Practical Modbus: exam questions with worked answers

Questions for the "Practical" topic

1. Q01

   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

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   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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2. Q02

   Modbus (RTU and TCP) has no native security: no authentication, no encryption, no signatures. The accepted good practice is strict OT VLAN isolation, an industrial firewall and no direct internet exposure.

   TrueFalse

   6. Practical· Cybersecurity· Hard

   ▸

   Correct answer

   True

   Learning tip

   Modbus was designed in 1979 with no security in mind, so any direct internet exposure brings: spoofing (anyone on the network can issue a Write Register), sniffing (all frames are in clear text) and denial of service (flooding requests saturates the slaves). The hard truth is that a Modbus Secure variant (TCP over TLS, adopted in 2018) does exist but its deployment is extremely limited because field devices do not support it. The industry standard remains network-level isolation through industrial firewalls (Hirschmann, Phoenix mGuard, Siemens SCALANCE), no internet exposure, and dedicated OT monitoring (Claroty, Nozomi).

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