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With no experience with Modbus, I'm creating a python script running on a slave computer receiving Modbus RS485 data from master ( a Siemens control computer). Then store it into a text file.

The whole system is on a ship which is not so convenient to reach. Therefore, I want to be able to simulate it in an office, using 2 computers and some connectors probably? Some advice? Thank you very much.

Yang
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  • If the modus is RS485 then there is a good chance it will also be using RTS assertion which involves the node that will be transmitting to assert the RTS line high in order for it to take control of the bus. The issue you may have is that RTS assertion requires spot on timing, what is the host operating system? – SPlatten Dec 20 '17 at 10:34
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    I don't really understand you, but the operation system of the slave computer is windows 7, of the master I'm not sure. – Yang Dec 20 '17 at 14:01
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    Windows is very bad at timing, RTS is Ready To Send, its the way devices on an RS485 network indicate which device is in control of the bus. Windows can't time accurately assertion of the RTS which is why there are devices available that will do this for you when they automatically detect data on the bus, see KKSystems for such devices, they work great. – SPlatten Dec 20 '17 at 14:03

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Give the pymodbus package a try and get some USB-modbus adapters so that you can try loop-back tests. Just be careful to find adapters that support both slave and master modes of operation.

Jon Cage
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