You wrote a Modbus TCP server

Point FieldKit’s client at your board.

Find devices confirms that your port answers, with Modbus or without it.
Quick read shows whether your byte order and addressing match what you intended.
Bad requests show up by exception name. Advanced raw request lets you test the edge addresses and quantities for writes.
Live monitoring polls at 1 to 60 second intervals, handy for spotting a leak or a stall over a long run.

If your server should accept 0xFF or 0 as the Unit ID as well as 1, try all three. What the guide recommends.

You wrote a Modbus TCP client

Start FieldKit’s Server on the iPhone and point your board at the address on screen, port 1502. Now you have a server you didn’t write. Edit values on screen and check that your code sees them. Writes from your code appear in the register map about twice a second, and counters show requests, responses and exceptions.

Ask for an address outside the map and you’ll get exception 02. Ask for a function it doesn’t support and you’ll get 01. That lets you test your error handling, and you won’t have to break anything real.

Read the frames

The frame decoder splits a hex dump from your serial log into fields and checks the length and CRC. The frame byte by byte explains each one. For automated tests, a pymodbus server runs on a laptop.

Limits to know

The FieldKit server runs in the foreground and speaks Modbus TCP only, without RTU over TCP. It’s best for interactive checks. Use pymodbus in a test suite or for runs that should go unattended.