The M4RTUX is an I/O expansion option for the obsolete M4RTU that adds eight G4 digital and four G4 analog I/O channels. Included with the M4RTUX is an M4RTUXCAB cable and connector assembly, used to connect the M4RTUX I/O Extender to the M4RTU base unit.. Opto 22. Opto 22. M4RTUX.