Instructions

Table Of Contents

PSI-MODEMSHDSL/SERIAL

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PHOENIX CONTACT 104275_en_c00

1.5.3 RS-485 W2

The RS-485 W2 interface is similar to the RS-422 interface. The electrical levels and their

logical assignment are identical to those of the RS-422 standard. In addition, the RS-

485 W2 interface offers the option of multipoint connections.

In the case of a multipoint connection, all devices are addressed and identified via a single

address. Only one device may transmit at any given moment, all other devices are in "listen-

ing mode".

In the case of the RS-485 W2 interface, W2 stands for 2-wire technology. Data transmission

is based on the half duplex method, where data is alternately transmitted and received.

Figure 1-4 RS-485 W2

The 2-wire bus cable can be up to 1200 m long. It must be terminated at both ends by a ter-

mination resistor (100 ... 200 ohms). The individual devices can be located away from the

bus cable using branch lines. The length of the branch lines depends on the data rate. We

recommend that the branch lines used are as short as possible.

The transmission speed is up to 2000 kbps when using a twisted pair shielded data cable.

The RS-485 standard only describes the physical properties. RS-485 interfaces are not

necessarily compatible with one another. Additional parameters such as transmission

speed, data format, and data encoding are defined in system standards such as PROFIBUS

or Modbus.

Properties of RS-485 W2

Standards ISO/IEC 8482/DIN 66259-4, TIA/EIA-485 (draft)

Transmission speed 2000 kbps, maximum

Transmission length Depends on the data rate

Method Voltage difference, twisted pair

Principle Half duplex, multipoint

For additional information, please refer to "Connecting the serial interface" on page 2-9.

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