDSK is printed on the label of every KNX Secure device, often as a QR code that can be scanned. When the device is added to the project, ETS asks for the FDSK to authenticate the take-over. Once the device is in operation, ETS derives and uses runtime keys stored in the ETS keyring (a password-protected file). The FDSK itself must be kept safe — losing it makes later recovery of the device significantly more complex.

   Source: KNX Support — Data Secure

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5. Q05

   Activating Data Secure on a Group Address adds a per-telegram overhead (authentication and encryption) that reduces the usable application payload.

   TrueFalse

   9. KNX Secure· Data-Secure-Overhead· Hard

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   Correct answer

   True

   Learning tip

   True. Each Data Secure telegram carries additional bytes for the sequence number, the MAC for authentication and the encryption envelope. Those bytes occupy room inside the standard telegram and therefore shorten the application payload that can be transported in a single frame. For long DPTs such as 14-character DPT 16 strings, this overhead can force fragmentation into multiple telegrams, which is an important sizing consideration for high-rate visualisations.

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6. Q06

   In an installation predating 2018, a maximum of 64 sensors, actuators, line couplers and backbone couplers can be connected to a TP main line.

   TrueFalse

   2. TP Topology· Offizielle Musterprüfung 4· Hard

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   Correct answer

   True

   Learning tip

   True. Before the 2018 update of the KNX TP specification, a main line was limited to a single segment of 64 devices (sensors, actuators, line couplers and backbone couplers combined), with no possibility to extend it via repeaters as a regular line could. After 2018, this restriction was lifted and the modern rule allows up to 256 devices in total per main line, in line with the rest of the topology. Legacy installations therefore still operate under the older 64-device cap.

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