User Guide

AXIS 2490 User’s Manual The Unit Connectors

communication is initiated. RS-485 is similar to RS-232, but without a signal ground and

with double-wired transmit and receive data. The wires are TxD+, TxD-, RxD+ and RxD-.

Data detection is done by measuring the voltage difference between the positive and

negative line. This eliminates disturbance by interference, because interference appears

identically on all lines simultanously. This means greater distances can be covered than

with RS-232.

RS-422

RS-422 is similar to RS-485, but uses 2 pairs of wires (4-wire RS-485). This provides a

full-duplex communication line that supports cable lengths up to 1220 meters (4000 feet).

RS-422 is a so-called 'differential' interface. This means that the signals are transported

over balanced circuits. In practice this means that you will have two dedicated wires for

each signal. Data is coded as a voltage difference between the two wires. Each signal has to

be transported over one pair. Because these wires have to be a twisted-pair, any noise

picked up will influence the voltage level of both wires in the same way and the difference

between the wires will not be affected. Thus the information is much less receptive to noise

than when using RS-232.

Delimitations for RS-485/422

RS-485 and RS-422 are similar, although there are differences in the electrical

specification. Most driver circuits are compatible with both, at least for the receiver part.

The AXIS 2490 Serial Server is designed primarily to work with RS-485 devices, although

it will probably work with most RS-422 devices as well. A terminating 100-120 Ohm

termination resistor might be needed between RX+ and RX-.

Physical Connections on the I/O Connector Block

Screw terminal connector with 8 poles:

• (Phoenix MC 1.5 - 3.81 mm)

• 4 pins for RS-422/485 TX+, TX-, RX+, RX-

• 1 pin for RS-485/422 ground (connected to GND with 100 ohm resistor)

• 1 pin for GND

• 2 pins Alternative power (9-24V AC)