@Bill-Lee The protocol used on the SPI is not exactly the same as Modbus. So you should use the TCP Modbus Server that we provide (maps of registers to IOs can be found at http://downloads.unipi.technology) it will create the /dev/extcomm/x/y serial line. The /dev/extcomm/0/0 and 1/0 both points to the same serial line (since there is only one). It is only for some developers to ease acces to it.