Add modbus slave/server support

[pazzarpj]

Hey @nicomt, Do you have any need for a modbus server? If so what do you want to see?

[nicomt]

@pazzarpj Hi thanks for reaching out. We are just using a Modbus client right now and there is no immediate need. But I do think it would be interesting to have a Modbus server, It would be cool to allow other people to integrate into our system over a standard interface. Let me know if there is anything I can do to help.

[pazzarpj]

@nicomt Hi Nick, thanks for responding. I think I'll do my own thing at the moment but when I'm close to ready you could have a try and test it out. I'm trying to solve the following use cases:

• Full python device (Useful for simulation of devices and behaviors through hooks)
• Gateway (Parse the requests through to other modbus devices by unit id. Eg. Modbus TCP to ModbusRTU pass through to multiple devices.)
• Remap (Kind of like a gateway but registers might need to be transformed between devices. Eg. Multiple devices relevant data might be remapped to a single modbus table)