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HMIs, human machine interfaces ► ET-208 ► Electrical Installation ET-208 ► Connection to ► X2 - COM1 RS-422 / RS485
The X2 ports of our devices provide you with the option of using an RS-422 or an RS-485 bus, depending on the wiring.
The RS422/RS-485 interface of the devices is provided at terminals 1-4 at the X2 and X3 terminal blocks. The signals have the following meaning:
Pin
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Name
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Definition
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1
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TxD-b
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Data/Out - the data is sent from here. Negated signal
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2
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TxD-a
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Data/Out - the data is received here.
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3
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RxD-b
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Data/In, this is where the data is received. Negated signal
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4
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RxD-a
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Data/In, this is where the data is received.
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Circuit as for RS-422:
In the RS-422 the only thing that is defined are the electric characteristics of the interface (OSI reference model, level 1). No standard exists for the connector pin assignment or the flow control protocol (control of data exchange, level 2), and you have to refer to the assignment information contained in the manuals of the connected devices for the exact wiring. The connected devices furthermore have to support the same protocol.
The RS-422 is mainly intended for the realisation of point-to-point connection. In process technology it is also used to realise a bus system (multi-drop) with (n) participants.
Classic point-to-point connection:
Multi-drop connection:
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Device 1
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Device 2
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Device (n)
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Circuit as for RS-485:
Below is an example of how this works with an RS-485.
The RS-485 (today: EIA-485) provides a data transfer interface with differentiated signals. Please note that the RS-485 bus is a bi-directional interface. This means, the interface sends and receives data. Connected devices therefore have to ensure communication can take place.
The following illustration shows how it should be connected to the device's interface:
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The maximum length of the data transmission cable is 1000m.
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The EOL resistor (120 Ohm) of the receiving circuit is switched on in the  Communication parameter menu under "Termination".
The PIN configuration of the other unit will be detailed in the interface description issued by the unit's manufacturer.
cable type:
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Data connection at least UTP [unshielded twisted pair] in rough environment STP [shielded twisted pair]
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See also:
 RS 485 Informations
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