Recently, I needed to check a Modbus TCP server. I went through several client utilities, and I liked one by Schneider most of all:
It also works with Modbus RTU via the COM port. The only drawback is that it only works with default port 502.
With the ability to specify custom port, I liked the utility:
It has slightly less convenient user interface, and .Net framework should be installed, but there are sources in C #, Java, Python and a DLL in C # so you can use them in your programs.
Here is a direct link to the binaries: https://sourceforge.net/projects/easymodbustcp/files/latest/download
Modscan is another useful utility that allows continuous cyclic reading:
Modpoll is a nice small command-line utility that supports Linux: