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.
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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.
Keep a one-line mbpoll command in your laptop notes that targets your most common test slave: a single ENTER on site beats firing up a full SCADA package just to check connectivity.
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