For the C0135 relay board there is now a beta release that implements the following FCs:

FC01     Read Coils
FC02     Read Discrete Inputs
FC03     Read Holding Registers
FC04     Read Input Registers
FC5     Write Single Coil
FC6     Write Single (Holding) Register
FC15     Write Multiple Coils
FC16     Write Multiple Registers

I ran tests with QModMaster and also made sure that the installation is feasible for people who don't care much about the implementation :-)

The entry point for "general audience" is the link to MODBUS RTU releases on the C0135 Relay Control Board page in the STM8 eForth Wiki.