medium Modbus exam questions

Medium level questions

1. Q01

   A Modbus RTU bus on RS-485 supports up to 32 nodes per segment (extendable with repeaters), requires 120 ohm termination resistors at both ends of the bus, and biasing resistors to define the idle state.

   TrueFalse

   3. Modbus RTU· RS-485 Multi-Drop· Medium

   ▸

   Correct answer

   True

   Learning tip

   The RS-485 standard natively supports 32 nodes per segment using "1 unit load" drivers, and up to 128 or 256 with modern "1/4 unit load" drivers. 120 ohm terminations are required at both ends of the bus (unlike KNX TP1) to match line impedance and prevent reflections. Pull-up/pull-down biasing resistors (typically 680 ohm to 1 k-ohm), usually placed at the master, hold the idle state stable when no node is transmitting. The recommended cable is a shielded twisted-pair with 120 ohm characteristic impedance, and the maximum length is around 1200 m at 9600 baud (less at higher data rates).

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2. Q02

   Which Modbus function code is used to READ the Holding Registers (16-bit read/write registers) of a slave?

   • A.0x01 (Read Coils)
   • B.0x02 (Read Discrete Inputs)
   • C.0x03 (Read Holding Registers)
   • D.0x04 (Read Input Registers)

   4. Function codes· Lesen Coils/Register· Medium

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   Correct answer

   C — 0x03 (Read Holding Registers)

   Learning tip

   0x03 (Read Holding Registers) targets the 16-bit read/write register space and is by far the most used code for reading analogue values such as temperatures, pressures, counters and setpoints. 0x01 is wrong because Read Coils returns 1-bit read/write outputs, not 16-bit registers. 0x02 is wrong because Read Discrete Inputs returns 1-bit read-only digital inputs. 0x04 is wrong because Read Input Registers targets a separate 16-bit READ-ONLY register table, not the Holding Registers. The full picture includes 0x05/0x06 for single writes and 0x0F/0x10 for multiple writes.

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3. Q03

   Modpoll (focus-sw.com) and mbpoll (Linux/macOS) are free command-line tools used to query a Modbus slave from a PC, indispensable for Modbus diagnostics and integration commissioning.

   TrueFalse

   6. Practical· Modpoll / mbpoll· Medium

   ▸

   Correct answer

   True

   Learning tip

   The standard set of field diagnostic tools for Modbus includes Modpoll (focus-sw.com, Windows/Linux) -- a free command-line utility for Modbus RTU and TCP; mbpoll (sourceforge, Linux/macOS) -- the open-source equivalent; QModBus -- a cross-platform GUI; and CAS Modbus Scanner -- a Windows GUI. Their typical use cases are answering questions like "is this slave responding?" or "what value is currently in its register 100?". They are indispensable when commissioning a KNX/Modbus gateway, and fluency with mbpoll is essentially a baseline expectation for BMS integrators.

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4. Q04

   Most modern industrial variable-frequency drives (VFDs) such as ABB ACS, Schneider Altivar, Siemens SINAMICS and Danfoss VLT support Modbus RTU and/or TCP as a control interface, with a register map documented by the manufacturer.

   TrueFalse

   11. Practical· Antriebsanbindung· Medium

   ▸

   Correct answer

   True

   Learning tip

   The standard "drive over Modbus" pattern is: the drive exposes registers for start/stop command, speed setpoint, current reading, speed reading and fault status; a Modbus master (PLC or BMS) writes the commands and reads the feedback. A typical cycle is 100-500 ms for slow applications (water pumps) and 10-50 ms for faster ones (machine spindles). The reference document is the vendor "Modbus mapping table" or "Communication protocol manual". Always verify the byte order and the available sub-functions before integration.

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5. Q05

   The typical Modbus 'hub and spoke' architecture has a central master PLC periodically polling N RTU/TCP slaves and aggregating the data, which it then forwards to the BMS over a different protocol (BACnet, OPC UA, MQTT), enabling scaling without loading the BMS directly.

   TrueFalse

   22. Architecture· Hub-and-Spoke· Medium

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   Correct answer

   True

   Learning tip

   The spokes are the N Modbus slaves (meters, drives, instruments), the hub is the PLC or smart gateway that polls and aggregates, and the backend is the BMS or SCADA reached through a 'modern' protocol such as BACnet or OPC UA. The advantages are decoupling (the BMS is never directly exposed to Modbus), performance (local polling is much faster than reaching slaves across the Internet), security (the hub can act as a firewall and sanitisation point) and scalability (adding slaves is transparent on the BMS side).

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6. Q06

   Smart pumps such as the Grundfos MAGNA3 and the Wilo Stratos MAXO natively support Modbus RTU/TCP, exposing around 50 parameters (power, flow, temperature, status, alarms) and accepting commands (speed, mode), which enables BMS integration without a separate external drive.

   TrueFalse

   24. Use cases· Pumpen und Antriebe· Medium

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   Correct answer

   True

   Learning tip

   Several pump ranges embed Modbus directly: Grundfos MAGNA3 / SCALA2 (Modbus integrated), Wilo Stratos MAXO (Modbus and BACnet), KSB MyFlow / Ecoflow (Modbus). Typical exposed measurements are instantaneous power (W), flow (m^3/h), pump head (mCE), fluid temperature, operating hours and start counters, and alarm codes. The control side typically offers mode selection (Auto, Constant Pressure, etc.) and a setpoint. This avoids fitting an external VFD for pumps up to 7.5 kW.

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