Modular, cheap (~ 220 EUR basic modul + TCP/IP modbus), well distributed on various markets. It can operate either in PLC mode, using programme, or as relay switcher, with outputs fully controlled by TCP/IP modbus. TCP/IP modbus is supported by RTS2 on connection level.
Schneider provides software for programming. Once you will find out advanced features (=macros), you will be quite happy with it. The software runs on Linux under Wine (ask Petr Kubánek for details if you fail to run it). Software allows quite nice visualization of the results. Roof control in Zelio, which I designed and tested in ~ 5 hours (ask email@example.com for file), allows: