Modbus, explained for people who have to deal with it

Start here

If Modbus is new to you, read what is Modbus? first. It explains the one thing that causes most of the difficulty: a Modbus device will happily tell you the contents of slot number 7, and will never tell you what slot 7 is.

Using the app

• How do I use Easy Modbus to read values from a device?
• How do I write to a register, and put it back afterwards?
• How do I build a custom remote for a device?

Guides

What the app does

• Finds equipment — sweeps the local network for anything answering Modbus, because Modbus has no discovery of its own. Or add a device by typing its address.
• Two modes. Easy mode asks what kind of reading you are after — a temperature, a kilowatt figure, an on/off status — and picks the settings for you. Pro mode exposes raw registers in hex, word order, function codes and the frame log.
• Works out what registers mean — reads a block, shows it in hex and decimal, and offers the plausible interpretations with the reasoning, so a wrong guess is visibly wrong.
• Remembers the map — name a reading once and the address, data type, word order, scaling and units are saved for next time.
• Imports and exports — paste a vendor's CSV register map in rather than retyping forty rows; export the whole thing with live values to hand on.
• Writes carefully — off by default, off again every launch, confirmed against a summary, bounded by limits you set, read back afterwards, and with a Put It Back action that rewrites what the register said when you arrived.
• Custom control screens — build a drag-and-drop panel of readouts, setpoints and switches for one device, sized for a gloved finger.

Notes for anyone connecting to building equipment

• Get onto the same network as the equipment. Controls gear is usually on its own VLAN, and guest Wi-Fi will not reach it.
• Modbus TCP is port 502. 503 and 5020 are both seen.
• Finding equipment means sweeping the subnet, because Modbus devices never announce themselves. On a monitored network that looks like a port scan, so tell whoever runs it.
• Reading is harmless. Writing has no undo — no priorities, no timeout, no release. Read the writing guide before changing anything on live equipment.