FotS

See this post on TBS for an interesting project by member @Volhout.
Modbus Terminal on TBS

SNOOPY
Snoopy is a modbus spy. You connect it as a device to an existing MODBUS RTU system, and it lists all traffic on that bus. It is not sophisticated, but just does (only) that. Snoopy runs on a picomite. Connect a 485 driver chip (i.e. MAX485) to COM1 of the pico (GP0/GP1) and connect RE and DE pins to GP2. Same as shown in the first post.