The controller is located in a specialized enclosure and consists of three main parts: Mini ITX computer, FPGA controller, and the connection. There are also a number of ancillary components such as power supply, fans, and memory components.
Note that there are two different Modbus network systems in use, MTA wiring and CAT5 wiring. The CAT5 system is more recent. Ensure you are choosing the correct parts for your wiring system.