We have connected many different brands of HMI to the Trio using Modbus RTU on RS232.  Typical serial communications setup below.

baudrate = 9600

ADDRESS = 1

SETCOM(baudrate, 8, 1, 2, 1, 4) ' 8 bits, one stop bit, even parity

The HMI will use Modbus function 3 to read "Holding Register" values.  The VRs are used in the Trio as Holding Registers.  Note that there is an address offset from official Modbus, so Holding Register 40001 is mapped to VR(0).  So for reading VR(100) use Holding Register 40101.