Pymodbus - modbus rtu over tcp

[Leonid Polenov]

Help me please!

I understand how to use modbus rtu over tcp when my program creates connection to remote host (serial server), which has connected a slave device by RS-485 (it's good when there is a static IP). But in my next task I should send requests to slave modbus rtu device, when remote host will work as client to my program, because really IP is dynamic and I don't want to use DDNS-service. 

I try to describe as diagram:

[MyProg as server (should take incomming connection and after that send request to remote device)] <-ethernet-> [TCP-client(device tcp to serial)] <-rs-485->[Device with Modbus RTU in slave mode]

How can I do it?

Thanks.