Data communication interfaces - WXT530

With RS-485 and RS-422 interfaces, if the baud rate is 9600 or higher and the cabling from the transmitter to the host is 600 m (2000 ft) or longer, you must use termination resistors at both ends of the line.

The WXT530 series transmitters with serial communication interface have built-in termination options. Plain resistor (R) termination or termination with resistor connected series with capacitor can be selected with jumpers. By default, no termination is selected. In RS-422 mode, built-in termination is only between RX- and RX+ lines.

If external line termination is used, resistor range 100 … 180 Ω is suitable for twisted pair lines. Resistors are connected across RX- to RX+ and across TX- to TX+ (with RS-485 only one resistor needed).

1

NO, no termination

2

R, 121 Ω termination

3

RC, 121 Ω series with 4.7 nF capacitor termination

The termination resistors increase power consumption significantly during data transmission. If low power consumption is necessary, connect a 0.1 uF capacitor in series with each external termination resistor or use internal RC termination.

Note that the RS-485 interface can be used with four wires (as RS-422).

The main difference between the RS-485 and RS-422 interfaces is their protocol:

• In the RS-422 mode the transmitter is constantly enabled
• In the RS-485 mode the transmitter is enabled only during transmission (for allowing the host’s transmission in the 2-wire case).

The RS-232 output swings only between 0 ... +4.5 V. This is enough for modern PC inputs. The recommended maximum for the RS-232 line length is 100 m (300 ft) with baud rate 1200. Higher rates require shorter distance, for instance, 30 m (100 ft) with baud rate 9600.