RS-485 Information

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HMIs, human machine interfacesET-208Electrical Installation ET-208RS 485 Informationen

 

The RS-485 provides a data bus for the connection of several devices. The communication of these devices with one another is realised via the protocol used for the bus.

Usually, there will be a master inside the bus that controls the dataflow on the bus. For more details on communications control please refer to the documentation of the protocol you are using.

 

Determine which protocol is to be applied in SPSPlusWIN's Project Explorer under Symb_Internet Set port and driver.

 

The optimum connection of a bus system based on RS-485 is to connect the participants (devices) in a chain (known as "Daisy Chain" in transmission technology). The bus is terminated with a resistor in the first and last device in the chain.

 

Principle of a Daisy Chain:

Fgr_DaisyChainPrinzipal

Depending on the application, connectors can be stacked or the devices have an input and output. This way, the individual devices can be connected in a chain.

 

Although a star-based network is also possible, long transmission lines and the lack of EOL resistors in them may result in signal reflections in the line which in turn may interfere with the wanted signal and cause data errors.

 

The maximum cable length is 1200m, but the maximum transmission rate of 10 MBits/s can only be achieved with cable lengths up to 12m.

Longer cables may cause interference from potential differences. This may be remedied by also connecting the ground cable. For really stubborn cases we recommend you use a galvanic separation (e.g. by means of an optocoupler).

 

 

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