This is a fork of NModbus (https://code.google.com/archive/p/nmodbus/downloads) version 1.10, that is the last stable version (1.11 has a bug documented here http://www.mesta-automation.com/modbus-with-c-sharp-libraries-examples/#comment-392 and here http://www.mesta-automation.com/modbus-with-c-sharp-libraries-examples/#comment-562).

If you are looking for something similar, I would recommend NModbus4 (https://github.com/nmodbus4/nmodbus4), which is a more up-to-date and maintained fork.