KNX Modbus Gateway: Weinzierl KNXGW-MODBUS und Register-Mapping

KNXGW-MODBUS Spezifikationen

Interface:
  KNX TP bus connector (bus-powered, 10mA)
  RS-485 Modbus RTU: A, B, GND screw terminals
  Modbus TCP: RJ45 Ethernet (on -TCP variant)

Modbus RTU parameters:
  Baud rate: 9600 / 19200 / 38400 baud
  Parity: None / Even / Odd
  Stop bits: 1 / 2

Capacity:
  Up to 512 Modbus registers mapped to KNX objects
  Up to 32 Modbus slave devices on RS-485 bus
  Slave address range: 1 – 247

Configuration:
  ETS6 only — download product database from weinzierl.de
  No separate software required
  Parameters and group objects configured in ETS6

ETS6-Parameterseiten

General parameters (applies to all slaves on RS-485):
  Baud rate:   9600 baud (EM340 default), or 19200/38400
  Parity:      Even (EM340 default), None, or Odd
  Stop bits:   1 (standard for Even parity)

Per-slave parameters (one set per connected device):
  Slave address:    1 – 247 (must match device front panel)
  Slave type:       RTU (RS-485) or TCP (Ethernet)
  IP address:       Required for TCP slaves only

Per-register parameters (one entry per mapped register):
  Register address: Decimal or hex, 0-based or 1-based
  Register type:    Holding (0x03) / Input (0x04) / Coil
  Data type:        INT16 / UINT16 / INT32 / UINT32 / FLOAT32
  Byte order:       Big-endian / Little-endian / Byte-swapped
  KNX DPT:          DPT 9.001, DPT 12.001, etc.
  Polling interval: 1s – 3600s

Polling- und Totband-Strategie

Polling interval recommendations:
  Active power (kW):    5s – 10s  (fast-changing, demand monitoring)
  Energy total (kWh):   60s       (slow-accumulating)
  VFD speed feedback:   2s        (control feedback)
  Temperature sensors:  30s       (slow physical process)

Deadband recommendations (prevents telegram storms):
  Temperature:    ±0.2°C  (avoids 0.1°C floating-point noise)
  Active power:   ±10W    (noise on lightly loaded circuits)
  Energy total:   ±0.01kWh (send every 36Wh increment)
  VFD speed %:    ±0.5%   (avoids resolver dither noise)

Without deadband on a noisy FLOAT32 register:
  Gateway sends a new KNX telegram every poll cycle
  even when the physical value has not changed.
  On a 512-signal configuration at 1s polling,
  this can saturate the KNX TP bus at 9600 baud.

RS-485-Verdrahtungsregeln

Signal polarity (EIA-485 standard):
  A terminal = non-inverting (+)  →  positive when marking (idle)
  B terminal = inverting (-)      →  negative when marking (idle)

Termination resistors:
  120 Ω resistor at BOTH physical ends of the bus only
  Never terminate in the middle of the bus
  Total bus impedance = 60 Ω when both terminations installed
  Most Weinzierl gateways have built-in switchable termination

Cable requirements:
  120 Ω characteristic impedance twisted pair
  Belden 9841 or equivalent (22 AWG, shielded)
  Shield: drain both ends to panel earth (same earth reference)
  Maximum bus length: 1200m at 9600 baud (EIA-485 spec)
  Maximum 32 standard unit loads (UL) without repeater

WAGO polarity note:
  WAGO 750-653 Modbus RTU coupler labels
  A and B REVERSED vs EIA-485 standard.
  A(WAGO) connects to B(EIA-485) and vice versa.
  Always verify with oscilloscope on first connection.

Inbetriebnahmesequenz für Multi-Device-Bus

Pre-connection preparation:
  1. Assign unique slave addresses 1–247 to each device
     via front panel programming (DIP switches or menu)
     BEFORE connecting to shared bus
  2. Verify each device address with a Modbus RTU tester
     connected directly to the individual device
  3. Document address assignments in panel schedule

Connection sequence:
  1. Connect gateway to device 1 only (install 120Ω at both ends)
  2. Verify gateway polls device 1 successfully in ETS6 diagnostics
  3. Add device 2 (remove termination from device 1 if not end of bus)
  4. Verify both devices poll successfully
  5. Repeat for each additional device

Bus capacity (EIA-485, 9600 baud):
  Maximum 32 unit loads (UL) — one UL per standard transceiver
  Many modern meters present 1/4 UL → up to 128 devices possible
  Verify UL rating in device datasheet before exceeding 32 devices
  Use RS-485 repeater (e.g. Weidmuller RS-485 REP) if needed