What is a good free alternative to Modbus Poll?

Updated 10 November 2026

Short answer

Modbus Poll is a good, mature Modbus master, and if you live at a Windows desk it is worth what it costs. But it is paid, Windows-only, and a master only — simulating a slave is a second product (Modbus Slave) bought separately — and its free evaluation stops communicating after about ten minutes per session. The free tools people most often use instead are QModMaster, modpoll (command line) and CAS Modbus Scanner. They read and write registers fine. What none of them can do is tell you what a register means — for that you need the device's register map.

What people are really asking

“Modbus Poll alternative” is almost always one of three wishes: I don't want to pay for it, the trial keeps disconnecting, or I want it to also decode the value, not just show me a raw register. It is worth being clear which one you have, because the honest answer is different for each.

The ten-minute trial

Modbus Poll's free download is a fully featured evaluation that disconnects after roughly ten minutes, so you reconnect to keep going. That is how the trial is meant to work, not a bug, and there is no legitimate way round it other than buying the licence — which is the right call if it is your daily desk tool.

The free tools people compare

If you specifically want a free desktop master, the names that come up most are QModMaster, modpoll (a command-line tool) and CAS Modbus Scanner. They read and write fine. What they mostly do not do is the part that eats your afternoon: telling you that a raw 0x2A3D across two registers is 12.75 as a word-swapped float rather than nonsense. See why your Modbus value looks wrong for why that step matters.

The part no tool does for you

Every Modbus master, paid or free, shows you the contents of a numbered slot. None of them knows what the slot is. That knowledge lives in the manufacturer's register map, so the fastest way to a working setup is usually to find the map first:

The free register-map library lists verified addresses, data types, word order and scaling for common meters, drives, generator controllers, inverters and HVAC equipment.
The float and byte-order decoder shows two raw registers in all four word orders at once.
The address converter turns a 40001-style address into the 0-based number your tool sends.

This is not a “Modbus Poll is bad” page. It is mature, fast, scriptable, and excellent for desk and bench work, and if that is your world the licence pays for itself.